95/96 Ls400 Low Idle, Engine Dies And Jerking Thre

[LEGENDICON]

Hey, sorry fellaz but I got too fed up with this problem so i gave up and traded in my LS400. I Just picked up a new CADILLAC Deville DTS black on black. Wow this car is incredible. It has so many features i dont even know what to do with them. the ride is better than the lex, more power and handleing is twice as good as the lex, and between both these V-8's the cady gets better gas mileage. I thought The interior on was nice on the lex but the cady puts it to shame with more compfortable seats, softer leather, more room and user friendly features. Dont get it twisted I did like the lex alot except for the constant problems, but to me the cady just blows it away. And good luck trying to fix this studdering problem if u'z are still experiencing it because 2 months of mechanics not being able to fix mine was too much and i had to give up. so good luck

[pfleyva]

Thanks for the info:

I have a 94 california LS400 stalling when the A/C cycles off or is switched off. I'm being told it's the throttle body also, but it has been happening since the car was new.

Is what your saying relavent to my problem.

SKPerformance, can you please comment?

[95 ls400]

I have a '95 with the California ECM -231. It has the stalling problem that the rest of you have. It has not been a problem all summer because the A/C has been on most of the time, but it just stalled again yesterday coming off the freeway in 50 degree weather. I can't take a chance that my wife will be driving when this happens, so I just found a 49 state -224 ECM on car-part.com and it is being shipped to me for $200.00, which I think is a pretty good deal if it works. I will install it this weekend and let you all know how it goes. I expect success, judging from the other posts. Thanks everyone for your information.

[cardona6569]

You're the first to have a 94 with all the identical symptoms that I know of. Most guys with earlier models cleaned the throttle body or replaced the idle control and solved the problem.

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Was waiting for the weather to clear up and attempt the throttle clean up as the first step before hitting at the ECU.  I found major carbon deposit on the inside. Using a household oven cleaner, I patiently dissolved all the hardened carbon deposit on the valve disk. Now when the valve disk is fully closed, I can see a nice clean clearance gap for the air to flow through at idle.  I suppose this was the reason for the engine dying after a long haul.

I notice my cold start idle speed went up slightly - 1100 rpm from 800rpm previously. I tried getting off the ramp with A/C off - no dying this time! Will see if I can keep repeating this test and never fail.

My reasoning - there appear to be 3 cables keeping that valve open - cruise control, accelerator and perhaps idle control for cold start. So after a long haul, when getting off the freeway, none of the three cables are pulling on the valve to increase its opening and you need that minute clearance to sustain the air flow for idling the engine.  With carbon deposits blocking it, no wonder the engine was choking and dying.

Now I need to deal with the hesitation at 45 mpg when accelerating uphill. That obviously has nothing to do with the throttle body cleanup as at 45 mph, the valve disk is open quite a ways.

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Hello: My opinion on that jerking at 45MPH is to first clean check the air filter, if not done regularly and change the fuel filter if you have more than a 100,000 miles on the stock one. That is usually the trade mark of a dirty fuel filter.

C. PR

[nc211]

the jerking is called "Off Throttle Shock" as identified by lexus. they attribute this to the faulty ecu sending a signal to the engine that drops the fuel supply to the enging too much when you take your foot off the gas.

[LS400jockey]

My ls400 95 (144000 mile) is right now at the Lexus dealer with this issue, this is the second dealership that will be looking at it. The first did not fix it because they could not reproduce it.

I drive it on the freeway every day.

When I drive it for a distance greater than 30 minutes, speeds over 65, brake at a right turning off ramp, idle goes down and it stalls. The car immediately starts back up. Drives fine on the city streets, no more stalling after restarting the car.

I get the "off throttle shock" as well. Thanks for giving me something to try.

[cardona6569]

My ls400 95 (144000 mile) is right now at the Lexus dealer with this issue, this is the second dealership that will be looking at it.  The first did not fix it because they could not reproduce it.

I drive it on the freeway every day.

When I drive it for a distance greater than 30 minutes, speeds over 65, brake at a right turning off ramp, idle goes down and it stalls.  The car immediately starts back up.  Drives fine on the city streets, no more stalling after restarting the car.

I get the "off throttle shock" as well.  Thanks for giving me something to try.

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HI: If fuel filter/airfilter is good, injectors clean, I would go with the mass/airflow sensor, have it cleaned first and then replaced, it might just be corroded in the electrical parts.

C. PR

[prix]

thanks for all your hard work. i have this problem intermittently, where if i turn off air the car quits. starts right back up, always, though. by intermittently, i mean it goes through phases, where it might do this five times over a period of a few months, then nothing for another 6 months. then it comes back for a time. v. strange. i will store this info on the offchance i ever need it. really good detective work on this one.

]

Here's the results after a 70 mile run at 80 mph non stop.

Be sure to read the entire thread. These are the results of replacing my Calif ecu 89661-50231 with a 49 state ecu 89661-50221.

The car idles 600 rpm..absolute steady..not a waver on the tach. Even after the sustained highway driving (it used to die at idle.) This proves my problem was the ecu.

The car has much better power everywhere from stoplight to stoplight or accelerating on the highway. This shows the Cali ecu's are as suspected...cut back on engine performance.

The jerking that happened when closing the throttle and then getting back on the throttle at around 45 mph has only occured once with the 49 state ecu and it was nowhere near as harsh.

So I will now return this borrowed ecu and hunt down a 49 state unit and install it in my car...or will go for the "improved" ecu if I find one at a reasonable price.

Hope this helps someone in the future.

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[cardona6569]

thanks for all your hard work. i have this problem intermittently, where if i turn off air the car quits. starts right back up, always, though. by intermittently, i mean it goes through phases, where it might do this five times over a period of a few months, then nothing for another 6 months. then it comes back for a time. v. strange. i will store this info on the offchance i ever need it. really good detective work on this one.

]

Here's the results after a 70 mile run at 80 mph non stop.

Be sure to read the entire thread. These are the results of replacing my Calif ecu 89661-50231 with a 49 state ecu 89661-50221.

The car idles 600 rpm..absolute steady..not a waver on the tach. Even after the sustained highway driving (it used to die at idle.) This proves my problem was the ecu.

The car has much better power everywhere from stoplight to stoplight or accelerating on the highway. This shows the Cali ecu's are as suspected...cut back on engine performance.

The jerking that happened when closing the throttle and then getting back on the throttle at around 45 mph has only occured once with the 49 state ecu and it was nowhere near as harsh.

So I will now return this borrowed ecu and hunt down a 49 state unit and install it in my car...or will go for the "improved" ecu if I find one at a reasonable price.

Hope this helps someone in the future.

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Hi: You are in the right track, it must be something electronic if you ruled out things like dirty fuel filter, because the problem at 45 MPH looks like bad filter. Second, I had a similar problem with a Brandnew GM car, it would run ok, suddenly started jerking and not moving, when I turned it off it would run like a dream. Conclusion bad mass airflow sensor, it did not do it all the time either, I had to let it cool down so the dealer would start it in the morning so the problem would happen. I would go for cleaning the Maf sensor, and see what happens, in addition to the CPU thing. I think the problem has two parts the ECU and the other less obvious at this time.

C. PR

[lexluxury]

having similar problem. if anyone who owns a 94 ls400 has figured out what the 'real' part number for the ecm (ecu?) w/b for this year, please post. i'll wager that if everyone on this board with this problem contacted their state's atty general (consumer division), that lexus might do something about this. i agree it is dangerous. happened to me again today in heavy traffic...i was able to pull off to the side, but since i have power steering, that was VERY hard to do. started right up again, as it always does.

I am trying to make a thread here for the many of those with low idle, engine quits after an extended highway cruise when coming to a stop, and a mysterious drivetrain jerk when you let off the throttle and get back on....sometimes...and usually happening over 40 mph.

My 95 Ls400 built april of 95 which now has 150,000 miles has these problems.

I've done full diagnostics. I've gone after the throttle body and the Idle control valve. All of which were working perfectly. After 3 days of reading threads at several different forums of similar problems with 95's, several emails and follow ups on who found what the problem was and how solved I wanted to put all the information here so the next guy could find it easier.

Some guys are driving around with the a/c on to keep the engine running at a faster idle to keep the car from dying.

I'm convinced all 3 problems are related to the ECM and it's controlling of the idle air control. While there were a couple of TSB's on this it was never a recall. Those of us who bought used or have high miles are out of luck for any dealer help...at least I would think.

Your Ecm is located behind the glovebox on the passenger side. If you want to read what number Ecm you have..you can do that by not removing it. Just remove the lower plastic panel below the glovebox and look up in there. You'll see "Engine Computer Label" and the numbers.

For 1995's:

Your original Ecm is either:

89661-50220 or 89661-50221 if Federal (49 state). Lexus new replacement on the TSB says to replace with 89661-50224

If California then your orginal Ecm is either 89661-50230 or 89661-50231. The TSB calls for 89661-50234

But searching a local Lexus parts site shows that number has been replaced with 89661-50235-85.

The cost of these Ecm's new is just total crazy. But there might be another solution.

One member bought a used pcm from a wrecking yard #80661-50224. He paid about $150 for it and it solved his issues. I did some searching in wrecking yards and there are some of these #'s out there.

If you have a California car then there seems to be few out there. Mine is a California car. I don't know if a 49 state Ecm will work...I suspect it will...but unknown at this time.

Another member bought a remanufactured Ecm for his 49 state car. He paid $300. for it and it has a 1 year warranty. He also stated that it cured all problems. The company he purchased it from was Auto & Truck Electronics in West Palm beach, Florida. He said the part number was #95 Int 56834. (That was a 49 state unit).

There are other places on the web where rebuilt Ecu's are sold. The problem is most are listed by the old number. We have no way of knowing if the old units are rebuilt and then "reflashed" to the later specs which we are looking for to solve these issues.

I'll try to find out about that and post here what I find out.

For 1996's: The Ecm you want is 89661-50303. Original's were 89661-50302. A wrecking yard buddy did a nationwide search and there are used 89661-50303's out there. Evidently these work in all 50 state cars.

Ps: A 1996 Ecm will not work in a 95. It turns the check engine light on. That's been tried.

Update: I just received an email from the gentleman who bought the remanufactured pcm from Auto & Truck Electronics(mentioned above). He confirmed with them that the Ecu had the latest software upgrade before he bought the Ecu. He also commented the car shifted smoother than it ever did.

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[nc211]

Man, this thread is like the freakin' energizer bunny.

If we want Lex to own up for a recall, you've got to notify the national highway safety and transportation board. There are a few notices on the issue "I think 10" to them already. I used this fact, and their own recommendation of the fact that out of all the TSB's issued for the car, this is the ONLY one LEXUS HIGHLY RECOMMENDS BE DONE! That, a few calls to Lexus USA, and some legal mumbo-jumbo to the manager of the dealership, and they caved in, paid for it to be fixed. But, my story also included a 90 day / 3,000mile warranty. I have noticed however on the 95-97 used models that dealership gets....it's now "AS IS" on the sticker! Hahahaha. Sorry guys.

[lexluxury]

will clean fuel filter first, then the throttle body(?), and see what happens. might try the ecm solution rec here if these things don't take care of it.

Man, this thread is like the freakin' energizer bunny.

If we want Lex to own up for a recall, you've got to notify the national highway safety and transportation board. There are a few notices on the issue "I think 10" to them already. I used this fact, and their own recommendation of the fact that out of all the TSB's issued for the car, this is the ONLY one LEXUS HIGHLY RECOMMENDS BE DONE! That, a few calls to Lexus USA, and some legal mumbo-jumbo to the manager of the dealership, and they caved in, paid for it to be fixed. But, my story also included a 90 day / 3,000mile warranty. I have noticed however on the 95-97 used models that dealership gets....it's now "AS IS" on the sticker! Hahahaha. Sorry guys.

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[cardona6569]

will clean fuel filter first, then the throttle body(?), and see what happens. might try the ecm solution rec here if these things don't take care of it.

Man, this thread is like the freakin' energizer bunny.

If we want Lex to own up for a recall, you've got to notify the national highway safety and transportation board. There are a few notices on the issue "I think 10" to them already. I used this fact, and their own recommendation of the fact that out of all the TSB's issued for the car, this is the ONLY one LEXUS HIGHLY RECOMMENDS BE DONE! That, a few calls to Lexus USA, and some legal mumbo-jumbo to the manager of the dealership, and they caved in, paid for it to be fixed. But, my story also included a 90 day / 3,000mile warranty. I have noticed however on the 95-97 used models that dealership gets....it's now "AS IS" on the sticker! Hahahaha. Sorry guys.

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Hi: I would do both, clean the Throtle Body, extensive threads on it getting carbon up, even mine was, and the fuel filter change, which is not a bad idea either, even as they hardly go bad on the Lexus. But since you have some problems that might point to it, you may have to change it.

Sincerely,

C. PR

[95LS400Bob]

It's an ECU problem.

Throttle bodys that are dirty don't wait for 30 min or more of highway driving before showing a problem....nor do they fix themselves as soon as the car restarts.

Probably the best question is the "why" the ECU starts doing this. It certainly didn't do it when it was new. It has to be an aging process...maybe something to do with the circuit board itself...from the many hot and cold cycles it has done over it's lifetime.

And this could very well be why many of these same cars get the "P0" check engine lights after a cruise.

[lexluxury]

well, replacing the fuel filter did not solve the problem. a day or two later, after i shut off the air, kaput. bob, just a note, but my car does this just in 'around town' driving. in other words, i'm not moving at high speeds on any freeways, just stop and go traffic. on occasion, if i run the air and then turn it off, this happens. not every time, but even once in a week or two is a problem that i do not want to have. how do i find one of these 'reflashed' ecus? might be worth a shot. or, rather, how do i figure out what # part i need? can someone point me to where i read up on service bulletins by lexus? my car did not start doing this until summer ... so the ageing process makes sense.

uote=95LS400Bob,Nov 6 2005, 12:10 AM]

It's an ECU problem.

Throttle bodys that are dirty don't wait for 30 min or more of highway driving before showing a problem....nor do they fix themselves as soon as the car restarts.

Probably the best question is the "why" the ECU starts doing this. It certainly didn't do it when it was new. It has to be an aging process...maybe something to do with the circuit board itself...from the many hot and cold cycles it has done over it's lifetime.

And this could very well be why many of these same cars get the "P0" check engine lights after a cruise.

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[95LS400Bob]

If your car dies or low idle and does not require a "highway cruise" to do this, then I would definately look other places first.

The car dying or low idle around town would sound like another issue. First place I would look would be the idle air control valve. Perhaps it is sticking or bad. Throttle body could cause these problems also.

The idle air control valve is an air control valve that contains a pulse motor. The ECU pulses that motor to open the valve to regulate idle speed. Any change in engine load is sensed by the ECU and the ECU pulses the IAC to open or close slightly to allow more or less air to the engine to maintain the proper idle speed.

But the engine dying after a highway cruise but acting totally normal otherwise has been found to be an ECU issue. I have no way of knowing the programming Lexus is doing to the ECU but it is my bet that after "X" amount of rpm in high gear...the ECU commands the IAC valve to close. Maybe Lexus does this for better signal through the MAF sensor...or maybe they just program it to close (or nearly close) since idle is not needed. But for some reason it fails to open when we return to idle after the highway cruise. If we restart the car....all is normal again..for the ecu resets and we idle normal again till the next cruise.

That's the ecu issue and doesn't sound like you have that.

[LS400jockey]

My ls400 95 (144000 mile) is right now at the Lexus dealer with this issue, this is the second dealership that will be looking at it.  The first did not fix it because they could not reproduce it.

I drive it on the freeway every day.

When I drive it for a distance greater than 30 minutes, speeds over 65, brake at a right turning off ramp, idle goes down and it stalls.  The car immediately starts back up.  Drives fine on the city streets, no more stalling after restarting the car.

I get the "off throttle shock" as well.  Thanks for giving me something to try.

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My cars ECU part number is 89661-50233. The dealer was able to reproduce the stall probem twice in 2 drives by letting a mechanic drive it home. He lives 60 miles from the dealer Their fix for the problem was to clean the throttle body and the IAC. So far this seems to have resolved the problem as he drove it home twice after the cleaning and the problem did not repeat. So far I have not driven it far enough to confirm it does not happen to me, I'll do that soon.

[95LS400Bob]

Please keep us posted on this. My throttle body and IAC were clean and I could not catch the IAC malfunctioning.

Also...that ECU that you list 89661-50233. I don't seem to find that part number anywhere. I do show the latest 95 ECU was 89661-50234. California ECU.

[Kickers]

I had 95 Lexus LS400 that bought used 7 years ago. I had not taken cars on long drives until just recently. I also had problem with stalling after extended freeway drive when exit freeway. Lexus told me that they believed ECU issue and $2,300 repair. In researching this issue came across this board and posts advised me of others having this problem and that Lexus had issued a TSB on this issue. The local dealer (had not bought car from them) advised that as car out of warranty they would not cover. I made point that this was an original defect with car. Discussion did not go anywhere so spoke with Lexus customer service. After much discussion they told me they would give 25% discount. That was not acceptable as felt should be 100% cover as this is not a warranty item that just wore out but a defect in the car. Had numerous calls and letter where advised that I would report to NHSB, wrote to them about their potential liability if they allow this to continue and somebody gets hurt. I also referenced this board and number of posts and would report level of problem to the NHSB. Bottom line was after fair bit of effort on my part Lexus agreed to replace the ECU at no cost to me. So they did the right thing, I just wish it had not taken so much prompting.

[prix]

-Bob,

Actually, this is happening on my '94 with just 'around town' stop and go driving. I can be zipping around town in the 30-40 mph range, and then if i turn off the AC (especially when I am slowing down or stopping), the car dies. Not every time, but just once in awhile. The steering always gets stiff for a few days before I am going to have an 'episode', I assume because it is not getting proper electronic assist? I have also noticed lately some fluctations in the idle...seems to go off on its own looking for the right idle. It's not too bad, but something I've noticed lately. I don't do a lot of freeway driving, so do not know for certain, but assume that it would die then as well (coming off the freeway as you described). Looks ot me like I am in the same boat as others here...haven't noticed any jerking, and don't do freeway driving, but other than that, symptoms are exactly the same.

Quite a deal isn't it? LOL. We have these wonderful quiet smooth running cars until we take a highway cruise and then they quit at a stop light. Restart and all is normal again. Till the next highway cruise.

Meanwhile you drive around town and all is normal.

Drove me nuts. I had to find it.

For the record...see if you can find out what number is on your present ECU and what number is on the one they replace it with.

And we are all glad we are training your dealer. Instead of throwing parts at it.

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[lexluxury]

Can you give us some follow=up? Did you finally replace the ECM and if so, did this work?

Ok. Im the guy with a 93 Ls 400 with the same problem as you 95ers and im on the very of insanity now fellas. So here we go; so far in effort to correct this problem i started small and inexpensive and worked my way up. I have replaced the spark plugs, wires, oil filter , fuel filter, air filter, caps, and rotors, NOT FIXED. So I cleaned the throttle body, and replaced the Air Flow Meter. NOT FIXED. now i just had the dealer put in a new idle control valve where they said that was definalty the problem. funny, they were wrong, because I stalled out on the way home from the dealership. So back the car went, they are currently running all types of tests and diognostics on it. hopfully there genius mechanics will resolve this problem and I can report good new with the cure to all of you. but any1 come across some answer please keep us all posted.

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[prix]

Just wanted to add that i have had the throttle body cleaned, replaced the fuel filter. The car still dies with around town driving off and on, usually when I am slowing down. As I said, the way I know it's going to happen is the steering starts getting firmer/kind of stiff. Sometimes it will do that for days before the car actually dies, then it seems to 'reset' and the steering is fine for a time. Sounds like ECM?? Ideas?

-Bob,

Actually, this is happening on my '94 with just 'around town' stop and go driving.  I can be zipping around town in the 30-40 mph range, and then if i turn off the AC (especially when I am slowing down or stopping), the car dies.  Not every time, but just once in awhile.  The steering always gets stiff for a few days before I am going to have an 'episode', I assume because it is not getting proper electronic assist?  I have also noticed lately some fluctations in the idle...seems to go off on its own looking for the right idle.  It's not too bad, but something I've noticed lately.  I don't do a lot of freeway driving, so do not know for certain, but assume that it would die then as well (coming off the freeway as you described).  Looks ot me like I am in the same boat as others here...haven't noticed any jerking, and don't do freeway driving, but other than that, symptoms are exactly the same.

Quite a deal isn't it? LOL. We have these wonderful quiet smooth running cars until we take a highway cruise and then they quit at a stop light. Restart and all is normal again. Till the next highway cruise.

Meanwhile you drive around town and all is normal.

Drove me nuts. I had to find it.

For the record...see if you can find out what number is on your present ECU and what number is on the one they replace it with.

And we are all glad we are training your dealer. Instead of throwing parts at it.

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[95LS400Bob]

The ECU and dying problem was more related to the 95 and 96's from all the research I did. And that was after a highway cruise....otherwise it never happened.

As I remember, the IAC is different on the 94's than the 95 and up. The IAC would be the first place I looked with the problem on your car. Not saying it is not a ECU issue..but I would go for the IAC first.

We know the engine computer controls the idle function through the IAC.

Opens that valve to let more air in to speed up the idle depending on load.

First thing to watch for is cold start and warm up rpm. If the engine goes to high rpm and then tapers down in rpm as it warms up...then the IAC is working. Should also be a difference when turning on the a/c.

If it is doing all that normal, I think I would look at the connections at the IAC and also remove it and make sure everything is clean.

A new IAC is a $510.94 item...so it would be nice to find a used one for sure if it is suspect. 1993 and 94 Ls400, SC400 use the same IAC.

A new ECU would cost close to $1000. Again 1994 is different. And I show 1994 ecu is by itself as far as part number.

Searching a used parts site, the numbers I come up with for an ECU for 1994 Ls400 is:

89661-50142 if it is a 49 state car.

89661-50151 if it is a Cali car.

ECU from a 93 might work and probably would but a 95 up will not for sure. 93 ecu numbers are 89661-50141, 89661-50140 for 49 state. I don't find any different numbers for Cali cars in 93.

Hope this helps a little. These intermittent problems can be a bugger to find.

[prix]

Bob -- V. nice of you to post those parts numbers. Those prices are a tad scary. A grand for a computer chip? I'll have my work cut out for me (finding it cheaper, that is, if that's the issue). I take it you never tried that $99 rebuild (saw an ad on this thread). Also, was curious if you ever resolved your issue, and, if so, how. Maybe I missed something, but I thought the last info I read was that your problem came back. Well, again, thanks. Will post back here when I know more.

The ECU and dying problem was more related to the 95 and 96's from all the research I did. And that was after a highway cruise....otherwise it never happened.

As I remember, the IAC is different on the 94's than the 95 and up. The IAC would be the first place I looked with the problem on your car. Not saying it is not a ECU issue..but I would go for the IAC first.

We know the engine computer controls the idle function through the IAC.

Opens that valve to let more air in to speed up the idle depending on load.

First thing to watch for is cold start and warm up rpm. If the engine goes to high rpm and then tapers down in rpm as it warms up...then the IAC is working. Should also be a difference when turning on the a/c.

If it is doing all that normal, I think I would look at the connections at the IAC and also remove it and make sure everything is clean. 

A new IAC is a $510.94 item...so it would be nice to find a used one for sure if it is suspect. 1993 and 94 Ls400, SC400 use the same IAC.

A new ECU would cost close to $1000. Again 1994 is different. And I show 1994 ecu is by itself as far as part number.

Searching a used parts site, the numbers I come up with for an ECU for 1994 Ls400 is:

89661-50142 if it is a 49 state car.

89661-50151 if it is a Cali car.

ECU from a 93 might work and probably would but a 95 up will not for sure. 93 ecu numbers are 89661-50141, 89661-50140 for 49 state. I don't find any different numbers for Cali cars in 93.

Hope this helps a little. These intermittent problems can be a bugger to find.

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[prix]

Meant to add that yes, the RPMs at start up are normal -- 1200=1400, then drops to 650 after a few minutes or so. The only time it drops to 200-400 or just plain dies is when I'm slowing way down to stop, or sometimes when I turn off the AC. Other than that, it's normal. It does idle up when I turn on the AC, the RPMs jump from 650 to 850 or so (very rough guess). So it seems to be working. Might have cleaning done of throttle and IAC valve for starters. I'd much rather it NOT be that computer chip.

Bob -- V. nice of you to post those parts numbers.  Those prices are a tad scary.  A grand for a computer chip?  I'll have my work cut out for me (finding it cheaper, that is, if that's the issue).  I take it you never tried that $99 rebuild (saw an ad on this thread).  Also, was curious if you ever resolved your issue, and, if so, how.  Maybe I missed something, but I thought the last info I read was that your problem came back.  Well, again, thanks.  Will post back here when I know more.

The ECU and dying problem was more related to the 95 and 96's from all the research I did. And that was after a highway cruise....otherwise it never happened.

As I remember, the IAC is different on the 94's than the 95 and up. The IAC would be the first place I looked with the problem on your car. Not saying it is not a ECU issue..but I would go for the IAC first.

We know the engine computer controls the idle function through the IAC.

Opens that valve to let more air in to speed up the idle depending on load.

First thing to watch for is cold start and warm up rpm. If the engine goes to high rpm and then tapers down in rpm as it warms up...then the IAC is working. Should also be a difference when turning on the a/c.

If it is doing all that normal, I think I would look at the connections at the IAC and also remove it and make sure everything is clean. 

A new IAC is a $510.94 item...so it would be nice to find a used one for sure if it is suspect. 1993 and 94 Ls400, SC400 use the same IAC.

A new ECU would cost close to $1000. Again 1994 is different. And I show 1994 ecu is by itself as far as part number.

Searching a used parts site, the numbers I come up with for an ECU for 1994 Ls400 is:

89661-50142 if it is a 49 state car.

89661-50151 if it is a Cali car.

ECU from a 93 might work and probably would but a 95 up will not for sure. 93 ecu numbers are 89661-50141, 89661-50140 for 49 state. I don't find any different numbers for Cali cars in 93.

Hope this helps a little. These intermittent problems can be a bugger to find.

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