90 Ls400 Misfire / Bucking / Jerking, Worse Under Load

[90Duchess]

Hello to all you awesome LOC members! Thank you so much for all your advice and life-saving tutorials that have helped us all out immensely over the years; you guys rock! I am starting a new thread on this issue because after reading through many relevant posts, I was not able to find one where the issue was resolved, despite it being a seemingly-common issue on these cars.

I have a well-maintained 1990 LS400, with approximately 250.000 miles. Last July, in the heat of summer, my car developed a misfire, or bucking. As far as I can tell, it happens all the time to some degree but it is most noticeable under load, climbing a hill. Vehicle speed does not seem to matter as this happens in town and on the highway. For instance, I can coast down a hill, and upon climbing the next, the car will misfire / buck several times in a row, run normal for a few seconds (surge), and misfire / buck a few more times, in no discernible pattern, until I let off the accelerator. While idling in park, I can also listen to the exhaust in back and barely discern something akin to misfiring in the resonance. The car also lopes slightly while warming up in the morning, and goes away when it reaches operating temperature, and the exhaust smells rich and burns my eyes if I get too close.

I have replaced the spark plugs, wires, caps and rotors, twice. I used factory parts the second time, and routed the wires correctly following one of the online wiring diagrams. Reading that the coils can go bad, and knowing that mine were original, I ordered factory replacements and just finished installing them and road-testing the vehicle. I had the throttle body cleaned by a mechanic before this issue started, less than 8,000 miles ago. I replaced the EGR valve a couple years ago, but disconnected and plugged the vacuum line to see if it would have any effect. The fuel filter gets replaced yearly, and the air filter about every other oil change. Nothing so far has solved the issue. There are no stored codes, no warning lights illuminated, but I have reset the computer a couple times anyway to see if it would help. No such luck. I am at a complete loss as to what else to do. I called my shop and they think it is due to worn bushings in the rear, so obviously I am not taking it to them. I am calling on all of the Lexus guys out there, all of whom are infinitely smarter than my shop when it comes to these cars, to please help me figure out what is wrong with my car before serious damage results and I end up losing my most favorite car I have so far owned since 1995. Please help; I will post a solution to benefit others as soon as we figure it out. Thanks in advance!!!

[Yamae]

Sounds like an ECU issue to me.

[90Duchess]

Thanks, Yamae. I am tracking down a confirmed-working ECU to diagnose with. I am also getting it in to test the fuel pressure this week, will update with the results.

[BangEmBoy]

I had a similar problem with one of my LS's, and this is what I did to resolve it; this may or may not help . ... After trying the same things that you did, I took my EGR valves off and cleaned out the passage way's, next I cleaned my throttle body intake, lastly; I cleaned my intake and valves with a bottle of Sea Foam . (Here's how you do that) : look at your intake, in the front you will see two hose's, one big and one small; the big one feed's vacum to your power steering pump . Disconnect that line and get you another piece of hose about two(2) foot long that fits that vacum port . Next start the car and let it idle, then insert the hose that you connected to the vacum port on the intake into the can of Sea Foam . The vacum of the idling engine will suck the Sea Foam out of the can and into the engine . If the engine tries to stall, give it some gas . Let the engine suck about half of the can of Sea Foam, and then let car idle around 1500 rpm's until all the white and or black smoke stop's coming out of the exhaust .

The process that I just described, cleans the intake and valves of deposits that build up over the years . I use this process on the vehicles that I service when there is a lope or misfire that isn't attributed to a vacum leak or tune up . I have had to do the procedure twice on vehicles that had alot of build up .

I hope that this helps you .

[landar]

If it were mine, I would want to hook up a strobe light to the main high-tension wire of each coil (one at a time) and look for a 'hiccup' in the strobing sequence. It might be the ECU, or it might be an ignitor (the driver circuit for the coil). In any case, I would want to do some troubleshooting before replacing parts blindly...that cost a lot of money. If the spark looks ok, you would go after fuel delivery but try to 'divide and conquer' with some logical path first.

Unfortunately, the early OBDI systems such as yours, do not show misfire codes for individual cylinders like the later OBDII systems.

[90Duchess]

Thank you guys for your help!!! It took a bit to get my car in to a "recommended", major third-party shop today, and my service liaison said that the mechanics can't spend all afternoon diagnosing it because they had other work to do, despite my week-in-advance appointment!!! I set up the appointment last week, talking to the same liaison for a good 20 minutes describing the issue and the measures taken to correct it, and he assured me that despite the age of the vehicle that the shop had many older and current diagnostic tools to find most any issue. I went in today and the shop had my car for about 30 minutes before the same liaison that set up the appointment came back and told me that my car was not able to show any error codes that their mechanics could follow. I told him that there weren't any codes last week before I made the appointment! The liaison knew it was a 1990 when I talked to him last week, and prior to OBD-II, but having no stored codes for the mechanic to follow was the excuse I got. I told them fine, but still want them to at least test the fuel pump while it as in, and they did, showing that it has some weakness internally by the EKG-type graph they showed me. However, they assured me that the fuel pump was working; just that the regular spikes in the output-graph had a few in-a-row that were lower (weaker) than the following spikes, and occurred often. The details the shop put on the invoice are that, " too early to have Mode 6 data or misfire counters; Current Ramped the fuel pump and see the pattern is a little uneven, communicators (12 in all) and brushes may have wear, and amperage was about 7.1 amps). I am back to square-one.

***Yamae: I noticed mention to repairing the ECU on Club Lexus during a search; it is the original ECU and I have owned the car since it was a mere six years-old. I believe as you do, that buying another used, old, ECU would have similar or worse problems if not immediately, eventually. I would like to have mine rebuilt with the capacitors you recommend. Is there a service that will rebuild mine, or do you perform this service? Please let me know. If my ECU is not to blame, then someday soon it will fail and cause problems for something else. Obviously, I continue to plan keeping the car for the long-run, so I would be interested in having my capacitors replaced with the new, high performance, non-quarternary-ammonium caps as you recommend. Please advise me; I very much wish to keep the car and stay abreast of preventative maintenance, such as this.***

Thank you all for your suggestions; I have been following them and working them into my diagnoses. I had high hopes for this shop but, alas, their techno-bots only proved that they are not able to think for themselves and can only read a computer screen. You guys are far more talented than the techno-bots I must endure every time I take my car in for something I can't diagnose! I believe that since it is the original fuel pump, I should replace it before it outright fails, but because the techno-bots can't measure fuel pressure or volume through a Banjo fitting ( they took off my throttle cable cover that has TRAC displayed on top of it , LOST it and DENIED ever removing it! looking for a shrader [sp] valve to hook up to ), or even try to improvise an adapter, I am left with the electric graph they gave me as to its performance. What do you guys think?

Also, my research shows that ethanol-based gasoline, even at 10%, can cause damage to the pump, much less the entire aluminum-based fuel system and aluminum heads. In fact, it is warned of in the Owner's Manual as Gasohol in that concentration or higher, and in 2005, Lexus did a recall on the ES, LS, and GS for fuel-related problems because of "gasohol." But my gas station says they have the lowest concentration in town, (for what THAT is worth), and say it is about 8%. (I'm sure....) I never had these problems before I started using their premium, exclusively, a year ago. Could this be a factor? I mentioned my concern to the shop today, but it seems they dismissed it, because they never tested my fuel After the techno-bot road-tested my car, (without me, I might add, driving it "HARD" as my liaison said he told him), they "were not able to duplicate my concern," and they recommended I take it to a transmission shop. Please help guys!!!

[fsuguy]

Hi,

You probably are on the right track with the ECU test. Were you able to get a test unit? I have also read Yamae's pretty comprehensive and informative analysis of the capacitors used in the early versions, and also the reports from members who have followed his advice and their results - all basically good! The pictures with leaking capacitors, however, are very much more convincing. Would really like to know what you come up with, since my LS is an older one as well (1992) and I like it very much - a much better built automobile than the many other types I have owned and driven, except for its electrical quirks!!

By the way, while ethanol is not the best for our cars, I believe the big issue is its impact on the rubber components - hoses, etc., something about the size of its molecules being smaller than the usual non-adultrated gasoline, but I may be wrong about that.

Good luck with the diagnoses!!!

[Yamae]

Sorry for the late reply.

I live in Japan and I only fix ECUs at nearby repair shops as a volunteer. To those who live in USA, I suggest you to purchase right capacitors (only low ESR type capacitors) as explained by LScowboyLS who is a good friend of mine and replace them by yourself or ask someone who is good at soldering. Usually the parts cost is only about $10 and you can fix your ECU.

LScowboyLS has started his thread about a year ago after succeeding to fix his ECU. He is kind enough to write the details. I am familiar to electronics affairs but it is not easy for me to write the detail in my limited English. Now he is helping people to fix the ECU next to next. You can find the thread here below.

http://www.clublexus.com/forums/ls400/656360-all-my-crazy-lexus-issues-solved-ecu-leaking-capacitor.html

[90Duchess]

Thank you guys, for your help! It took a bit to get my car in to a "recommended" shop today, and they said that they can't diagnose it because they had other work to do! I set up the appointment last week, talking to the rep for a good 20 minutes describing the issue and the measures taken to correct it, and he assured me that despite the age of the vehicle, they have many diagnostic tools to find most any issue despite the age of the car. I went in today and they had my car for about 30 minutes before they came back and told me that it was not able to show any error codes that their mechanics could follow. They knew it was a 1990, and prior to OBD-II, but that was the excuse I got. I told them I want them to test the fuel pump, at least, and they did, showing that it has some weakness internally by the EKG-type graph they showed me, but assured me that it was working, just that the regular spikes in the output graph had a few in a row that were lower (weaker) than the following spikes, and occurred often. The details the shop put on the invoice are that, " too early to have Mode 6 data or misfire counters; Current Ramped the fuel pump and see the pattern is a little uneven, communicators (12 in all) and brushes may have wear, and amperage was about 7.1 amps). I am back to square-one.

Yamae: I noticed mention to repairing the ECU on Club Lexus during a search; it is the original ECU, and I have owned it since it was a mere six years-old. I believe as you do, that buying another used, old, ECU would have similar or worse problems if not immediately, eventually. I would like to have mine reb

Sorry for the late reply. I live in Japan and I only fix ECUs at nearby repair shops as a volunteer. To those who live in USA, I suggest you to purchase right capacitors (only low ESR type capacitors) as explained by LScowboyLS who is a good friend of mine and replace them by yourself or ask someone who is good at soldering. Usually the parts cost is only about $10 and you can fix your ECU. LScowboyLS has started his thread about a year ago after succeeding to fix his ECU. He is kind enough to write the details. I am familiar to electronics affairs but it is not easy for me to write the detail in my limited English. Now he is helping people to fix the ECU next to next. You can find the thread here below. http://www.clublexus.com/forums/ls400/656360-all-my-crazy-lexus-issues-solved-ecu-leaking-capacitor.html

Thanks, Yamae, I will order them today and plan on having them replaced this weekend. I have one question, though: On the list of recommended capacitors, clicking the links to the specific ones on Digi-Key, the voltages are not 12v. I will have an electronics technician do the work, but since I am not an electrician, I just want to be sure I am ordering the right ones for my car. Are the ones listed on the page the right ones to get?

***UPDATE***

I spoke too soon; reading through you and LScowboyLs's posts on CL, I was able to educate myself that the voltages on the replacement capacitors must be AT LEAST the stated voltage on the originals, and what is most important is that the capacitance values must match. Today I received the exact capacitors for my 1990 LS400 that you guys specified, ordered from Digi-Key, and am having a computer repair guy replace them for me this weekend. I also ordered a new Lexus fuel pump, but it wont be here till next week. I will update with my results, first from the ECU repair, and later, the fuel pump. I can't thank you guys enough for the time and effort you both have put into the postings on ECU diagnosis and repair; you guys are the best!!!

[landar]

Hi,

By the way, while ethanol is not the best for our cars, I believe the big issue is its impact on the rubber components - hoses, etc., something about the size of its molecules being smaller than the usual non-adultrated gasoline, but I may be wrong about that.

Good luck with the diagnoses!!!

There are some stations that sell 100% pure gasoline. No ethanol mixed in. Of course the fuel is more expensive but if you want pure stuff, you gotta pay. Here is how to locate these stations in your area( covers US and Canada...click on your state or the map for the entire country) -> http://pure-gas.org/

[90Duchess]

Thanks, landar, that site was very helpful! I've found several stations that sell unadulterated premium, and will be switching to it as soon as I'm ready to fill up again. In the mean time, I've been using Marvel Mystery Oil in my gas to help offset the corrosive nature of ethanol. While the shop didn't confirm the concentration of ethanol in my tank, or if I have any fuel-related degradation, they did, at my insistence, test my fuel pump and it showed internal wear. I'm going to run my tank empty, replace the fuel pump with a new Nippon-Denso unit, and fill up with pure premium from one of the stations on the list you provided and stick with it, since these cars can quickly drain your bank account in assorted repairs, especially from careless mistakes, such as maybe continuing to use gasohol when the manual clearly states not to for extended periods. Thanks again, and good luck with your '92! I'll keep posting updates on my repair progress; hopefully our experiences can save others a lot of time, trouble, and expense!

[90Duchess]

Ok, I replaced all my capacitors according to Yamae and LSCoyboyLS directions, and to be honest, I cannot really tell much difference. Maybe it is because the bucking is still present at part-or-higher throttle, I don't know, but the capacitors were not the brownish-colored Nichicon caps in the tutorial directions that failed on so many people; they were all black Rubycon caps, except one that was a purple-ish blue. I know that it doesn't mean much, but none were leaking. The next two avenues i'm trying this week are the TPS and the fuel pump. Let me know what you guys think, thanks!!!

[landar]

Do you cry 'uncle' yet? ^_^

Lets perform a restart here. You can do a lot of this leg work yourself and save the mechanics charge.

You have a lot of miles on this engine so this misfire could be several things. But let's try to isolate the cylinder(s) that is misfiring. That will help a lot. You mentioned that the engine does misfire and 'lope' when idling. That's actually a good thing. What I would like you to try is to ground out each spark plug wire, one at a time, and listen for a change in engine performance. The theory is that grounding out a spark plug will disable that cylinder and you will notice a sudden worsening of the engine performance with a good cylinder. However, when you ground out the plug of the cylinder that is misbehaving, you will NOT notice much change at all, if any. Its an old trick that a lot of mechanics use and is very effective. It does not yet tell you the exact cause but is very important in isolating the problem. You can use a dental pick with a ground wire connected to the chassis or a sharp grounded pin to pierce each plug wire but be careful that you are not in the path to ground or you will get a jolt. You can also pull the plug wires, one at a time(either at the distributor cap or at the spark plug) to see if one makes a difference. Takes more time but gets the job done.

So, are you game? After you isolate the cylinder(s) that is at fault, we can discuss ways to determine exactly what is wrong (fuel, spark or compression).

[90Duchess]

Thanks, Landar, I'm definitely up to trying anything to narrow down my possibilities. I think I'd prefer to take the dist. cap covers off and disconnect the wires one-at-a-time that way, versus piercing them. I'll get it done in the next couple of days and post my results and we can go from there. My research has led me to suspect that my problems are either related to the TPS or the fuel pump. The pump was tested electrically and showed internal wear, but it wasn't pressure our volume tested. Both the TPS and fuel pump are 23-years old, after all, so their failure wouldn't surprise me. I will try your suggestion first before replacing those expensive parts, though, and post my findings in a couple days. Thanks for all your help and suggestions!!

[landar]

You could also pull each spark plug and label them as to cylinder position. Place them on a clean white piece of paper and examine them very carefully. Maybe post a pic for us (get a lot of "eyes" on it). If a particular cylinder is misfiring, you may see that evidence from the plug. If the issue is the fuel pump, which is common to all cylinders, you probably will not see a difference. From what you have described so far, I have a hard time believing it is the fuel pump (especially at idle). The grounding or disabling of each cylinder one at a time, is still a better test but examining plugs *may* provide additional help.