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1993 Ls400 Starting Problem


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I have a 1993 Lexus LS400 that I haven’t driven for 3 years due to a starting problem. Just maybe someone may have experienced the same thing and have an idea what is wrong.

The problem is that about 10% (and increasing) of the time while starting the engine, it tries to “kick back” due to the ECM sending the firing signal to the wrong igniter, causing the next 3 firings during that crankshaft cycle to be out of the correct igniter sequence. At the next (third) camshaft signal, the ECM corrects the firing order of the igniters. This problem only occurs during the one revolution of the crankshaft cycle between the second and third camshaft signals to the ECM. The “kick back” is caused because four cylinders are firing 90 degrees advanced. If it wasn’t for the starter torque and the engine inertia, the engine would stop rotation during the “kick back.” Obviously engine damage can (and has) occur.

The engine can be started and will run normal using the signal from just one of the two camshaft sensors. The malfunction only occurs when the ECM is receiving signals from both camshaft sensors.

As a background, I have many years of auto repair experience. Lexus doesn’t have a clue what is wrong. No fault codes are stored in the ECM, therefore Lexus’ attitude is to “throw parts at it until it’s fixed.” (Their remedy could cost more than the car is worth.) I have spent hundreds of hours trying to diagnose the problem. I have videotaped the malfunction on my 4 channel scope, and I know exactly what is going on. I just don’t know which component is causing the problem. The following items have been replaced:

Distributor caps and rotors.

Crankshaft sensor

Engine ECM (tried rebuilt, then replaced)

Timing Belt

According to test equipment information, all sensors are functioning correctly. Since the malfunction occurs less than six times consecutively, the ECM will not record it. This problem began happening occasionally and over time (1 year) progressively got worse.

I would appreciate any information regarding the cause of this malfunction.

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Welcome to the club!

I surly do not have the skills that you have. But I have thought about what I would look at. Any corrosion at the connectors. Or at any ground points. Move any sensor/igniter wires around. Sounds like a intermittent short.

I also have a 93ls with ECM/TPS code 41 problems. Replaced the TPS, now planning to have my ECM rebuilt.

Good luck


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Your good suggestions have already been tried. :rolleyes:

Regarding a rebuilt ECM, I learned a lot when I had mine "rebuilt". Generally, "rebuilt" only means that the electrolytic capacitors are replaced, and any damage caused by leaks is repaired. I was informed that the capacitors can/do leak and cause problems. I checked my ECM capacitors thoroughly before sending in the ECM. All capacitors and other components looked good. The salesman guaranteed me that if their “rebuild” didn’t solve the malfunction, (he was confident it would) he would refund the rebuild cost. Well, it didn’t, and the salesman left the company right after shipping me the “rebuilt” ECM. The company wouldn’t refund my money without me sending the ECM back to them for further testing. I did. They claimed it worked fine, which, as it now appears, was working fine when they first got it. I finally got my money back after a fight with the company, but the multiple shipping costs was almost the cost of the “rebuild.”

After much more testing, I thought that maybe the programming in the ECM was at fault, so I got another ECM. As mentioned in my original post, the second ECM didn’t solve the problem.

The moral of the story is this: if one has good electronic experience, then they can open the ECM, check for faulty (leaking, swollen, etc.) electrolytic capacitors and replace them and any circuit board damage. Also, check for any burnt or otherwise damaged components. The capacitors can be purchased on line for under 25 cents each. As a side note, defective capacitors were unwittingly used by some auto manufacturers for a few years, but Lexus was not one of them. A Google search will reveal more information.

Good Luck with your car….

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I like 93ls400walt, do not have your expertise. But, since it sounds as tho you've covered all the logical stuff, how about attacking the problem from different angle?

Are there any other electrical malfuntions that have occured?

Two that come to mind, which are ls400 issues, that may or may not be a contributing factor are:

1-the nefarious, notorious trunk hinge wiring harness which is a known design flaw. The shorting that occurs there will cause these symptoms, and probably others as well: balking, wild rpms, uneven idle, bucking while driving, loss of dash lights, radio shut down when applying the brakes, etc. The great thing is, it's a snap to fix. Just strip off harness and expose the bundle, re-tape, and let the whole mess hang free so that there is little/no tension on the bundle.

2- long shot. Not sure if a short here would be related- but on the theory that any short, anywhere in the system could cause weird things to happen, check this out:

The rusted connector there may cause other issues is my thought.

Post any other electrical issues you may have had.

Good luck.

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do you know anyone that owns a working 1ls nearby? Swapping parts seems to be the next logical step at this point.

that's a very strange issue you have, especially not being ECM related. what else can alter the output of two different ECMs in such a goofy way without affecting normal driving?

here's a longshot, maybe it's an arc inside the wiring harness caused by a pin that's not making a completely solid connection. this would also explain the increasing frequency, as arcing destroys delicate wiring.

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@Alexander the o.k.

Thanks for your suggestions. I will check into the trunk hinge wiring harness, and also the possible connector corrosion.


Good idea about swapping parts, but I now live in the country, a long way from a big city and a Lexus dealer, so I rarely see one. Regarding a faulty pin connection, I have tested numerous connections at the ECM with my scope, looking for any irregularities in the signals. Haven’t seen anything except those that occur as a result of the malfunction (not the cause).


As a note regarding electrical connections, I purchased the car in 1999 from a private owner with all books a records. Up until the current malfunction, there has been no other engine/electrical problems. I will mention something interesting. Soon after I purchased the car I noticed that the speaker (woofer) in the rear window deck wasn’t working. (previous owner hadn’t noticed) Long story short….I found that the Nakamichi circuit board in the amplifier (located in the trunk) was completely hand soldered (instead of flow soldered, unbelievable!), and that one leg of the output power transistor to the woofer wasn’t soldered! (soldering it solved the problem) Obviously the circuit board or amplifier had never been tested at the factory. The quality of the circuit board construction was junk. I couldn’t believe what I saw!! I would never buy their products again.

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@Alexander the o.k.

Sorry for the delay, but I had to get the requested info from my video archives.

The "event" is generally midway between the first and second left bank cam signals, during the right bank cam signal generation. The fourth cylinder that should fire during the crank revolution following the left bank cam signal, is omitted and replaced by the first cylinder of the next sequence of 4 to fire during the next revolution. Hence, the next 4 cylinders to fire are doing so 90 degrees advanced. When that next left cam sensor signal generates, no cylinder fires at that time. (The cylinder that should have fired, already did.) When the ECM receives that left cam signal, it corrects the firing order from that point on.

The peak to peak voltage signal of the left cam sensor at its first signal generation is 5.8volts. The peak to peak voltage signal of the right cam sensor at its first signal generation (at the point of firing sequence replacement) is also 5.8volts. The second signal generation of the left bank is 11.74volts. The second signal generation of the right bank is 9.8volts.

The peak to peak voltage is RPM dependent to a point, starting low and then getting higher as the starter turns the engine faster. This is normal, of course.

Hope this info helps and makes sense. Thanks.

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  • 2 weeks later...
MP. Did Pat fix you up or point you in the right direction?

Please post any answers so that we all may learn.

Pat is giving me some ideas which I try out. So far they haven't worked. I replaced the timing belt tensioner today and that didn't solve anything. I'll just keep trying new ideas.

I did a search of this forum before I posted, but somehow missed this post. Found it yesterday. It's the only info I have seen on the web about this problem. My starting symptoms do not include the "silence" part, but the rest is the same. Definitely caused by an out of sequence firing order, not engine mounts, and etc. One can see the serpentine belt jump (in my video) as the engine attempts to kick back.;hl=kickback

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The solution to my problem in that old post was the driver's side motor mount. It was so old that the rubber had compressed to a point where enough slack between the mount and hardware was causing a thump. New mounts fixed it and it never returned.

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  • 1 month later...

This is an update on my quest to solve the kickback problem.

7-23-08 Replaced timing belt tensioner. Didn't fix problem.

8-18-08 Tried another (third) ECM. Didn't fix problem. (Also tightened ALL pins in the connectors to the ECM)

9-1-08 Installed new camshaft sensors. Didn't fix problem.

And the saga continues.... :wacko:

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The engine can be started and will run normal using the signal from just one of the two camshaft sensors. The malfunction only occurs when the ECM is receiving signals from both camshaft sensors.

Wow, what an interesting problem to have! Ok, I have several things to mention then you can dismiss my remarks as bogus ;)

You mention that the engine will run normal on just one cam sensor. If that is so, just leave the one disconnected and drive it! Life is too

short to wonder why it wont run as engineered. sorry, couldnt resist :lol:

And if it runs on one, does it matter which one? If you can isolate the sensor causing the erroneous timing then you can concentrate on that side.

I am also wondering if something is loose such as the sensor or mount(I admit to not knowing much about how it is mounted). A dynamic shift of the sensor while starting might

cause erratic timing behaviour.

How is the cam timing mark on each camshaft WRT the mark on the crankshaft? Is it possible that there is a factory mismarking of the timing mark on one of the cams?

Could the camshaft itself be flexing enough to cause a problem? (defective camshaft). Is the correct camshaft in the car?

Fire away!

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Damn MP. This has got to be driving you nuts!

Just had another thought since you've done everything else it seems like. Can you put your scope and video setup on another correctly running LS and look for any slight differences? Just another long shot, but maybe it'll give you a clue to look in some other area. Compare the vids to each other.

Also, try checking all ground wires, everywhere. Tighten all.

As always, good luck...

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Thanks for your reply. What I meant by “normal” is that the engine will start and run like “normal.” No kickback during startup and engine itself runs OK. Btw, once the engine has started, it will run without either cam sensor connected. The ECM gets its timing exclusively from the crankshaft sensor after startup. However, disconnecting one or both cam sensors triggers an error code in the ECM which turns on the check engine light and disables the traction control. It also puts some of the engine operation in the default mode. Not good, sorry. The ECM requires a consistent signal from both cam sensors. The sensor is positioned in a machined hole in the distributor. It cannot move. It is held in the hole by a retaining screw which is secure. All markings and items on the engine are original except those I have changed after the problem started. I have recently discovered that the position of the camshafts at startup, hence the camshaft fingers that trigger the sensors during the starting, have a major influence on the problem. I will be investigating this to see where it leads.

@ Alexander the o.k.

Thanks for your reply. I live in the country, miles from a dealer and I rarely see another Lexus. So checking with another Lexus (much less the same as mine) is out of the question. Rechecking the ground wires is something worth looking at. Might have missed something.

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  • 4 weeks later...

After additional testing I have decided that 3 years trying to solve the kickback problem is long enough. It’s time to dump the car. It’s the first time in 45 years that I haven’t been able to fix a mechanical/electronic problem within a reasonable time. The biggest problem here is that I failed to investigate the design and engineering of the Lexus before buying it. I clearly understand (now) that Lexus designed the car to only be repaired by a Lexus dealer. It’s bad enough to repair a reasonably engineered car, but a deliberately engineered mechanics’ nightmare is not something I want to voluntarily repair. Obviously Lexus determined that if one could afford a Lexus, they can afford to be ripped off in repair bills. I also understand that Lexus doesn’t have a monopoly on this method of operation.

It is true that generally a Lexus will operate for several years with little or no problems. However, the poor sucker (like myself) who buys one used really gets shafted. Just a routine dealer “major tune-up” costs over a $1,000. I’m going to pass this future money pit to someone with a bigger bank account than me. Fortunately I could do my own major tune-ups and fix the burned out speedometer needle light, another engineering nightmare money pit. (Another dealer $1,000 bill). I’m sure that if I kept working at it, I would eventually find the cause of the kickback problem, but life is too short for that.

It’s onward and upward………

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  • 3 weeks later...

Two guys on here (one being me) have both said that their kickback problems were completely solved with new motor mounts. I saw where you assumed you weren't having motor mount problems, but I don't see where you've replaced them, or posted a date in the car's service history when they were replaced. If they are the originals (now 15 years old), I practically can guarantee you that they're ready to be replaced. I would stop the electrical hunt, and start with the mount hunt. If you don't address that possiblity, you could very well replace the entire engine, on the original mounts, and still have the kickback/clunk. It's the driver's side mount that wears out first, and when you start the car, the enigne torques inside the bay. When the mount gets old, it will force the metal supports of the mount against the frame of the car, causing a kickback sensation and clunk noise. That's all the advice I can give you amigo. I had a kickback problem too, and was solved for good with two new engine mounts. Unless if your car was heavily tweaked by the previous owner, I honestly think you're chasing your tail if you ignore the engine mounts.

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