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Billy, Trainer/ Master Technician
Category: Lexus
Satisfied Customers: 3498
Experience:  Trainer 8+ years, master tech. 10+ years, SAE member
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Lexus LS400: Have a 1992 Lexus LS400 that is running bad .

Customer Question

Have a 1992 Lexus LS400 that is running bad . It was driving fine, and now all I can get it to do is idle in the driveway, if I give it any kind of gas, it shuts off . Checked codes, and codes 14(ignition no. 1 signal) & 31(air temp sensor) . Air temp sensor has been replaced, and BOTH coils fire(igniters are good) . I have been told that code 14 means that the ECU isn't getting a rebound(confirmation) signal, and that I need to check the pins at the ECU for a refrence signal which would indicate a short . I can do that if I knew which pins to check . I was sent a diagram, but none of the connectors on my ECU has 10 pin holes . Can somebody help me with this issue ? Thankx
Submitted: 5 years ago.
Category: Lexus
Expert:  Billy replied 5 years ago.
Hello and welcome! I'm Billy and i will assist you with this. Feel free to ask if further info is needed.

My apologies for the delay, thank you for your patience.

First here's the part of the diagram you need:
click here for details

The wiring goes from the coils to the igniters to the engine control unit connector A.
I would check all contacts at control unit connector for moisture, dirt build up etc...

Having checked these, and given the symptoms, i would make sure to have good ground (chassis) point on engine, and near the engine control module. And then i would check the fuel pressure, as it sure seems like a fuel shortage problem (checking the filter as well.

Thank you for your trust! Get back to me if you need more info...Thank you in advance for the positive rating...

Regards, Billy.
Customer: replied 5 years ago.

First off I would like to thank you Billy for you help concerning this matter, and before we go any further I want to let you know that I am also an A.S.E. Certified Automotive Technician with over 20yrs in this field . With so much experience, why am I on this page ? Simple, because while I am GOOD, I am NOT the ABSOLUTE BEST mechanic in the world; nor am I one not to seek the advice of other's who may have more knowledge about a particular vehicle than me . I use whatever mean's are at my disposal to help me with a problem that I am not familiar with or that I can not solve ....

Let me give you alittle background to my problem . I puchased the LS earlier in the year because I got a GOOD deal on it . It wasn't running when I got it, but I figured that I could fix it . The previous owner parked it because it kept blowing the no. 2 coil every few months . When I got the car and started doing diagnosis on it, I found out that it didn't have an ECU, so I purchased the CORRECT one and a spare just in case. I did a COMPLETE tune up (plugs,wires, caps,rotors and igniters) . Then I changed the fuel pump and all the related filters, as well as changed all fluids . And because I didn't know the history of the car, I also changed the timing belt and water pump . When I finished, the car ran fine for about 2 weeks, and then on my way to work one day, I noticed white smoke pouring out of the tail pipes . A check revealed that the vacum pressure switch on the p/s pump had gone bad and was letting the p/s fluid be sucked back into the intake . I fixed that problem, and then on my way to work the next day, the car started to loose power . I had it towed home and did some diagnotic on it . I ran a fuel pressure test, a leak down test and a compression test . All were within specs, except for # 8 compression being down and some leakage in that cylinder as well . When I drove the car the next day it was fine for about 20 miles then it started loosing power again . Since that time, all the car will do is idle . If you try to give it gas, it stalls; and the ONLY way that you can get it to idle again is to take out a spark plug and let it idle before replacing it back; the car runs SUPER rich .

I also have a 90LS that runs just fine, and I have been swapping parts between the two vehicles to try and find the problem because none of my diagnostic skills are working . I have swapped the ECU, the Mass Air Flow sensor, the Intake Air Temp sensor, and the TPS between the 2 vehicles with no change . All these parts work just fine when switched to the 90LS.

The diagram that you sent me was not the right one, thanks anyway . When I requested that, I had those 2 codes in the ECU; they must have been old ones, becase after I cleared the ECU and let the car run for 20 mins or so, and I kicked the computer back in diagnostic mode; no codes were to be found . I have taken the upper intake manifold off to clean the throttle body, because it was gummed up bad from the p/s fluid being sucked back into it . I will install it tomorrow after I find another Idle Air Control Valve; I ohmed mine out today, and it was not within specs.

You have given me some advice, and eventhough I have already done these things, I will recheck tommorow; but I believe my problem lies elsewhere . Any additional feedback or asisstance that you can give me concerning this matter would be gratefully appreciated . Thank you very much for your time, patience and help with this matter .

Expert:  Billy replied 5 years ago.

A heck of a job you have done on this one!

But first, you need to understand that the info you provide in the question, is all the info i get. Hence i had no way of telling whether any of these checks was performed. I had no way to know the level of expertise working on vehicles, and no way to know that the codes were old ones. You always should erase the memory, and then check for the codes again.

Now having this new info, replace the idle air control valve, and recheck the fuel the pressure regulator has a vacuum line that gets...well, p/s fluid in your case.

Get back to me with your findings....this just got mighty interesting...

Regards, Billy.
Customer: replied 5 years ago.

Thanks for your response Billly..
I think that we may have a miscommunication here . The p/s fluid that I refered to in my reply to you, actually came from the valve on the back of the p/s pump . That valve feeds off of the intake manifold via two vacum lines to provied the asisst for the steering . When the valve went bad, it leaked internally and the p/s fluid was sucked back into the intake manifold via the same two vacum lines, which caused the smoking out of the tail pipe .....

I will replace the fuel pressure regulator as you suggested, and replace the vacum line(s) as well since the fuel regulator feeds off of vacum . And I will also replace the Idle Air Control Valve, as soon as I complete these repairs, I will report back to you with my results .

Once again, thank you for your time and asisstance .


(Note : It may take a day or so for me to locate a Idle Air Control Valve . Do I have a time limit in which to respond back to you ?)

Expert:  Billy replied 5 years ago.
You are most welcome Sir!

You can take your time, and get back to me on this page. If for any reason you find it closed, just contact customer service to get it back open. However, in a few days or weeks, this should not get closed.

Waiting for your findings...Regards, Billy.
Customer: replied 5 years ago.


Expert:  Billy replied 5 years ago.
Customer: replied 5 years ago.

First off I want to thank you for your time and advice concerning my issue . I have found my problem ,after I started to look at the LS as a car instead of this Mysterious being .

I didn't get a new IAC, I switched the one from my running 1990 LS, and after getting the same results, I finally sat down and use my mechanical common sense to come up with the proper diagnosis . I redid the compression and leak down test's; with the cams and ALL spark plugs pulled . On the leakdown test, no.s 6 & 8 were leaking, with no.6 leaking back into no.8 . At that point I knew what was wrong, but I did the compression test anyway just to verify my therory . All cylinders were up to par, except for no.8 again . I then tore the engine down and found out that I had a blown head gasket between the no.6 and no.8 cylinder ,