raydleoni

In my original note I described 1990 LS400 problem as the engine seemingly running out of fuel and RPM winding down while driving and sometimes stalling. But never a problem restarting. Most of the time the engine would surge back as if the fuel supply suddenly returned which did turn out to be the case. There were no engine codes and the problem was intermittent and no fun to experience.

The local Toyota dealer in Hyannis MA (no Lexus dealers within 50 miles) cleaned the throttle body that he said was badly carboned up after 160,000 miles but that did not solve the problem and he gave up because during his checkout the car ran fine. While I was driving home the intermittent operation came back and I limped back to Toyota.

With the engine idling badly and going from high to almost stall speed by itself, the Service Manager (Brian Scarpellini, heard a faint clicking noise in the right front fender area that would come and go as the engine RPM fluctuated and be in sync with the surge frequency.

He found the clicking to emanate from the fuel pump relay however the relay checked out OK. He then disconnected the fuel pump resistor, also in that area of the engine compartment, and bypassed it with a jumper wire. The engine immediately smoothed out. He found that the resistor's resistance changed greatly as the unit heated up and he measured the local temp reaching 200 degrees.

Replacing that resistor completely solved the problem. It is P/N 23285-50010-LX. Unit price $63.28 plus $95 for labor.

I thank those of you who took the time to help and thought you might be interested in the final (I hope) solution.

Ray

RA40

iTrader: (6)

Thanks for the update. Some of these weird occurrences are hard to nail down. This may save others some heard searching and parts replacement.

larrry

am having somewhat the same problem i have been running my car on e a lot one day i ran on e and just made it to the gas station i put about 30dollars of gas in may car the next dat my car started to miss real bad rpm started to surge up and down, car take off slow i have to keep taking my feet off the gas to get more power to go faster while am driving its take a while for it to pick up speed i can fill while am driving regular speed that the car is backing off i have been thinking maybe the gas i put in was bad gas now when i start the car while its sitting still i can press the gas and it seems like something is holding back the power it is missing real bad rpm is surging up and down i dont know what it could be need help engin light comes on and off i have an 91LS400

CantSeeMe

Hi Larry, Welcome to Club lexus. First , you need to check your engine codes.

Chin

iTrader: (34)

Would a bad resistor prevent the fuel pump relay from energizing?

zukikat

I know this is a really old thread/post but I'm trying to figure out if that part is also my problem or if I should look elsewhere for other possible causes?

I bought this 1994 LS400 one week ago in need of a timing belt & water pump service interval (184k miles) and some other moderate tlc but running smooth and driving well overall with no apparently major problems I wasn't comfortable trying to fix myself and only a Code 31 diagnostic error code stored that I felt should be easy to fix with my spare AFM Module of the exact same part number from my 1992 Turbo Supra.

I test drove this car twice before buying it and other than the Code 31 throwing a CEL on decel sometimes and some light smoke on cold start-up (valve stem seals most likely) and really warped rotors and flaky gauge cluster (lighting and gas gauge not working) and a moderate power steering leak that didn't appear to be rack related it actually passed carfax and autocheck with flying colors and has all original vin tags that I could find and the paint and interior are still very good so when they haggled down to a price I was okay with, knowing about how much those problems and the servicing should cost me, I said why not and bought it.

Of course on the way home the BRAKE light began coming on every time I pressed the pedal even with a full reservoir, the power steering pump began groaning, and as I got the 2 miles from the dealer to a friend's house the coolant level warning light came on and then as I tried to go get power steering fluid from a nearby auto parts store it stalled as I turned into the parking lot and again as I tried to turn into a parking space, yay, none of which it was doing the first day I saw it or the TWO days I test drove it after that...

Then 2 days later I got brave and decided to try and drive this LS400 nearly 200 miles to a friend who is familiar with the 1UZ Lexus V8 engine to do the timing belt and basic servicing and check the car out more thoroughly for me.
I topped off the coolant reservoir and power steering fluid and double-checked all the other fluid levels and began my journey and to my initial relief it drove normally other than the BRAKE warning light issue and gauge cluster lighting problem and Code 31 lighting the Check Engine light during decel/stop conditions.

The check engine light had been coming on during decel and stopping and went off while moving and was reporting the Code 31 AFM signal during both days of test driving so I figured the Karman Vortex Air Flow Meter sensor module was probably bad and causing the stalling which he agreed with and the car also did to him several times during his post-servicing test drives (stalling) but he said it would also hunt and stall while driving without throwing any codes which it hadn't done to me at all...

So when he was finished with everything I drove the car all the way back home 2.5 hours on the interstate and it seemed to run even better and have more power with the new spark plugs, disdributors, etc but it would occasionally seem to shudder or stumble or even buck a little on mild accel or decel requests at any speed occasionally which it hadn't done to me before at all, then today while getting off the interstate it began acting like I was very rapidly turning the key on/off/on/off/on and lit the check engine light each time the tach fell to zero and the engine WAS actually cutting out each time the tach dropped to zero, it was NOT a tach problem! Then it suddenly cleared up and drove normal again like nothing bad had happend so I headed to a friend's house nearby.
It did that again twice in the 3 mile trip to his house and finally stalled as I turned the steering wheel mid-stumble to turn into his neighborhood off a 40mph road.
So I bumped the shifter to Neutral, hit the key, it fired right up and started to go, made it through the turn, it stumbled again a couple of times about 1/2 block down the street so I backed off the throttle and hoped it kept running (which it barely did), then once I turned the corner to get to his house I threw it in Neutral, coasted to his driveway with the engine in neutral choosing to idle really high at 1,500 rpm's, and then I got out and put the diag jumper in to check for trouble codes.
When I first got there before I opened the hood the engine continued to idle at 1,500 rpms, the check engine light was no longer on, and when I put the jumper in to check for codes the check engine light was flashing the all good pattern with no codes stored at all like nothing was wrong at all...

I had just replaced the Karman Vortex AFM module off the AFM housing with a spare one from a 1992 Turbo Supra that uses a smaller diameter Karman Vortex AFM housing but the IDENTICAL sensor module as the Supra guys actually use the larger V8 LS400 Air Flow Meter housing and larger fuel injectors to run upgraded Turbo's on their Supras!
I also pulled the EFI fuse to reset the ECU and clear the code 31 before I unplugged the bad AFM Sensor and reinstalled the EFI fuse after I installed the replacement AFM Sensor module and then drove the car for a while to let the ECU "Learn" the new sensor and engine operating conditions...

Note that the rapid hunt for idle and hesitation and stalling problems were happening to my mechanic friend (who did the timing belt/water pump servicing) BEFORE I replaced the AFM Sensor module and the problem continued AFTER I replaced it so there appears to be some other issue at work that's causing this problem.
Since I swapped out the AFM Sensor I drove about 20 miles with mixed interstate and city traffic deliberately with at least 10-20 minutes of alternating freeway and city driving with almost no traffic and the replacement AFM sensor seems to have corrected the Code 31 problem as the only time the check engine light comes on now is when the car is acting like I'm constantly turning the key off and on really fast and it isn't triggering any trouble codes at all now.

Today was my first time experiencing this even with two 2.5 hour interstate road trips to and from the mechanic's place for the timing belt job and power steering repairs and such, and let me just say that I went from calm and happy with my new to me LS to extremely worried that I'd be stranded on the side of the road at any moment with no warning whatsoever and it really scares me because I bought this car for my 50 mile (each way) work commute which I often do after midnight and I can't afford to be stranded by this problem, nor would I want to be at like 3AM in the winter!

Someone suggested a faulty 93-94 LS400 Engine Control Unit "Computer" but before I go trying to find a used Non-Trac 94 LS400 "ECU" or send mine off to be "repaired" I wanted to know if anyone else is having this type of problem and what their successful solutions have been.

Thanks in advance for any assistance anyone can offer for this problem!

zukikat

So the next day I tried to take the car out on a lunch run and backed down the driveway, then as I put it in Drive it lit the check engine light solid, then stumbled and stalled as I began to press the gas pedal.
It started back up so I tried to drive it and it would barely move in Drive and took its' time building any speed to speak of. Literally I could run the 1/8 mile to the stop sign faster than the car could get there and it was running crazy rich (the exhaust made my eyes water) but in Reverse it would "burn rubber" on demand.
The Check Engine Light was on but when I inserted the diag jumper it refused to flash any codes or even acknowledge that I'd inserted the jumper at all, and the ECU wouldn't "reset" either. Essentially the car was totally undrivable.
This was completely different from the symptoms I experienced in the above post.

It was basically in "limp it home" mode, not managing the transmission at all, yanking the timing back and running the air/fuel ratio super rich, and it couldn't or wouldn't tell me what it thought was wrong.

So now I know why the lady traded in this car...
Even with all the other problems it had that were somewhat expensive to fix, they weren't preventing it from being drivable, but the ECU getting flaky was threatening to leave the car stranded on the side of the interstate in the middle of nowhere and on rural roads without any help readily available and that would be bad.
I went to a junkyard about 50 miles from here and out of the FIVE 93 LS400's only ONE still had its' ECU and someone had ripped the dash open, taken the covers off of the ECU, and left it there to get rained on. Then I stumbled across a 91 LS with two different ECU's loose in the trunk, googled their P/N's, found that one was from a 90 and the other from a 93, and of course the one from the 93 had junkyard paint pen markings on it like someone had already bought and returned it, and I had no way to tell which car it was out of so I couldn't tell exactly which of the two model years it was out of or if it was a TRAC version or the Non-TRAC version I needed for my base-model 94 LS400...

So I opened my dash up, discovering that someone else had also done this before in this car, found the ECU actually missing one of its' two cover plates and a curcuit board was totally exposed, and it was a different Part Number than the one I got out of the junkyard yesterday.
I decided to try it anyway, it didn't report any error codes when I first turned the key or when I inserted the diag jumper, the car fired right up with no complaints, and after 1/4 tank of mixed freeway/city driving with no signs of the prior problems the junkyard ECU still reports "all good" although the transmission is shifting a bit lazy and the idle is 1,200 - 1,400 rpms even after it warms up so this ECU appears to be in the early stages of problems as well but at least so far the car is totally drivable again and I've managed to fix everything except the seat belts at this point, YAY!

My work commute is 50 miles each way and I plan to begin trying to use this car for that within the next week or so and time will tell what happens next...

Kyary

Hi, I'm kyary not sure I am posting this correctly. I read where fuel pump resistor gets hot. 200 deg. ? I have a 91ks400. There is a plastic piece covering the fuel pump and relay. At front pass wheel well under hood. That plastic cover also happen to be right over that resistor. I unsnapped the plastic piece. And have been running on 2-6 gal for over a week. Seems possible the heat off the heat-sink resistor was being trapped by plastic piece. To much heat. Cooks the resistor maybe. Anyway no stalling so far. I really hope this works. And helps someone else with this problem.

GS4_Fiend

iTrader: (22)

It would help others if you could post up the pictures of where the resistor is and how it looks like

LEX400LS

I have a very similar problem as my 1994 LS 400 was doing very well until three days ago. While driving I started to suddenly lose power and finally the engine quit. I tried to start it that night but no success. Next day I got up and the car started right off, no rough running at all. I let the car idle for about 10 to 15 minutes but then the engine quit again. I called the local Lexus Dealer and they told me the brain must have gone out and/or it could be a thousand other types of problems. They wanted me to have the car towed in and allow them to run test on the Lex. I started to study the groups and have found a lot of information I did not know. I do not feel the brain is out because of the way the car acts when it does start up and runs. It is very smooth with no roughness at all and this is the same thing they told me two years ago when it did not pass a state inspection. They wanted 2500 dollars to fix all the things they said were wrong with the car. I took the car, did a good tune-up for fewer than 200 dollars, then went back and asked them to do another inspection. The car passed the inspection with flying colors and was well below the standards of Texas. There was nothing wrong with the brain at that time and I feel the same right now. The car seems not to be getting fuel but the pump is putting out good pressure and the flow of fuel was checked and seemed to be good all the way to the racks. I will replace the fuel filter and am trying to find all of the relays, switches and control units that are placed on the car. I will take a serious look at the items you mentioned and will report back as soon as I can do all the testing. Thank you for providing this service. LEX400LS

Unclebob9

The fuel pump relay resistor is not inside the fuel relay switch enclosure, it is underneath and to the rear of it. It is about 2" x 3", aluminum, mine has a 4 stamped on it. It is held in place by 1 10mm bolt and has 2 wires leading to it. Mine was getting too hot to touch. I replaced it with one from Pick N Pull ($5) and it ran fine for a week and then the used one went out, so I have now just cut it off and connected the wires together (thus bypassing it), the car runs fantastic like this. But from what I read, this may caure pre-mature fuel pump failure, so I am ordering one from Discounttoyotaparts.com, for $52.52.
Another side note is that my radio had not been worth listening to due to too much static. Once I bypassed the fuel pump relay resistor my radio came in clear again.

Kyary

I bypassed the relay by jumpering the wire in other words just cut the wire and wire nutted the two wires together leaving the relay off completely no problems have driven the car over 10,000 miles zero problems hot weather or not

RA40

iTrader: (6)

A good thread to read through is this one in the LS400 section:
https://www.clublexus.com/forums/ls4...capacitor.html

alnevala

my lexus runs great then as im going down the road it stalls. It takes about 5 minutes before it will start again. It does this to frequently to drive. please help

zukikat

It could be the fuel pump or some other issue but there's a good chance your ECM (computer) is going bad.
There is a small like 1" x 3" sized "Diagnostic" connector box on the top of the engine towards the driver side center of the engine.
You need to research on here to find out which pins inside it do what for diagnostics...

If you're not comfortable working on cars, have someone more knowledgeable help you, but this should probably be the next step to try and find your car's problem...

Inside its' cover once you've opened it you'll see a small grid of white label boxes with 2-3 letter combination descriptions showing for identifying each pin of the connector itself.
NOTE: This procedure only applies to the 1st generation LS which is "OBD-1" model years.
The newer "OBD-2" models of LS might possibly also have the labels for these pins in their diagnostic connector's cover but will likely be missing the actual pins in their diagnostic connector itself and instead an OBD-2 code reader should be used on those cars!!!

There are many pins inside the Diagnostic connector but there are two specific ones that when a piece of wire is jumpered between them and the key is turned to "ON" (not start) the Check Engine light will flash in patterns to report any fault codes. Pins TE1 and E1 (center and a certain corner of the 3x3 grid, TE1 may just be T1 on some years of these cars) are the two for running engine diagnostics if memory serves me right...
When my car's ECM was failing, whenever the car was stalling, the jumper wire did nothing, couldn't get the check engine light to come on at all for anywhere from a few minutes to an hour or more which was a good indicator that the ECM was the problem.
In that case, usually anywhere from 5-30 minutes later the ECM would finally start responding to the jumper wire but at that point it always reported that there were no faults and the car would start right up and run normal again for a while.

There are also 2 other pins in the diagnostic connect that using a jumper wire in will cause the fuel pump to run when the key is turned on, which is an easy way to tell if your fuel pump is failing. Pins B+ (POWER!!!) and FP should make the pump run with the key on, engine NOT running! When inserted you'll hear a faint click and within a couple of seconds begin to hear the sound of liquid running into the fuel injector "rails" similar to filling a glass with water from a faucet. If that's what happens your pump is fine. As soon as you hear the sound of fuel running into the system remove the jumper, which may be hot and can burn your fingers if left in too long.
That test bypasses all resistors and relays and pretty much directly connects the pump to the battery for full power. Do NOT drive the car like that!!! BAD idea!!!
(BE CAREFUL and don't leave the wire in those pins very long as the wire can get very hot!). NOTE: You can blow fuses or damage other parts if you use the wrong pins for that test!
When my fuel pump was going bad and the car wouldn't start at all, nothing would happen for 10-15 when I inserted the jumper but if I removed and reinserted the jumper a few times quickly the pump would usually kick in at some point and the car could then be started.
After a couple of weeks of having to do that once or twice a week to get my car to start for the first drive of the day, the car got me all the way to work, 50 miles, let me park the car, and that was the last time that fuel pump ever ran again...