WetBrain

Hi all,

I have a riddle for all of you and I would really, really appreciate if anyone can help solve it.

About 3 weeks ago, my 1994 Lexus SC400 started stalling on me when I would accelerate from a complete stop. Additionally, sometimes when the car was idling in park, it would rev up and down between 1,000-3,000 RPMs. This did not happen all the time. Some times the car would work perfect for the whole trip and some it would stall out at nearly every light.

I brought it into a local Lexus repair technician. It is a specialty repair shop that only works on Lexus’, and the guy has always known his stuff in my dealings with him (I wonder if he is active on these forums...). He tried to replace a couple sensors at first but those didn’t work (the throttle sensor and one other). He finally diagnosed that the ECU was bad and we waited a few days to get a "new" refurbished unit from the OEM (Denso). He said we were all fixed and I took my car back on the road.

Some weird things happened on the drive home (I will list all the symptoms below), but I thought maybe it was the car adjusting to the new computer or something. A few days later, the car starts its old tricks again. I take it back to the repair shop and after a few days with it, he determines that the spark plugs were fouled by the old computer and replaces them. He says my car is good to go.

Nope.

The past few days I am getting the same symptoms plus a few new ones. Here is everything that has happened so far. Some happened before repair, some after, and some both before and after. The worst part is that some days, none of these problems occur and the car works flawlessly all day. This has made it difficult to pinpoint the problem.

1. When idle, the car may rev up and down. Sometimes this results in the car stalling.
2. Sometimes when accelerating from a stop, the car will immediately die when I push the gas pedal.
3. The car may take 3-4 times to turn over. However, most of the time the car starts right away.
4. Once, upon starting the car (while in park and without my foot on the gas), the engine started revving up very fast and did not stop until I turned off my car. It got to about 5,000 RPMs before I shut off my car. Another time this manifested itself while I was driving. I was at a stop light and when I let go of the brake, the car immediately jumped to 20 mph. My foot never touched the gas.
5. At times I notice a flickering on the illuminated speedometer and odometer. The only parts flickering are the needles, not the number readouts. I am not completely sure whether this coincides with other issues or if it is a separate, unrelated issue.
6. Just yesterday I was driving and it my power steering seemed to stop working. I took three (very scary) turns during this time and the car was accelerating fine between the first two. On the third turn (and with the wheel still incredibly hard to turn) the car stalled immediately after making the turn. It took me 4 tries to start the car after this, but the rest of my drive home was fine.

That is all I can think of right now. Given that these symptoms presented themselves before and after changing the ECU, I am fairly certain that he misdiagnosed the problem and the original ECU was okay. Now, it is also possible that we received a bad ECU and it is the same problems continuing. However, what I don’t believe is his claim that not only did I have a bad ECU, but now I have other failing components causing the same exact problems. Occam’s Razor and all… Instead of 5 things failing, I tend to think it is more likely that we have one issue causing all these problems. We just haven’t found it yet.

Has anyone ever encountered some or all of these symptoms before? If so, what did the problem turn out to be. At this point, I have sunk too much money into getting this car repaired. It also sucks to be without this amazing car for close to a month now. I would really appreciate any help you may be able to give.

EDIT: Also, the car does not give any codes with the jumper method.

V8drummer

iTrader: (5)

Idle Air control Valve would be my first guess. Then maybe throttle position sensor. Has he tested these yet?

WetBrain

Thanks V8! Actually, the first thing we did was replace the throttle sensor. We tried it before he ran any sort of diagnostic because it was also his first guess and is a really quick install.

As for the idle air control valve, I will check with him first thing in the morning. This seems to fit a number of the symptoms. The only one I can think of that is outside of this is the power steering issues. Is there a way that the IAC Valve may have an effect on the power steering? I had my power steering redone 2 years ago and I know all the parts are in good working order.

sj408

iTrader: (6)

yeah it sounds like the IAC for a few of your symptoms, but neither of these lexus techs has hooked up datalogger? Sounds like your just throwing money at it when 5 minutes on a datalogger will probably find your problem without all the guess and check.

WetBrain

Okay, I brought the car back in and spoke with the technician. The IAC Valve was cleaned during the initial service, so that doesn't seem to be the issue. Also, he told me that the datalogger wasn't showing anything wrong.

The good news is that my car will no longer start, so the problem may be easier to diagnose now. The problems were so sporadic that he could never recreate the issues I was having at the shop. He put about 100 miles on my car last time I brought it to the shop and he still didn't get any of the stalling issues.

I am beginning to think it is electrical, but we are going to try two more hardware fixes before we begin checking each wire. I think the first step wil be the fuel control computer and, if that doesn't work, the fuel pump will be next. Are we missing any other potential causes here?

speed6

iTrader: (1)

Check the ECU, i know the capacitors leak in them.

WetBrain

As I mentioned in the original post, we already put a new ECU in. Thanks though.

Ali SC3

iTrader: (10)

This has classic sc400 symptoms that fit 2 main categories that most of the time are the issue.
First is the fuel pump ecu going bad. this starts the way you are describing, then eventually leads to a no start when it fails completely. If you jumper the ports in the diagnostic connector for the fuel pump to turn on, and the car starts and runs, then you need to replace the fuel pump ecu, or do the 12v mod and bypass it. very common problem.

Other thing it could be is one of the 2 coils, or the rotor and cap on the 2 distributors. the coils eventually fail on every sc400, and not always at the same time. a worn down rotor or cap can also cause the same symptoms. If you haven't done it inspect and replace the rotor and cap if they look worn. You may notice a lower idle rpm than normal if you have a coil out, or misfires if you have a worn rotor/cap.

CANDYAPPLE

iTrader: (1)

like you said thank god it finally died, now you can find the culprit. the first thing i would do is pull the fuel line and see if your getting fuel, then check the spark. those are the two easiest things to do. the cam position sensor also raises a red flag for me, because it stalls and sometimes hard to start. last but least check you crank sensor and the harness, might be loose or damaged wires.

WetBrain

This is really great info, thank you! We were going to order a new fuel control computer, but I will see about bypassing it first to see if that is the issue.

groverwood

I say Fuel filter. My sc400 did the same kinds of things. I couldn't get the old fuel filter off without breaking fuel lines so I did the the fuel filter hot wire trick( 12V mod) so it(filter) fires up wide open every time. Never had a problem since.

speed6

iTrader: (1)

has this been resolved?

OG Dada

iTrader: (10)

https://www.clublexus.com/forums/sc-...on-issues.html

Ali SC3

iTrader: (10)

wow thanks for that link I forgot about that thread. much easier than me writing it out each time.