Off Idle Stumble

[pishta]

Sorry if this has been covered, but I have an off idle stumble that is slightly annoying. If I just ease onto the gas, painfully slow, I can get a uniform acceleration, runs like a top. But if I drive it like a normal person here in So. Cal, I get a slight hesitation right off idle, lik ethe car is downshifting or something. If you could graph the RPM's, it would be a steady rise with a flat spot at around 1300 for about 1/4 second. As if the trans is doing something. It runs fine besides that. Any idea? Maybe a slack throttle cable, or out of adjustment? I can press the accelerator down about 1/2 inch without any response. Just hit 100K Miles.

[eatingupblacktop]

Quick guess, TPS, MAF, vacuum leak?

[Graham]

Quick guess, TPS, MAF, vacuum leak?

I have the same problem in a 1994 SC400. Driving like a normal person causes a surge, and also on the highway at certain throttle settings there is spark knock. The engine is 6 months into new plugs, wires, caps + rotors along with timing chain, already tried Seafoam, but with 243k miles, it could totally be a worn-out TPS.

The TRAC light and Check Engine light come on sometimes. I figured out how to read OBDI codes here:

http://www.lexls.com/tutorials/intro/codereading.html

Procedure:

1) Chalk wheels and set E-brake and put transmission in neutral.

2) set the desired test using the under-hood connector

3) insert key, turn to ON position

4) follow code-reading procedure from LexLS.com

My SC400 reports these codes:

TRAC code: 51

TRAC code: 45

ENGINE code: 41

These codes and symptoms point to failure of the "main" EFI TPS (IMPORTANT: TRAC TPS is a different P/N.)

I ordered a Main TPS from PartsGeek.com, & I'm looking around for the installation procedure right now. B)

[Blayd]

I dunno but my cars in the shop for the same thing. Exept mine is worse which your problem will eventually be to they all do mine started out like your describing. Ill let u know what the prob was with mine when it finally gets out.

[93ls400walt]

My 93ls is similar. There is a slight shift jerk between 1st and 2nd. And if I try to accelerate other than painfully slow the rpm drops but does not stall. And the car loses any forward motion. If I press the gas pedal further while this is happening there is no engine response at all. If I let up on the pedal then try accelerating more slowly the rpm comes back and I move forward, though slowly.

I have a code 41 and have replaced the TPS as well as complete tune-up, fuel filters, coils,seafoam, cleaned the throttle body a few times.

I think it is the ECM. Someone a while back had this prob. on a 93. And had his ECM rebuilt for around $300.00 And his prob was resolved.

Side note: every so often the car will run fine for a few days. No loss of power and man does it run soo much better and no shift jerk either. When running bad it failed the smog with high Nox. Re-tested during a good running period and it passed with flying colors.

I plan to send my ECM out for repair as soon as there is some extra cash.

[N.C.O.]

My 93ls is similar. There is a slight shift jerk between 1st and 2nd. And if I try to accelerate other than painfully slow the rpm drops but does not stall. And the car loses any forward motion. If I press the gas pedal further while this is happening there is no engine response at all. If I let up on the pedal then try accelerating more slowly the rpm comes back and I move forward, though slowly.

I have a code 41 and have replaced the TPS as well as complete tune-up, fuel filters, coils,seafoam, cleaned the throttle body a few times.

I think it is the ECM. Someone a while back had this prob. on a 93. And had his ECM rebuilt for around $300.00 And his prob was resolved.

Side note: every so often the car will run fine for a few days. No loss of power and man does it run soo much better and no shift jerk either. When running bad it failed the smog with high Nox. Re-tested during a good running period and it passed with flying colors.

I plan to send my ECM out for repair as soon as there is some extra cash.

Has anyone check the fuel pressure when it is acting up? A dip/drop in fuel pressure can certainly cause problems like that.

N.C.O.

[93ls400walt]

I have not checked the fuel pressure. I also have read from a British car mag that the ls has a 2-speed fuel pump. That it has a higher output under faster acceleration. Don't know if this is true.

I first thought it was a fuel issue, changed the fuel screen in the tank and the underbody filter. No change. On my ls I have sort of ruled-out the fuel system as the result of the emission test. Unless the NOx would go up under a fuel starving situation.

Any thoughts?