Sometimes it idles perfect, mostly it idles erratically
 

Series 5 NA with TB mod:

Erratic idle - sometimes 1200 steady, sometimes 800 steady, sometimes surging 500-1400, sometimes stalls.

Disconnect TPS - steady idle, even under switching loads.
(I think this rules out an air leak)

My TPS signal only shows only .02 volts with initial set connector grounded & Ignition on.
(like maybe my ECU is screwed)

TPS ohms = 1K ohm at idle.

Driving with TPS connected - normal performance - ECU cuts fuel while decelerating.
(The ECU sees the closed throttle, but I'm not sure how with only .02v signal)

Driving with TPS disconnected - normal performance - no deceleration fuel cut & nice afterburn (pow).

Questions:
Will driving with the TPS disconnected affect the OMP or does the ECU use the AFM to drive the OMP?

If the OMP uses AFM signal, can I just clip a 5K resistor on the TPS connector to prevent getting a check code?

Re: Sometimes it idles perfect, mostly it idles erratically
 

I'd also like the answers to these. I've been driving around for a couple of days with my TPS disconnected. With it connected the engine bogs down and won't accelerate pass 2500 rpms.

Chuck

SureShot (Bill) brought his car over my house yesterday and I wanted to add a few things to this thread. With TPS disconnected the engine idles normally, when load is applied the rpms drop but the engine holds steady at 500rpm. With TPS connected and load applied the engine just dies. We did a compression test on the engine and got 100+psi bounces.

Strange that the his TPS has any effect on deceleration since there was no voltage to be seen. Does this sound like an ECU problem or short in the wiring harness?

Brian

"Happy Easter!"

Re: Re: Sometimes it idles perfect, mostly it idles erratically
 

Pull your check codes, because you may have an OMP problem. If so run pre-mix until it's fixed.

Re: Re: Sometimes it idles perfect, mostly it idles erratically
 

BTW your symptoms give me a good clue that the ECU uses the TPS as source to control the OMP.
Without the TPS your are bypassing the OMP "limp mode".

So - does anyone know how the ECU can supply almost no signal voltage at the short range TPS, but still appear to see when the throttle is closed and you are decelerating?

The TPS in the 89+ cars is used to control the metering oil pump. This is the major reason the S5 cars have a full range TPS.

How can you get 1K at "idle"? Resistance must be checked with the car turned off and the TPS disconnected.

Have you tried another TPS? Or at least testing this one to make sure there aren't any dead spots? Really sounds like a TPS related problem to me.

Unless, of course, your throttle plates are becomming physically stuck.

Thanks Aaron - I think you & Hailers know as much about these ECU issues as anybody.

TPS ohms were checked per the FSM (ign off TPS plug pulled)
Sweep on the short range (orange to black wire) was smooth from 1k to open, and repeated within 10 ohms.

When I test the voltage by probing the orange wire, the TPS plug is in, initial set is grounded, Ign is on.
(I have no thermo-wax or fast idle cam.)
Am I looking in the wrong place for the 1 volt reading?

I don't have my wiring diagrams handy, and don't have that in my head so I can't honestly tell you. :) But your TPS reading basically sounds good...Using an anlog meter or scope is really the best way to test.

My fear is that it's something wacky, like a short in the harness that only happens when the TPS is plugged in, and might kill a replacement ECU.

So - how much are used ECU's these days?

^