Occasional High Idle Speed - $8D 383 fully modified TPI

[zaut]

I am writing/asking this because of two reasons. One my son has his laptop at school so I can’t data log and two I am looking for confirmation of my theory. Occasionally my TPI engine will start idling very high. I do not have a vacuum leak. One time while it was idling at about 1600 rpm, I unplugged the connector to the TPS sensor. When I re-plugged it, the idle went back to normal. It has not gone into the high idle since then. I was never getting a check engine light. (Except when I unplugged the TPS). I am thinking that I was not getting a good ground to the TPS. This in turn was giving the ECM a 5 volt signal to the TPS signal input (Circuit 417). Because the ECM was seeing a wide open throttle signal, the IAC follower function was causing the IAC to open fully allowing a lot of idle air in. Does this make sense? I would think if the ECM was always seeing a WOT condition I would have other issues during the problem but I wasn’t seeing any during. The car definitely has better acceleration now. I am thinking that the acceleration enrichment function was not working with the TPS ground problem. Anyhow, I just wanted pick some other brains on this issue. I haven’t been here for awhile because I got the car running great thanks to everyone’s help. Thanks.

[RobertISaar]

it's a possibility. i recently had to replace a TPS due to it acting badly after the engine warmed up.... when it did, after closing the throttle, the voltage dropped too far below the normal TPS closed voltage position and the ECM learned that position as 0%, so when the throttle was actually closed, the ECM thought it was open by 3% or so and caused a ~1000 RPM idle.

[EagleMark]

Most ECM will set to 0% when the engine is started. Or does some have another learn?

You seem to have found an issue with the TPS, but it could be the sensor itself or a ground. Can check with voltage meter at idle around .54 volts and swing slowly and smoothly to around 4.5 volts. Any glitch or sticking and you found the issue.

[RobertISaar]

with the MPFI and SFI 60V6 applications i'm familiar with....

they check voltage at key-on, and the lowest voltage it sees before cranking is considered the 0% TPS point. with the SFI stuff, this is done with every key-on, but with the MPFI, it only does it if the NVRAM failed(battery disconnected).
while running, if at a high enough RPM and low enough MAP, the TPS 0% point can be changed in a downward direction, but not upwards.

[zaut]

I need to keep my mouth shut. I think I jinx myself. The high idle started again. I checked the TPS and it is working fine. 0.54 volts a closed about 4.5 volts at WOT with a linear increase as I open the throttle. I will have a laptop this weekend to dig deeper. What is funny is it is not consistent. I shut the car off and it gets back to normal. I hate intermittent problems. They are the hardest to diagnose.

[EagleMark]

That sounds more like an IAC issue. Dave and I both found bad IAC valve this week, checked wiring, Ohm sensor etc... IAC would hang... it resets when turned off and mine anyway would work it's way back up in IAC counts and stay there. Replaced and fixed.

[EagleMark]

with the MPFI and SFI 60V6 applications i'm familiar with....

they check voltage at key-on, and the lowest voltage it sees before cranking is considered the 0% TPS point. with the SFI stuff, this is done with every key-on, but with the MPFI, it only does it if the NVRAM failed(battery disconnected).
while running, if at a high enough RPM and low enough MAP, the TPS 0% point can be changed in a downward direction, but not upwards.

Since you found the throttle hanging up you may want to disconnect battery for awhile and let things reset... Robert quote above says it won't reset to TPS 0% without! Then see how it does.

For a modified engine sometimes IAC is not enough for your cold start issue? Do a data log and when warmed up see what IAC counts are doing? If like 40 or so, well you have plenty of room to open throttle blades so more air can get in at cold start. Do want to end up with some IAC counts for idle control at end.

[RobertISaar]

it may not be necessary with 8D, but id rather err on the side of caution.