I've got a 91 formula 305 tpi (speed density) and I've been chasing a high idle for the longest time. Finally got a reading(live data scan tool) off the aldl connector when it started to idle high. The ecm went from reading 0%throttle to 9% but the throttlebody didn't move. Checked the ground on the tps (new) and it seems fine but the voltage on the signal wire was fluctuating a bit. So for some reason, the ecm thinks the throttle is open when the tps is reading 0 and it causes the iac valve to open cause it thinks im on the throttle. And as sudden as it starts it starts idling normal. Block learn/fuel trim was as low as 116 during the high idle was as high as 126 during normal idle. Not sure if it's a bad ecm ground or what. Any ideas?

Edit: don't think it's a vacuum leak since the high idle only happens every once in a while. Also has new tps,iac, and the iac passage and throttle blades have been cleaned.

 

Check for play in the throttle shaft - can cause fluctuations in the voltage on the TPS as the shaft moves around. The SD cars update the 0% throttle reading every time they see a lower voltage reading from the TPS. You should check the input and output voltage from the TPS as well with a meter. Make sure it's getting a nice clean 5v that doesn't fluctuate, and snap the throttle a few times to see if the voltage changes when it's closed. Should be around 0,54v +/- 0,075v.

And check the grounds at the back of the cylinder heads.

GD

 

thanks for the reply, will check the ground. We also found the tiniest bit of play in the throttle body and it seemed to be a bit different when we opened and let the tb close. But, we left it to idle for a few minutes without touching the tb and it started to idle high and have a high throttle %reading. It returned to normal after idling high for a min or so and we didn't touch the throttle body. So, after checking ground if the issue persists should I get a new throttle body?

 

You're saying TPS voltage is changing without anyone touching the car? At this point we don't know if this is an electrical problem or it's a mechanical problem with the TB moving the TPS around.

Take the sensor off the TB, but leave it electrically connected.

Then repeat your test. When you start it up, the ECM registers the initial voltage reading as 0% throttle, so it won't matter that you don't have the factory ~0.6V setting.

If the voltage is still changing like before, it's most likely the TPS itself or some sort of ground or other electricalissue. If it doesn't repeat the same symptoms, put the TPS back on the car and repeat the earlier test. If the symptoms return, it's most likely something with the throttle body.