Hey everyone, I have a '91 Firebird, 3.1 V6, auto, 156k miles. So I have this weird high idle problem, the car will be running fine and then it will just start idling between 1500 and 2000 rpm. No specific time when this happens, sometimes it will start the high idle when I first start the car, other times it will start when I've been driving, and so on. Now the wierd part is that the only way that I've found to get the idle back to normal is to either disconnect the battery or pull the ECU fuse by the air cleaner. Then it will only idle correctly for a couple of days before starting the process again. This happened about 2 years ago, and I reset the computer and it went away completely, until about 2 months ago, and now I've been resetting the computer every other day.

So here's what I know. There are no trouble codes, no check engine light. I put a scan tool on it and everything looks normal except that the tps sensor shows a voltage of .56 or .58 volts (which according to Chiltons is normal for idle) but the scan tool is showing that the computer is seeing the throttle being 20% open. The throttle blades are closes, not sticking at all, there's no play, and I just cleaned the carbon out of there. The IAC is brand new and I replaced the ECU with a rebuilt one. The plugs, wires, cap, and rotor are brand new. The TPS and MAP sensor are not even 2 years old and are showing normal readings according to the Auto X-ray. The fuel fitler is not even 2 years old. I replaced almost all of the vacuum lines 2.5 years ago, and they all (even the ones I didn't replace) still look good, with no signs of cracking or anything like that.

So does anyone know why a car will just randomly start idling really high, with the computer thinking the throttle is open when it's not and the only way to get it back to normal is to reset the computer? Thank you so much for your help!!

 

'vaglio

Welcome to TGO

Where did you measure TPS voltage? My guess that you have measured right at the TPS sensor. Re-verify the same measurement right at the ECM connector while you have the weird idle condition. If it still 0.50 to 0.56 Volts you may have flaky ECM.

//RF

 

Hi RF, thank you for helping. I've measured the TPS voltage with the scan tool so I know what the ECM is seeing, and then I verified it at the sensor itself. I also thought it was the ECM so I put in a rebuilt one a couple of weeks ago. It's used, but I was told that it works and is a good computer. I know it's possible to get a bad computer, but what are the odds it has the same exact problem as the old one? Thank you again for your help, I appreciate it.

Vinnie

 

Check the ground lead on the TPS sensor - flaky sensor ground (by the thermostat may cause incorrect TPS reading.

I have a further question clarification - how did you measure TPS voltage? Scan tool reports what ECM reports to it. So if ECM sees 0.56 volts from TPS it should report 0.56 volts or about 1.8% TPS. In other words if X-Ray reports 20% TPS the reported voltage must be much higher (100% approximetley 4.6 volts). To get a to the bottom of this you'll have to get a DVM, set it to read DC voltage relative to ground and measure Pin B of TPS sensor (Dark-Blue wire) while simultaneously looking at X-ray readout.
Check grounds.

//RF

 

Hey RF,

I will definitely check the grounds either this evening or tomorrow, depending on what time I get home from work (got to love northern VA traffic).

To answer your other question, I measured the voltage of the TPS with a multimeter, I don't remember what wire I took the measurement from, might have been the dark blue wire. And yes, I was looking at the Auto X-ray at the same time, they were identical readings. Also, wouldn't WOT read 1 volt from the TPS? For some reason I remember seeing that in one of the manuals. I also remember seeing that the TPS is auto zeroing meaning there is a range of voltage that the computer reads as 0% throttle, and the voltage I was seeing from the TPS falls within that range. But yet something is telling the computer that the throttle is open, which is the part that I cannot figure out.

Again, thank you so much for your help, I really appreciate it.

 

Yep, TPS circuit in C3 ECMs has the auto zero feature. When TPS voltage at idle is about 0.5 to 0.6 ECM will assign 0% TPS. The TPS sensor is nothing but a glorified variable resistor. At WOT Pin B (Dark Blue) should be around 4.0 Vdc. Reference voltage Pin C (gray wire) should be 5.0 +/- 0.2 Vdc. Pin A (black) should be 0 Vdc and connected to sensor ground.

You can check TPS health while engine is not running with key in run position by slowly varying throttle position - there should be a smooth transition from 0.6 through 4.0 volts at WOT position. Any jumps in voltage may indicate tired TPS. If such is the case get a new one.

I hope this helps

BTW Orange county traffic can be a real PITA as well!

//RF

 

Hey RF,

I checked the TPS sensor and the grounds, everything seems to be working correctly. I also sprayed some TB cleaner around the intake manifold to see if the engine stumbles at all, figured I might get lucky and find a vacuum leak or leaky intake gasket, but not luck on that either.

I ended up resetting the computer to bring the idle back down to normal (save me some gas during the commute). Thank you again for all of your help, I really appreciate it!

Vinnie