G8T8A

Last year my stock 88 GTA started having an intermittent fast idle - usually 1,000 rpm. It would come and go with no rhyme or reason (hot, cold, at startup, during cruise, etc). Some drives, it would behave fine the whole time from startup to shutdown. Other times, it would be fine, then idle up as I was parking it. Other drives, it would high idle the whole time.

I replaced the IAC following instructions I've seen, and it behaved the same as above. Random high idle. I drove it with the TPS disconnected. It acted the same as before with random high idle. I kinda then dismissed the TPS.

A got a scan tool, and I saw that my TPS voltage was the correct 0.54 or so at idle, but the WOT voltage never exceeded 1.279. Yesterday, a buddy gave me two spare (used) TPS units to test on my car. I first fired it up to see how it would act with the old unit. It had the correct idle voltage and it would not go over 1.2xx V with the pedal floored. While doing that, I noticed the data now showed it hanging at 0.686 for idle. Pressing the TPS plunger down would only increase the voltage to 0.784. I have not idea what made that operating voltage range shrink.

I swapped the used TPS on, and I set it to 0.54V for idle. Then opening the throttle, I saw the voltage peaked at the same 1.2xx V. I pressed the plunger a few times, and it started acting just like the other - the range shrunk with 0.6xx at idle and not going over 0.7xx with the plunger pressed enough to simulate WOT. I went for a drive, and when I turned off my side street, the engine nose dived the instant it went above idle. I literally had to idle myself back home. Even giving it throttle in park/neutral would cause it to want to die.

With a restart, it seemed okay as I parked it for the day. Last I checked, it was still seeing 0.6xx to 0.7xx from idle to WOT.

I'm stumped as to the fact this 2nd TPS initially acted the same as the old one: 0.54 to 1.2xx range. Then "failed" the same as the old one to the point its operating range was 0.6 - 0.7.

Prior to swapping in the substitute TPS, it had a Low TPS code. It's also set a EST Function(?) code, too.

One thing of interest, back in the winter, when I would drive it, while the idle was behaving it, it drove fine with full power. If I were to go WOT while it was in "high idle mode," it would ping the worst I've ever heard from an engine. I'd instantly have to let of the throttle. That makes me wonder about EST code.

Any pointers or similar experiences are appreciated.

Edit: I just remembered one interesting thing. I've driven the car with my scan tool (1320electronics@gmail.com Bluetooth adapter) numerous times in the last few weeks. It has not high idled once while the ECM was communicating with the app. My power connection via the cigarette lighter is flaky, and a good bump kills power to the adapter. A few times when communication was stopped due to that, the high idle would occur.

G8T8A

Problem solved. The ECM was failing.

IROCU2

G8T8A,

Did you experience a bouncy tach needle when your problem occur?

G8T8A

No, the tach was always stable.

I did have faulty coolant temp data long before the idle issue arose. That was a result of the ECM, too. I suspect a bad ECM can cause various symptoms.