I have an 85 iron duke fiero with isuzu 5 spd that has a strange idle problem not exactly covered in other threads that i saw. The idle starts out at 2000 rpm and goes between 2000 and 2200 for a little while. After a minute or so the rpms will increase to 2500, 3000 and idle up in that range. I shut it off at that point. The only code the car is throwing is the self check code "12" so that doesn't help me. I checked the egr valve by pushing up the diaphragm while it was running and it ran rough barely keeping an idle high enough to stay running so that is most likely good. I checked all the tubes and didn't see or feel any cracks or weakness. The gas pedal has zero throttle response until about 3/4 of the way down. Then it has the throttle response equivalent to taking off full throttle from a dead stop. I had a helper pushing down on the gas pedal while I watched the throttle body. The cable moves just fine and the butterfly opens at the same time (checked while car wasn't running so far) any ideas? It is not that cold outside (38°) so it shouldn't have to increase idle to warm up. Thanks
1985 pontiac fiero 2.5 isuzu 5 spd 140k miles

quote Originally posted by Kwmiller: It is not that cold outside (38°) so it shouldn't have to increase idle to warm up. Thanks 1985 pontiac fiero 2.5 isuzu 5 spd 140k miles

Wrong. Idle can/will be high < 40°F
If colder over night even more so because take time for engine etc to follow weather temps because ECT sensor still sees lower temp then you.

Plus can still have vac leaks and/or iffy power ground or both. See my Cave, Vac Leaks and clean/fix all grounds in engine bay.
Maybe IAC is iffy when cold but don't seem so as posted.

Most times can hit the pedal to knock down fast idle after a bit. Does this happen?

Disconnect C501 power to ECM clear it and to force it to relearn idle.
It may not change much, but maybe a Code will come up after running.

Not sure where C501 is on 85 Duke, but on 84 it's in the wiring harness.



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[This message has been edited by fierosound (edited 01-25-2020).]

Pulling C501 likely won't help. Think 85-86 L4 is on the right side on head.

quote Originally posted by Kwmiller: The gas pedal has zero throttle response until about 3/4 of the way down. Then it has the throttle response equivalent to taking off full throttle from a dead stop. I had a helper pushing down on the gas pedal while I watched the throttle body. The cable moves just fine and the butterfly opens at the same time (checked while car wasn't running so far) any ideas?

Could be iffy/bend TPS plus above.
If has bend arm can see it w/ or w/o taking it off the TBI but need a Meter or ECM scan tool to check electric problems.

I sprayed all vacuum lines with throttle body cleaner and no change in idle. I plugged up the port for iac (just to rule it out) and the engine ran awful eventually about dying. Which means the iac is doing its job. I ended up turning it off and on again every few minutes and let the premium fuel go through it. The fuel in it previously was 4 month old unleaded. But after a few minutes of warming up stepping on the gas pedal to make the idle go down. it ended up at a 1000rpm idle. Perhaps it was just cold? Its 43° F out now. I don't know how these engines are supposed to sound at idle but it sounded like it might have been randomly missing? Not huge but enough to move the rpm gauge a little bit. Throttle response still isn't good but the idle is normal. Forgive me, never owned a GM vehicle in my life just dodge and diesels trucks but saw this and thought it'd be fun. But any more suggestions on the throttle response? The gas pedal could be tightened up, it's fairly loose. The batterys probably junk too I have to use a battery starter on it every time I work on it so I'll get a new one. Thanks for the help so far