Fiero GT Idle Surge after warmup (Page 1/1)
dexatron APR 28, 04:07 PM
Hi,

I'm having an idle surge problem after my engine warms up, when it is cold there is no problem. I read another post where it turned out to be fuel injectors.
Not really wanting to take the intake off unless I have to, are there other things I can check before jumping in?

Idle Surge - youtube

I've replaced the IAC and tested the TPS, the EGR tube has already been replaced with a stainless tube. No apparent vacuum leaks.

I can see that the engine is trying to idle down but then the rpms pop back up, if I rev the engine it idles fine for 10-20 seconds then goes back to surging.

Any help greatly appreciated, Greg
Gall757 APR 28, 06:22 PM
Welcome Greg!

The Fiero computer uses default values to run the engine when cold, but when hot, the computer uses signals from the sensors. So it could be that one of the sensors is not working properly. The MAP sensor could be the one because it is monitoring engine vacuum all the time, but other sensors could be at fault. Scanning software is best for this kind of problem....if you have that.

Another potential problem is electrical. Corrosion in the ground wires can cause idle problems. Check the ground cables and wires first, because it does not cost anything.
dexatron APR 29, 08:29 AM
Thanks for the help Gall757,

I'll check my grounds right away. I haven't checked the MAP sensor yet, I don't even know where it is! But I'll do that right after I do the grounds.

All the best, Greg



quote
Originally posted by Gall757:

Welcome Greg!

The Fiero computer uses default values to run the engine when cold, but when hot, the computer uses signals from the sensors. So it could be that one of the sensors is not working properly. The MAP sensor could be the one because it is monitoring engine vacuum all the time, but other sensors could be at fault. Scanning software is best for this kind of problem....if you have that.

Another potential problem is electrical. Corrosion in the ground wires can cause idle problems. Check the ground cables and wires first, because it does not cost anything.
claude dalpe APR 29, 08:53 AM

quote
Originally posted by dexatron:

Hi,

I'm having an idle surge problem after my engine warms up, when it is cold there is no problem. I read another post where it turned out to be fuel injectors.
Not really wanting to take the intake off unless I have to, are there other things I can check before jumping in?

Idle Surge - youtube

I've replaced the IAC and tested the TPS, the EGR tube has already been replaced with a stainless tube. No apparent vacuum leaks.

I can see that the engine is trying to idle down but then the rpms pop back up, if I rev the engine it idles fine for 10-20 seconds then goes back to surging.

Any help greatly appreciated, Greg


Hello dexatron,
For your trouble try to unplug the oxygen sensor when your engine is warms up if your problèm is gone replace it.
theogre APR 29, 10:22 AM
Ground(s) and/or Power can make Idle problems.

Need volt meter to check:
Volts at battery and elsewhere. Should see same Volts everywhere engine off and on. ~ 12.6v off, ~14.7 on w/ good alt and belt.
Voltage Drop test for Grounds and Power lines like Alt output wire. Use Google or YT.

⚠️ All "Grounds" bolted to engine etc are not Power Ground but Return of Signal to ECM.
Ω meter won't work because wire is to big or worse damage ECM.
O2 and ICM has these "Fake Grounds." See my Cave, Ground "Myth" notes O2 is a Tan "Ground" wire to engine.

Likely need ECM scan tool after that.
http://www.reddevilriver.com/ has some cheap.

Can use Ω meter on some sensors but ECM still reads 1 or more wrong because has iffy sensor plugs, wire to them, etc.

See my Cave, Sensors Quick Ref and Scan Tool Help

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

dexatron APR 29, 12:17 PM
Awesome, thanks guys, I'm heading out to the garage now!

Greg
dexatron APR 29, 01:21 PM
I checked the voltages, it's 14.7 at the battery, so far a few other checks seem to be ok, I will get under the car and check the engine grounds directly.

I unplugged the o2 and the car idles at 1400 without any surging. Is this because the o2 is bad or because it forces the car into closed-loop mode?

Thanks, Greg
dexatron APR 29, 02:46 PM
Another quick update:

I pulled the o2 sensor and it was pretty plugged with carbon. I cleaned it up and put it back in and now the car idles about 1200 rpm with no surge! I'd be really happy if I could get the idle down under 1000 but at least the car is drivable now!

Thanks for the help, Greg
Gall757 APR 29, 06:01 PM
If the searching idle is gone and the 1400 rpm is with a hot engine, you probably have a small vacuum leak or a bad tachometer. (they tend to fail by reading high as they get older).
claude dalpe APR 29, 08:24 PM

quote
Originally posted by claude dalpe:

Hello dexatron,
For your trouble try to unplug the oxygen sensor when your engine is warms up if your problèm is gone replace it.



I'm happy this help you, but change your oxygen sensor a new one AC DELCO it's cheap
Don't forget to clean the threads of the exhaust pipe because the sensor is grounded by the exhaust