Mechanics

4CYL STUMBLES WHEN COLD.. WHY?

1985 Pontiac Fiero • 110,000 miles

1985 Pontiac Fiero 2.5l 4 cyl. SE
When first starting, sometimes it idles fine. But when put under load the tach bounces all over the place and it runs rough. The whole car jerks.
Will continue to stumble for about 3-4 miles. Then its like nothing happened.
What is very odd - is no check engine light comes on and there are no codes stored in ECM.

I Replaced:
Distributor cap & rotor
Spark plugs & wires
Vacuum hoses
Battery
I checked the damper door under the vacuum motor to see if the Air Cleaner Temp Sensor was moving the door to regulate hot/cold air. And it is.
Mechanic suggested I look for leaks around intake manifold. But how do I get under by that to do that. Try to spray ether from underneath the car?

I really don't want to continue buying parts to guess at fixing it. The Air Cleaner Temp Sensor is $50 or more where I live.

Anyone else have any other ideas?
Much appreciated, thanks in advance.
Avatar
Jakscars
July 13, 2012.




Egr valves are common failures for this condition. You should check it for proper operation. Take off the vacuum line and see if the issue is gone.

85 is way too soon for codes for these issue.

Otherwise, could need injector cleaning, coolant sensor or tps.

Roy

ASEMaster6371
Jul 13, 2012.
Took off the vacuum line at EGR. Problem remains.
Is there anything I can do to test tps or coolant sensor?

Thanks for the reply

Tiny
Jakscars
Jul 13, 2012.
Another note. I'm noticing the idle is staying up around 1500 it used to be at around 1000. Maybe IAC valve is bad?

Tiny
Jakscars
Jul 13, 2012.
A scan tool can monitor the sensors. You need a shop with a scan tool for this portion.

Roy

ASEMaster6371
Jul 13, 2012.
I replaced IAC valve and so far it seems ok/better no stumbling at all. True test is tomorrow morning I guess
AD

Tiny
Jakscars
Jul 14, 2012.
Good luck

ASEMaster6371
Jul 14, 2012.
I ended up replacing the coolant sensor and ignition module too. Something has definitely changed. Now its' like when I hit a bump (maybe coincidence) and the engine is cold. Sometimes it will stumble when I first start it and idling, other times only when I put it under load. But the stumbling doesn't last as long and things seem to have somewhat improved. What the hell?

Tiny
Jakscars
Jul 15, 2012.

AD