[GenHawkin]

I have a 1992 Firebird with 3.1l v6 with 58k miles on it. When I first got it, I've had no problems. The first thing I did was put a cold air intake on it, still no problems. I put seafoam through it and noticed some smoke coming from the EGR valve, so I replaced that. I also replaced the IAC valve and Throttle position sensor and reset ECM. The car never got hot, so I had the thermostat replaced. Since then I've had problems. When the engine is cold, it idles perfect. I would say a little over 1k rpm. After it heats up the idle drops between 7 and 900rpm. When coming to a stop, the idle drops between 5 and 600pm and almost stalls or does stall. I'm curious what it could be. Also idles rough. My first thought it a previous owner put a really low-temp thermostat or took it out completely to fix the idle problems, so the engine wouldn't heat up. Smells like it's running a little on the rich side as well.

Edit: No check engine lights.

 

[Injuneer]

Manual or automatic transmission? The programmed idle speed for rhe auto trans is often lower than for the manual trans. On cold start the ECM elevates the idle speed and richens the A/F mixture, based on the coolant temperature sensor. The fact your idle speed drops with time (engine warming up?) would seem to indicate the temp sensor is working. But your idle speed still seems high.

That’s why trans type is important. Auto trans is often programmed at 550 RPM idle in gear and 650 RPM in neutral. Manual trans might be programmed for 800 RPM. I don't have access to a 91/92 3.1L V6 program, so I'm just judging this from some of the 4th Gens I've worked on.

The dash gauge is usually controlled by a separate coolant temp sensor. So you can’t necessarily judge accurate engine temp from the gauge. Appears there is also a “cold start fuel injection switch” that does not connect to the ECM, according to Chilton's.

If the engine is warming up, but the coolant temp sensor for the ECM is not accurate, it could actually be warming up, but the ECM still thinks the engine is only partly warmed up and is still setting a rich A/F ratio. Might be worth checking the coolant temp sensors for both the ECM and gauge. Perhaps you knocked a wire loose changing the thermostat.

Hopefully someone who actually owns a similar engine can correct any errors in my post.

 

[GenHawkin]

Manual or automatic transmission? The programmed idle speed for rhe auto trans is often lower than for the manual trans. On cold start the ECM elevates the idle speed and richens the A/F mixture, based on the coolant temperature sensor. The fact your idle speed drops with time (engine warming up?) would seem to indicate the temp sensor is working. But your idle speed still seems high.

That’s why trans type is important. Auto trans is often programmed at 550 RPM idle in gear and 650 RPM in neutral. Manual trans might be programmed for 800 RPM. I don't have access to a 91/92 3.1L V6 program, so I'm just judging this from some of the 4th Gens I've worked on.

The dash gauge is usually controlled by a separate coolant temp sensor. So you can’t necessarily judge accurate engine temp from the gauge. Appears there is also a “cold start fuel injection switch” that does not connect to the ECM, according to Chilton's.

If the engine is warming up, but the coolant temp sensor for the ECM is not accurate, it could actually be warming up, but the ECM still thinks the engine is only partly warmed up and is still setting a rich A/F ratio. Might be worth checking the coolant temp sensors for both the ECM and gauge. Perhaps you knocked a wire loose changing the thermostat.

Hopefully someone who actually owns a similar engine can correct any errors in my post.

It's an automatic I forgot to add before I had the thermostat changed, the heater would barely put out any heat.

 

[Injuneer]

What temperature does the dash gauge show? Heater core can plug up.

 

[GenHawkin]

What temperature does the dash gauge show? Heater core can plug up.

That's about what it gets up to when warm. The rpms fluctuate.

 

[ZumpTA]

Does the engine have an Idle Air Control?

 

[GenHawkin]

Does the engine have an Idle Air Control?

Yea. It has a new one on