I have read thru a ton of threads on the issue but wanted to throw my scenario out there:

I just bought a 92 Firebird 3.1L V6 for my son's first car. The car is in great shape inside and out. Has 150k miles. The person I bought it from said it wasn't running when he purchased it and did the following:

replaced the distributor,cap and button
changed the plugs and wires- at least he used autolite wires!
replaced the heater core
new brakes
new tires

The seller told me the car would sometimes stall when you came to a stop but would start right back up and that is exactly what it is doing!
When first started the car idles well however after the engine warms slightly you can hear what appears to be a miss and the car will occasionally stall in gear. The problem becomes more common the longer it runs- miss and stall in gear. The car only stalls when at a stop in gear. I can shift the car into neutral at a stop and have only had it cut off once.
The check engine light is not on.

What I have done after reading various threads:
replaced the tps and IAC
did the IAC manual relearn process--- see below
replaced the vacuum elbow on the FPR- elbow was dry-rotted and no fuel was coming out of the vacuum connector so the FPR appears good

When I attempted to do the manual IAC relearn process I noticed the cooling fan didn't come on at all. While watching the temp gauge in the car I noticed it didn't get much over approx 160 degrees. This tells me there is either no thermostat in the housing or it is stuck open as the OEM thermostat temp is 195 degrees. I have a new OEM temp thermostat waiting for a day when I have the time and the tolerance to deal with the cold to be installed.

Would a stuck or missing thermostat effect the IAC learning process or cause the stalling issue?
What else could be causing the issue?

I would suspect the ignition control module except it should be new along with the distributor.... I am currently trying to get more info from the previous owner regarding what parts were used.

I don't mind throwing a few parts at the car as I only paid $700 for it but I don't want to drain too much of the money I have set aside for my 84 Caballero LS swap.......

Any and all help would be appreciated!

 

Unless there's something else obvious you have to start with the injectors and fuel pressure regulator. There are many threads on how to check them but if they are original get new injectors (Southbay has a great deal) and a new fuel pressure regulator diaphragm. Once they are done you know you have a good foundation to start from.

 

Check all your vacuum lines and make sure you don't have any leaks. I replaced all of mine just to make sure. Ohm your injectors. I had two that were bad. Replaced all of them with some from Southbay. Double check your timing. Good luck.

 

Thanks for the info.

I have looked at most of the vacuum lines and they appear good.
The FPR appears to be good (no gas leaking from the vacuum line).

If it is a timing issue wouldn't the miss and stalling issue be present at start up and not just when the vehicle has warmed up?

Any insight as to the temp issue?

 

Check the PCV valve. It's usually easy and cheap to replace, but can cause the issue your describing.

 

PCV valve is on the to replace list.. Will give it a shot tomorrow.

 

My cooling fan doesn't come on until after the thermostat opens up at about 195 degrees because of the Coolant Temperature Sensor. Does your top radiator hose get hot at all or it it hot all the time. Should only get hot after your thermostat opens up, if you have one. And I checked the timing on all of my 'Birds when I got them, just to be sure. I know when it was off just a bit, it bogged my acceleration down. And, don't forget to ohm those fuel injectors.

 

Will do on the injectors.... Any way to check these without pulling the intake off? What is the proper procedure for testing them?

Sorry but I am an old school engine guy and fuel injection is some kind of vodoo that may suck out my soul.....

 

If you have a gauge, it might be a good idea to check the fuel pressure. See if that's acting up when it starts running badly.

The pressure should be about 42psi key on engine off, and drops to the high 30s when the engine is running. The fuel pressure regulator is referenced against manifold vacuum, so it should fluctuate slightly in relation to the manifold air pressure.
When the key is turned back off, it should hold pressure. You shouldn't lose more than a few psi in 15 minutes. If it drops fast then something is wrong.

You can check the coolant temperature sensor with a multimeter. Check the one that talks to the ECM, not the gauge:
210F = 185ohms
160F = 450ohms
100F = 1800ohms
70F = 3400ohms
40F = 7500ohms
20F = 13500ohms

I think this table also works for the intake air temperature sensor.

If you backprobe the TPS and/or MAP connectors, there should be a pin carrying a regulated supply voltage of 5.0V. If that's deviating, it's a problem.

When the engine is missing, you could try introducing fuel at the throttle to see if it helps the engine run better. If it does, it would imply the problem is fuel related. Otherwise it might be ignition.

It's possible you could have an injector that's electrically shorting when it gets hot. Each bank of 3 injectors is connected in parallel so if one of them shorts it can affect all 3 of them. I don't know if this fits with the problem being worst when idling in gear though. Perhaps, since in that situation there's little momentum to keep the engine turning through a miss.
I'm not sure if there's any easy way to check for this, especially if it's only happening when hot.
If I'm not mistaken somewhere around 12-14ohms is normal.

 

That's how I feel about carburetors right now

There are a few ways to diagnose these, youtube.com has a lot of videos about diagnosing injectors and cleaning them as well.

Heres one that helps you find suspecting injectors while they are still in the car. It's easy and quick to identify a fault that can become obvious...

 

My advice would be to thoroughly clean the throttle body if you haven't done that yet.
The carbon buildup in there only worsens such issues.
In my case it even was no longer necessary to teach the ECM its idle values.

 

Where is that coolant temp sensor located?

 

Should be just under where the throttle cable is connected (under the bracket), (drivers side), (next to the exhaust manifold between the dipstick and engine). It may have a circular rubber cover over it..

 

Thanks

Will be picking up a multi meter soon to check the injectors....

 

Sweet let us know what you find out.

 

Hi FBDad,

I know this post was a long time ago but I found it recently because my 1992 Camaro (same engine specs, 3.1L V6) is having this exact problem! She will run fine when starting up, but once she’s warm idles roughly and will stall in gear at stop lights. I replaced the Ignition coil, fuel filter, and IAC valve and neither of those things fixed my issue. Did you ever find out what was wrong with yours and what fixed it? Thank you!

all the best,
Glitterwave80s

 

For a temperature sensitive issue, first thing I'd check is the Coolant Temperature Sensor. Check the one that talks to the ECM, it should have 2 pins on it. You can measure the resistance across those pins using a multimeter as described above. You can start by checking it cold, but since you're getting the problem when the engine is warmed up, try checking it again after it's warmed up and see if the value still looks reasonable. If you have an infrared thermometer you could use that to corroborate what the sensor says.

When the connector is unplugged from the CTS, one of the pins on the connector should have +5V on it. Check that also. If it's not 5.0V it would be a problem.

You could also check the fuel pressure, again while in the conditions that it's misbehaving.

Is this an auto or a manual? Is it rough idling only when in gear, or also when shifted to neutral/park? If it's an auto, try unplugging the lockup TCC connector. If that fixes it then the lockup TCC isn't letting go when it should.