96 Lumina 3.1 rough idle ONLY WHEN WARM

I have a problem with my 96 Lumina 3.1L that is unique to me, hopefully someone out there will recognize it.
When the car is cold, it runs beautifully (except for that annoying knocking sound common to almost all 3.1Ls, as documented by Chevy). BUT when the car warms up enough to register on the thermostat, it starts to idle VERY ROUGHLY, almost as if it were not firing right. I have replaced the plug wires, the plugs are not very old, the air filter is clean, I have cleaned the MAF thermistors, I don't think there is a problem with the PCV valve, and the fuel filter is fairly new.
IF it were something that happened ALL the time, I think it would be easier, but it doesn't, only when it's hot. I have owned the car for five years, and this is the first time I am really stumped. Rather than throw a lot of money at it and HOPE that fixes the problem, I would like to hear from anyone first. The car has 172K miles on it, and the only thing that was done recently was the upper and lower intake manifold gaskets were replaced. When he was replacing the gaskets, could something have gone wrong then?
Thanks in advance.
Jim

Check the EGR valve. When it starts to idle bad, get a hammer and tap the EGR a few times to see if it clears it up. My '90 had this problem, exactly like your saying, and the EGR was the cause.

Well I did tap it, but that didn't help. Thank you though. That's not to say that the EGR valve is not the problem, it still could be. I'll run the Haynes checks on it this weekend.
I almost think I hear a vacuum leak on that side of the engine, but where there are so many hoses over there, it could be any of them. I will be looking at the coil's primary and secondary resistances today or later this week, and replacing the spark plugs with brand new ones this weekend.
Any other thoughts?

sounds like a bad cts(coolant temperature sensor)
make sure you replace the one the computer uses

amac,
I, also, was thinking either: the coolant temp sensor, the air temp sensor (on the intake hose), or maybe a failing ignition module. I tried to get to the coolant temp sensor yesterday, but that bugger is hard to reach, beneath the throttle assembly. I will try again, to see how that goes. I will also check the air temp sensor.

Oh, one other thing about that coolant temp sensor: Haynes shows pics of it, and they tell how to check it, but they only mention one with two wires. Mine has three. Anyone know the proper way to check this?

Question for anyone: What is the hose going into the upper left corner (as standing in front and looking at engine) of the intake plenum? I know it is a vacuum line, and I know that when I take it off, the idle is so bad the car almost stalls, but the other end doesn't go anywhere. Where is it supposed to go?
This could be related to my problem, as the line itself is cracked and broken in a couple of places.

any vacuum leak will make it run like crap. i'm not 100% sure what your looking at might be for the map/baro sensor, egr, fuel pressure regulator, cruise control, evap theres lines every where on these things.
i don't know off the top of my head exactly how to check it i know you can back probe the wires and measure the voltage drop or unplug it and measure resistance.and compare these values to a given table. almost all computer sensors use 3 wires. i check them with my scan tool it tells the temp the computer sees.

The line is right on top, very accessible and visible, and runs from the plenum, which is engraved 3.1 something, around the back of the engine by the coils, and down toward the inner fender area, driver's side. It appears to feed something into the air to mix with the fuel, and when I disconnect the line at the plenum (upper intake manifold) it runs REAL badly. If I plug the hole with my finger, it resumes its normal idle (cold=fine, warm=suck). All I can figure is that the line must be plugged somewhere before the break (and therefore simulating the finger plug), as it is certainly not getting fresh air, and I know it is not getting what it is *supposed* to be getting, because the line goes nowhere.

The sensor is a thermistor, so it will (in this case) drop resistance as the temp increases. Haynes gives the chart, and the values I should (roughly) see, and working in electronics, I know how to test a normal thermistor. But for some reason, this has three pins. I just don't know the pinout of this sensor.

Last night on the way home the car started to run so badly that it almost stalled. The Check Engine Light came on, and I decided to stop at AutoZone. They scanned it, and for the second time now it came up as MAF. This time, though, it also gave a misfire code. I figured the misfire code could have come from the MAF causing the poor running, so I bit the bullet and bought the damn MAF. To avoid a core charge, I replaced it there in the parking lot. I did see a difference, but not like I wanted. I cursed about having bought the wrong thing ($99.99!) and tried to console myself that it may have needed that anyway. Got home, unplugged the negative battery terminal to clear the code, hooked it back up, and there was the CEL again! It didn't seem to run any better, so I tried the "teach it to idle" approach by putting it in gear, stepping on the brake for 7 minutes, etc. No change. So this morning I pulled into the parking lot at work and it *seemed* to be running better. At lunch I went out and it seems very close to right! When I started it again after lunch, the CEL finally went away all by itself. Tonight I will change the plugs, just to make sure all is OK there, but I may have solved this particular problem.

just remember if you have a good vacuum leak it can set a Maf code.did you ever figure out where that vacuum line went?? if not just cap it off.

just remember if you have a good vacuum leak it can set a Maf code.did you ever figure out where that vacuum line went?? if not just cap it off.

amac209,
I was not able to figure out the vacuum line until this afternoon, after I had had the good luck with the MAF, so I don't think that was the problem, although I did reconnect that line this afternoon before leaving work.
I finally found a dealer service department who would take the time to answer and who knew the answer. They said that it is a vacuum reserve, used to move things like the shutters in the passenger compartment air distribution system. This would explain why the heat seemed to keep coming from the floor, even when I requested the panel on the switch. This would also explain why the hose appeared to go through the firewall; it may have! Good that I found this before summer, when I need the AC!

Just another thought: The Idle Air Control Valve is operated by the computer. There are 4 terminals in the valve. ABCD, the resistance between A and B and C and D should be between 40 and 80 ohms.

Has It Gotten Any Better? Where Exactly Did That Line Go? I Have One Unplugged That Looks Like It Goes Under The Battery Somewhere. Small Hose, Hard Plastic, Not Rubber About 3/16 Of An Inch Or So. Please Let Me Know.

Yes, the car is running much better now that I have replaced the maf and the plugs. I had another post that mentioned that when I went to replace the plugs, I found two of the rear plugs loose! They were so loose that I didn't even need a wrench to loosen them. While this may have been part of the problem, this didn't seem to be the full problem, as the maf replacement got things pretty much back to right.
The vacuum hose that comes from the upper plenum (intake manifold) passenger side, goes down around the back of the engine and to the driver's side. It meets a "T" where one side goes to the passenger compartment through the firewall. The other branch of the "T" goes under the battery and into the space between your driver's side front fender and wheelwell. In there, there is a black plastic "ball" that is about the size of a canteloupe. This is a vacuum reservoir, drawn upon when needed, for things like changing the shutter positions under the dash. When you want panel air, you move the dashboard switch to panel, and it tells the selector under the dash how to move the shutters to direct the air that way. The shutters move by the vacuum power.

Yes, the car is running much better now that I have replaced the maf and the plugs. I had another post that mentioned that when I went to replace the plugs, I found two of the rear plugs loose! They were so loose that I didn't even need a wrench to loosen them. While this may have been part of the problem, this didn't seem to be the full problem, as the maf replacement got things pretty much back to right.
The vacuum hose that comes from the upper plenum (intake manifold) passenger side, goes down around the back of the engine and to the driver's side. It meets a "T" where one side goes to the passenger compartment through the firewall. The other branch of the "T" goes under the battery and into the space between your driver's side front fender and wheelwell. In there, there is a black plastic "ball" that is about the size of a canteloupe. This is a vacuum reservoir, drawn upon when needed, for things like changing the shutter positions under the dash. When you want panel air, you move the dashboard switch to panel, and it tells the selector under the dash how to move the shutters to direct the air that way. The shutters move by the vacuum power.

THANKS FOR THE REPLY. MY 96 LUMINA STARTED YESTERDAY MORNING AND THEN DIED AT THE END OF THE DRIVE WAY. THANK HEAVEN, IN A WAY,, I DIDN'T HAVE TO HAVE IT TOWED, IT WAS THE FUEL PUMP. I REPLACED IT TODAY AND SO FAR IT IS RUNNUING FINE. IT MUST HAVE BEEN GETTING WEAK. AT LEAST I HOPE THAT WAS THE PROBLEM. WELL THANKS AGAIN.
TEKDVR