I've had really good luck here in the past, so trying again. :-)

1997 Thunderbird, 4.6L with 197,000 miles.

The symptoms are basically: Starting on a cold engine, things behave pretty normally for a few minutes. Idle is fine (650-700 RPM range), etc. However, after a couple of minutes, once things start to warm up a bit (in the area where the cabin blower may start to kick in, etc.), the idle while in gear drops to 500 RPM or below. This causes a relatively annoying shake, which is of course both audible and physical. It appears it can drop as low as 400 RPM, and comes very close to stalling (did a couple of times).

Kicking it out of any gear (park or neutral) and the idle is perfectly fine.

I replaced the Idle Air Control valve, and that improved it to some degree -- not the level of the rough idle, just the frequency. I also have a new throttle position sensor, but haven't yet installed it. We've searched for bad vacuum lines, hoses, etc. and no luck.

Obviously my curiosity piqued when it became obvious that the problem seems to arise after a couple of minutes after starting a cold engine -- as if a sensor comes online or a valve opens after the engine reaches an early temperature (?).

In any case, when it gets bad, I have to be in neutral while stopped or force a little extra gas into the engine. Coming into winter where we have a lot of freeway creep and crawl, this isn't a wise or safe thing to do when it's wet, snowy or icy.

Hoping that the symptoms sound familiar to someone to offer suggestions.

 

Speed sensor? spark plug wires? coil? I'm getting too tired to think what's most likely.

 

TPS sounds to be a good place to begin.

 

Coil is original, but plug wires were replaced by pretty high-end after market wires about a year ago, so hoping those aren't a problem. :-)

 

As a follow-up... Replacing the throttle position sensor had no impact at all on performance or behavior. Looking for other suggestions, as a good 99% of them have followed the IAC and TPS route, now both complete...

 

hadthe same problem changed plugs, wires, air filter , themostat .95% resolved ,check tranny fluid. i think mine needs a egr valve as well.

 

Was curious about this problem so i registered to add thoughts... have a 95 tbird 4.6L For the most part runs great. When i get to operating temperature i sometimes get rough idle but only in with drive engaged and stopped. The trans was recently rebuilt, plus new radiator.
Once in a great while i get a code coming back as egr valve overflow. Im planning on replacing it to see the effect. Sound familiar?

 

Have you tried running some Fuel Injector cleaner through the system? When I first bought my 97 Tbird it had a bad habit of stalling if I drove very slow at a coast (No acceleration) I ran a couple of tankfuls of gas with Fuel injector cleaner on the first tank and gas treatment for the second tank and I have not had a single stall since I did that.

Although from the description you gave it does seem to be sensor related because the problem does not occur until your engine warms up. You did not mention a check engine light, but if you do have a light on have you tried to check the codes for a hint to where the problem might be? I do realize that codes sometimes begat other codes but they can help track down the problem. And unless you have swapped out all the sensors getting new common failure sensors like O2, etc...are sometimes a good idea for engines with high mileage.

 

despite this thread being old, i saw it searching for something else and there was no definative answer yet. So i thought i'd register and throw in my 0.02$

Check the transmission selector position sensor. (I think it's bolted on the side of the transmission, where the linkage for the selector connects). It'll have an electrical connector on it, obviously.

If the sensor has gone bad, then the PCM will not know that you moved the lever and put the car in gear. The PCM needs to know this to bump the idle up enough to keep RPMs consistant when you have the engine under "idle-load".

I had a similar problem with my '94 GRMARQ, and this sensor/switch on the trans corrected it.

 

a rough idle could be the start of the end of your catalytic converter(s) there are 3 of them on this car. if the rough idle carries through to be a rough running engine with a feel as if it is missing a spark and running rough, it could be that the catalytic converter is clogged and not letting enough exhaust pressure through it. The material inside could be melted or broken and when approaching 100k miles this could be the culprit. New ones are available online with or without the pipes. I believe I saw a set with the down pipes for under $200 online. I need to buy this myself as I have a 94 bird and it is in great condition with 94000 on it and all of a sudden the rough idle started and now it is running rough throughout the throttle range. I know my engine is in tip top shape so I am going to pull the converters off and have a look inside to see if airflow is blocked off.