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Skjaeret -> 4.6L V8 idles rough in gear (11/12/2007 1:49:37 PM)
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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.
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