Buick V-6 3.8 Idles Fast in Drive (But Idles normally in Neutral)

...86 Buick V-6 (3.8) with 120,000 miles: idles too fast in "D"rive, BUT idles at normal speed when put in "N"uetral". TPS (throttle position sensor) had
been replaced previously, and still checks OK with an ohmmeter. I could not see any obvious vacuum hose leaks. I also had the battery disconnected for awhile to reset computer. (still drives at 20 mph with foot off throtlle). Does this OK-in-Neutral provide a significant clue?? Thanks for any suggestions.
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Check your motor mounts.
Dave
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I'm not a mechanic but was googling some problems with my car and it wasn't even 5 minutes ago I was reading an article that didn't apply to me, but sound a lot like the problem you're having. Read this: http://www.2carpros.com/topics/idle.htm . There are 2 or 3 different post on that one page describing exactly what you are saying. Those persons have had their IAC and TPS changed and it did NOT fix the problem. The answer to the problem was the below quoted suggestion. Good luck...
"This is a difficult problem to detect because the computer can not sense a malfunction, the vehicles computer system thinks everything is ok. In reality what is happening is the electrical component inside the MAF sensor becomes contaminated by air impurities. These containments imbed themselves to the electrical component that senses the air flow causing the sensor to be slightly off, but not enough to trigger the MIL (malfunction indicator lamp). Replace the MAF sensor with a new. NOTE: (Mass Air Flow MAF sensor or Air Intake Sensor AIS, depending on manufacturer. It does the same thing only with different names)."
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