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Hey everyone, I have a 97 tbird 4.6L 108,500 miles. Every so often I’ll get this weird idle where the rpm needle will go up and down while the car is parked. I’ve replaced the idle air control valve along with the gasket not too long ago so I’m confident that’s not the issue. Maybe the throttle position sensor? Sometimes it’ll fluctuate very minimal and other times it’ll fluctuate a couple hundred rpm’s. I don’t know if I’m able to post videos in the thread but I have a video of what it’s doing although it’s pretty self explanatory to imagine what the issue is. Again, this problem doesn’t occur every time I start the car. Only every so often. Thanks in advance!
 

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I would check all the vacuum lines. The plastic pipes for the PCV system to both valve covers don't always fit snug, or fall a little short. You can replace a couple of those hoses with longer pieces, mainly the one on the driver's side valve cover.

If that still doesn't do it, you may need to replace the 8 little pieces of hose for the EVAP system, located in the passenger wheelwell. There should be a tech article for finding all the hoses.

Al
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Vacuum leaks are definitely the starting point. You can test the TPS from the technical article: TPS Adjustment
 
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