While on cruise control my RPMs sometimes increase slightly. It is not a case of an incline and therefore more power is needed. I'll be on flat road and it happens. But when I say slightly, I mean perhaps 50-100 RPM. Could it be the IACV? I know if that goes bad the RPMs go crazy, so I don't know. Slight vacuum leak? Bad spark plugs? When I start driving on a cold engine, I can almost feel the cylinders pulsing. I don't have any codes, I seafoam maybe twice a year, and I put in new plugs in on 8/30/08 with about 55,000 miles. This is what my old plugs they looked like:

 

The only reason the RPM should rise is because the car is needing to give more power to maintain that speed. It has nothing to do with plugs or any thing else. That or the tq converter is slightly unlocking and locking.

 

So do you think I have nothing to worry about? It seems to have started doing this after I changed the plugs, so that may be a clue. I don't remember it doing this before, and my old TL didn't do this. Maybe I should re-check the gaps and just reinstall the plugs.

Like I said, it will happen even if I am on perfectly flat road and just maintaining the same speed.

 

This has happen to me before. It just does it every now and then. Never figured out why it did that but the car runs well though.