Chrysler Repair: 1997 Plymouth Breeze erratic idle, throttle position sensor, plymouth breeze

Question
I have a 1997 Plymouth breeze with the 2.0L 4-Cylinder engine with automatic trans. The car has 216000 miles on it. Recently I have been noticing that the idle has been erratic. The car normally idles at around 850 RPM. I am noticing that the idle will jump up to around 1400 RPM, run there for a while, then come back down to normal, then jump up again, then back to normal. There does not seem to be a pattern to this. I have changed the Throttle position sensor,  and cleaned the throttle body. I took the idle control valve off and made sure it was clean. I changed the MAP sensor thinking it may be related to the temperature being sensed or the manifold pressure. I also recently changed the EGR valve, but that also did not help this problem. The computer has no codes in it. I am stumped. It will do this when I am in park or Neutral, or in drive. I can feel it pull when I am sitting at a stop sign and the idle kicks up. Sometimes, it does not come back down to normal idle when I come to a stop.

So what would cause the idle to jump up like that. When it jumps up, sometimes it will stay up for minutes, sometimes, just a second and then back down.

I thought it may be a vacuum leak, and I am searching the vacuum lines, but have not found a leak yet Vacuum at the intake manifold is around 18.

I will also mention that when driving along at highway speed and the transmission is in overdrive, it will occassionally come out of overdrive and then go back into overdrive. It is not slipping, it is shifting down, then back up. I am hoping these are related somehow.

I am pulling out what little hair I have left. Any ideas?

Thanks for any help you can provide.

John

Answer
Hi John,
It appears that you have covered almost all the possibilities for the problem. I agree that the intake manifold leak is the most likely reason at this point. Have you tried using a spray can of starter fluid to do the search? It is very flammable so you have to be careful to just use short spritzs at each connection and along major manifold gasket interfaces while listening for a surge in the rpm with each burst. You might also want to check the speed control cable if you have cruise for malfunction/binding.
I would use the vacuum line diagram on the underhood sticker as a roadmap to check all the vacuum line connections and the hoses proper for cracks.
Please let me know when you find the cause as this is shared by others I am certain.
Roland