Hi, I'm trying to figure out what to look for next.

My 97 TSi RPMs have recently been fluctuating by a few hundred RPMs, where they should remain at a steady speed you can see the needle jumping about 200 RPMs. This maybe just a tiny amount but here's the kicker once you start it and put it in drive the RPMs will drop down to about 200 while in gear and foot on the brake and about 400 RPMs while in neutral.

For the most the Jeep runs/drives fine it's just when stopped. The IAC has been changed, plugs are looking brand new still. I do know there is a vacuum leak somewhere that I can't find because when I hit the highway and especially on hills the defroster will come on full blast with cold air.

I took a video of how the Jeeps acting while sitting still I shift from drive to neutral a couple times and gave it a quick rev twice I think. you can see after the rev the RPMs will drop to where they are supposed to be they'll pause there for a second then drop to 200 RPMs around 01:13 & 01:24 in the vid

Any ideas? Thanks

 

There is a vacuum reservoir under the battery tray (actually part of the battery tray), check to make sure the hose is not rotted off and that the battery tray is solid.

 

I did see a hose run under the battery, into a black plastic box. I didn't remove the battery though. From the box on back the hose still looks new.
Matter of fact the only hose that has any sort of wear to it is going from the engine to PVC valve, there's a crack along that hose but it's fully glued to the nipple where it's cracked.
Speaking of hoses, I found one in the rear of the Jeep that comes out of the floorboard then hooks into the axle on the passenger side. Any idea what the heck that hose is for?

 

Anyone else have any ideas?

 

The hose in the rear is your axle vent. I had the same problem and I took my throttle body of and cleaned it real good with carb cleaner and tooth brush and that solved the problem.

 

Ok, thanks. So I guess the axle vent will have nothing to do with the engine vacuum. but then again on Mercedes everything is controlled by vacuum including the door locks so, if one of those start leaking it could effect engine performance.
I looked down in the throttle body and it's still nice and shiny silver. My Jeep only has 57K miles on it.
Guess somethings may be age worn but not really use warn. I'll try taking the rest of the componets off of the TB and give them a clean. Maybe the the TPS is getting faulty. The IAC was just changed and I even unhooked the battery for a spell to reset the PCM. Still acting up, heck even my spark plugs still look brand new, except for some very light condensation corrosion on the nut part.
Good ol Daimler gremlins; all my Benzes had quarks and now the Jeep does too, especially the dissapearing radio display