1989, 4.0, auto

Have an issue. Been having it oh, for the past 5 months or so. My start up idle is spot on at around the 750 rpm mark. after a few minutes it drops to the first hash mark. doesn't stall or anything.

Then once up to highway speed (55-60mph) I can take my foot off the gas, coast, and the rpm's gradually drop to around 1100ish. it holds there for a couple seconds then starts to "bounce" between 1000 and 1200ish mark. if im going faster than 60, the rpm is steady....

Here's what ive done in the months its been happening (not in any order)

- New IAC
- New TPS (adjusted as per Cruiser's instructions)
- New crank position sensor (replaced by shop for no start when I first bought it, possibly related? didn't happen till after this) <--first thing that was replaced on the jeep since ive owned it that wasn't regular maintenance
- took off throttle body, cleaned it spotless
- vacuum harness
- cleaned battery terminals
- new oxygen sensor, not bosch
- cleaned all the grounds, tested grounds
- new map sensor
- full tune up
- checked for vacuum leaks, there are none. no lose intake bolts either.

I also had a shop with a very good rep look at it and they even said "everything checks out just fine. the fuel pressure, all the sensors, vacuum is great. its normal for early fuel injected cars to fluctuate rpm coasting like that. and the rpm is fine." ....yeah just fine my haha. my other renix's never did this either...

ANY IDEAS?

 

Let me clearify: Before the jeep had its intermittent no start issue, the rpms while coasting were very steady. the idle was at the first hash mark as well, so that hasn't changed.

 

Did they give you the fuel pressures by any chance?

 

No, they didn't. Probably my mistake to not ask for it after they said it checks out good.... guess I should double check that then....

its odd. below 55, the rpms are steady, and above 60 theyre stead. its only in that 55-60 range they bounce.

 

But thats only when you take your foot of the gas, isn't it?
tbh, I wouldnt worry about it, if its only in that little range, and it runs fine otherwise.

Surging (bouncing) at idle or something like that, I would try to chase down.

 

its one of those nagging things ya know haha. ive fixed just about everything else, or will be soon. but these are the 2 problems I cant figure out

 

Did you see if someone messed with the throttle body set screw? Post 14.

How about indexing the distributor? Post 13.

 

when I had the throttle body off, the butterfly valve looked to be where it should be. fully closed I could see a mere sliver of space. Havent indexed the dizzy. ill read that.