okay guys.
Im getting the occasional slow return to idle problem. occasionally it will just hang up about 1200 rpm for a few seconds after i let off the pedal, and shes not very consistent about it either.
otherwise she idles fine, the very very occasional stutter, but every other 4.0 ive had did it too.


here's where im at:
Fuel pressure is good.
TB is so clean you could eat off it.
New IAC
New TPS (have two that test and work the same)
New Coolant temp sensor
New downstream o2 (it had died a few weeks ago and threw a code)
new cap rotor and spark plugs

the only non OEM parts on there is i ended up trying the " injector upgrade" and its been great since. for me, i did end up with slightly better mileage and a smoother idle. nothing night and day though of course.



many many refined google searches were done before i made a post!

 

Just a few thoughts.. Upstream O2 sensor, check for vacuum leaks, check map sensor is good and the vacuum line on it is working properly.

 

Also, is the butterfly valve in the tb sticking? I have heard of ( very rare) the bearings for the valve shaft going bad.

 

I agree with 00jeepXJ that these are possibilities.

 

My vote is a vacuum leak. The IAC can compensate for a small leak after a couple seconds of searching. Check the vacuum canister.
Blow some cigarette smoke into a vacuum line on the intake and watch for any smoke escaping fom vacuum lines. As stated before - check the butterfly shaft where it passes through the throttle body. If it's extremely worn, you could be sucking air through there (probably less likely).

 

Throttle cable, TV cable(if auto), & cruise operate smoothly through out their entire length of travel? Is the floor mat or something under the dash hanging it up?

 

+1

Ive seen people adjust for slack in throttle cables before but accidently tighten things that shouldnt be thus causing a slower release usually a more consistent rather then sporatic problem but id start with the cables that are directly involved in throttle. pretty easy to check them out. just grab a buddy or unsuspecting wife.

 

An unsuspecting wife? They actually exist??

 

I've never personally met one but I've heard stories lol

 

might as well update.
its a 99 btw, i realised i neglected to tell you guys that.

replaced the O2 with a new ntk, no change at all, i dont think the old one was bad.
i dont think its the butterly valve bearing, smooth motion all the way through.

i searched for a leak, and i couldnt find one, i think my next step is to get some caps and plug them off right at the manifold, see if that helps.

i re-set the kicktown cable just to see, no change either.

thats the thing, it IS sporadic. happens with more frequency when winter breaking ( bumping her into N) then i really see the rpm's jump up and hold for a moment before slowly returning to normal.

i feel like im chasing a ghost.

edit: im going to go chase vacume some more.
double edit: lines are all solid. its not vac as far as i can tell.

 

When you cleaned the tb did you replace the gasket between it and the intake manifold?

 

its there.

 

What shape is it in? I had this issue and it was because of that $1 gasket

 

Do you have a stock Catalythic converter still.

 

no major damage to the gasket as far as i can see. a little flattened compared to new, but thats it.


yes, stock cat as far as i can tell. looks fairly old.