Last week I was having trouble with my 98 xj, it wouldnt start unless i held the gas, wouldn't idle. CEL showed code 0505 for the IAC. Today I replaced the IAC, now the jeep will fire right up and idle, but if i press the gas and then release it, there's a delay of a few seconds before the rpm's drop back to idle. And ideas what would cause this? Never had this problem until installing the new IAC today.
Thanks

 

The most simple possibilities are that the throttle body butterfly valve is not shutting completely when you let off the pedal due to it being gummed up / dirty, or some type of problem with the throttle cable not allowing it to get fully closed. The other is that the new IAC is shutting too slowly.

 

Could be an air leak as well. If the engine is sucking air into the intake stream, it'll lean it way out which will cause the symptom that you describe. Double check your installation.

I've never done the IAC on a 4.0, but I would assume that a bad seal (gasket or o-ring) would allow fresh air to be sucked in.

 

I didn't think of that, but this very well may be my problem. When removing the throttle body i noticed one of the 4 bolts holding it to the intake was stripped and could not be fully tightened upon installation. Perhaps air is leaking in between the throttle body and intake.
It is the threaded hole in the intake that is stripped, not the bolt itself. What would be the best solution for this, other than replacing the entire intake manifold?

 

Stripped, or damaged threads? You may be able to just clean it up with a tap. If not, you'd want to drill/tap/heli-coil or drill oversize and tap. Be careful to keep metal out of the intake.

 

following mine rev hangs like that too and is driving me nuts!

 

Did you get this resolved?

 

Not entirely...
I put new a gasket on the throttle body which seemed to help for a couple days... Then started it up and was idling at over 2000 rpm. CEL codes for both IAC and TPS. So i replaced them both (the IAC was less than a week old and they let me just exchange it. Now it idles fine, still hesitates a bit before returning to idle, but not as bad as it was. Stiil occasionally throws the code for the IAC, but without noticable symptoms, and just clearing the code makes it go away.

Sorry for the long response, wish i could be more help. Basically i got it drivable and thats good enough for me for right now.

 

Still having issues. IAC and TPS are both replaced and TB cleaned thouroghly. Idle is okay sometimes and other times so low it wont start without pressing gas. When driving and releasing the throttle the rpms sometimes seem to stick at about 1500 or so. Code p0505 comes on every few startups, but will go away for awhile if i clear it. Today also showed code p0320 for the first time.
Any ideas what could be causing these problems?
Thank You
Not sure if this is related, but as long as i've owned the jeep (before these problems started) the tach has been very "sticky"

 

Code P0320 can be a number of things.
https://repairpal.com/obd-ii-code-P0320

When I had an IAC issue even after cleaning the t/b and IAC which solved the problem the code was still tripped. I removed and cleaned the CTS (coolant temp sensor) and solved the code cleared.

 

How did you address the damaged threads for the throttle body mounting bolt? Are you confident that you have a good seal there? It's sounding less to me like that's the problem, but you need to fix it to rule it out if you haven't already.

 

Yes, I'm pretty sure the throttle body is well sealed now. I cleaned out the old threads and replaced the bolt with a new one the same size. Also used permatex gasket maker on the mounting surface and around all 4 bolts.

 

Any other ideas?
Supposed to be leaving on a 800 mile trip thursday and still having idle issues. Sometimes idles too low (wont start without holding gas) other times seems to "stick" at a high idle (1500rpm or so)
Thanks