High rev on cold start

[hgeranium]

An occurrence that has been happening for almost as long as I've had this MJ, about 3 years now, (88 4.0 AW4) is that, on occasion, during cold starts (sometimes when warmed up too, but more seldom) the motor will rev up hard to ~2000-2200 for a few seconds and come back down.

 

I also had another issue paired with that where the revs would go up and get stuck there, even while driving. This happened seldomly through the last 3 years but really ramped up last month. The throttle would get stuck open. Super jumpy, hard to brake, etc. like the throttle plate was cracked open. I cleaned the TB (no major carbon deposits), ran a can of sea foam, and replaced the IACV a few weeks ago and I haven't seen this problem come back thus far.

 

However, even with the throttle body cleaning and IACV replacement, that high revving phenomena still regularly occurs. I guess I would like to know if this a common occurrence for anyone else and I should just ignore it as an old vehicle quirk. It is my daily driver, so that's why it's more of a concern to me than if it were just a project.

 

TPS is brand new, voltage tested perfect and within that specified resistance range (Cruiser tip #8). Vacuum lines are solid all around, no leaks there. Resetting the IACV does nothing to change it. The truck runs flawlessly other than that. In park it idles steady at around 900rpm and in drive stays steady at ~700. The fact that the truck runs so well otherwise leads me to believe that it's just an old vehicle quirk but wanted to run it by y'all with much more 4.0 experience than I.

 

Thank you!

[Eagle_SX4]

It is probably your manifold bolts. They like to come loose and sometimes fall out. If you remove the air box you can get to all of them from the top of the engine. There are 11 bolts/studs total.

[hgeranium]

6 hours ago, Eagle_SX4 said:

It is probably your manifold bolts. They like to come loose and sometimes fall out. If you remove the air box you can get to all of them from the top of the engine. There are 11 bolts/studs total.

Thank you very much! I’ve read about that happening to people but stupidly never checked myself. 

[pizzaman09]

Do check that all manifold studs are actually present, particularly the lower one nearest the back.  Mine had backed out completely and was missing, I didn't realize it at first.