jeep runs funky

1999 Jeep Cherokee Limited. 4.0/AW4/NP242.

It has a weird/surging idle. It'll idle at 1200 RPMS and then randomly (i haven't noticed a pattern yet) drop down to either a real low idle and sound like it's going to stall (but hasn't yet) or idle just drop down and idle at 750ish or so.

It was puking coolant before so i started flushing the radiator out earlier. With the jeep running and the heat on full blast i proceeded to add water into the radiator and continue adding water as the jeep sucked it in. Once it got up to around 210 and stopped taking in water i left the radiator cap off and let it idle to burp out all the air and see where the leak was actually coming from.

5 minutes or so after it seemed to be burped out it began boiling over through the radiator cap spot (obviously i still didn't have the cap on). The second it started puking coolant out through there the idle seemed to change. No idea if it was related or just happened but worth the mention.

I swapped the entire throttle body off of another jeep that was running/idling fine onto this jeep to see if that would fix the problem and still no dice.

Any ideas?

Previous owner (this jeep never ran right since i bought it) recently had the head, the headgasket, along with the water pump replaced. I can visually see the new valve cover gasket, new intake/exhaust gasket, and new water pump.

It has those weird plugs in it with 4 grounding electrodes. Replacing with copper plugs that the 4.0s seem to like.

Oh, and i did a compression test; (dry - no wet test.)

Cylinder 1 - 122 PSI
Cylinder 2 - 130 PSI
Cylinder 3 - 140 PSI
Cylinder 4 - 140 PSI
Cylinder 5 - 140 PSI
Cylinder 6 - 150 PSI

Also just noticed when i went to move the jeep that for some reason shifting between park and any gear also makes the idle change.

In park it'll idle at 1200 RPMS.

If you drop it into any gear (not sure about neutral) the idle will drop down to 1000RPMS.

Video related.

https://www.youtube.com/watch?v=s3N18JiPof8

Is the Idle Air Controller and Throttle Body clean ?

Kingkong0192 said:

.... it started puking coolant out through there the idle seemed to change. No idea if it was related ......

Click to expand...

The TPS does not respond well to getting wet. If the idle symptoms do not clear up, replace the TPS. Use genuine Jeep parts.

Tim_MN said:

Is the Idle Air Controller and Throttle Body clean ?




The TPS does not respond well to getting wet. If the idle symptoms do not clear up, replace the TPS. Use genuine Jeep parts.

Click to expand...

The jeep it came off of ran fine. Didn't actually look at either (obviously the TB wasn't totally gummed up and filthy or I would of noticed.)

I'll clean the IAC and TB tonight and see if anything changes.

Update.

I didn't remember seeing the check engine light at all so I went and double checked. Turns out its either burnt out or someone tampered with it. The jeeps throwing three codes.

PO122

Throttle pedal position sensor a circuit low input

PO505

Idle air control system.

PO500

Vehicle speed sensor a.

Ideas?

Fix the codes ?

I know that.

I was wondering if anyone has ever heard of those same codes appearing at once and x was the cause of it.

Solve the P0122, and the other codes will more than likely not return.

Pulled the exhaust manifold and intake manifold off. Replacing the gasket to make sure it wasn't leaking.

What's the easiest way to torque the bolts back down with the head still bolted to the block?

Take the air filter housing out.

Torque spec isn't life or death, but I've replaced a lot of manifolds. If you don't know what 25-30lbs feels like, use a torque wrench.

Also, replace the freeze plugs while you're in there. Good time for it.

Looking through the thread it looks like 99% of your issues relate to a faulty TPS.