HudsonN

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

HudsonN

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.

stretch86

Mines doing same thing as what youve described so im gonna follow along with this have you taken the iac and throttle body off and give it a cleaning im gonna replace spark plugs tomorrow

Cheddarnut

Couple things, IAC valve on the throttle body. Pull it and take a look. Cheap to replace, 30 bucks new. Some guys clean them.

Coolant leaks cause a lot of trouble, you mentioned looking for the leak but didn't say if you found it. So if it's leaking find that, plus it wouldn't hurt to check the temp sending unit. Easy to knock wires off checking stuff.

Is it tripping the check engine light at all?

HudsonN

No. No check engine light.

Just put in a new battery and replaced battery terminals as those cause some odd issues on these.

I got the jeep running again today and the thing loves idling high. Like it gets stuck idling at 2500RPMs and wont idle for some reason.

When driving it seems to not enjoy shifting. Although i can't tell for certain it's like when i start braking the jeep doesn't realize and it doesn't downshift.

Also from a dead stop if you pin the accelerator you can hear what sounds to be a metal grinding noise. Maybe signs of something in the torque convertor?

Someones suppose to be coming to look at this thing as it's been sitting in the tennis court for six months now and i never got around to fixing it and my dad told some chick he knows that it was for sale for 4k so i'm trying to sort these issues like yesterday. Ha.

Sprayed starting fluid all around vacuum lines. No change in idle.

HudsonN

Thought i got the cooling situation situated apparently not.

Put a new radiator, water pump, radiator cap & thermostat in. Started the jeep up and let it run for 10 minutes as i was adding coolant and burping the system. Once done, put the radiator cap on and took the jeep for a 20 minute drive. The temperature gauge started at a line above 210, and as i drove slowly cooled down. Finally got down to a hair below 210, and then creeped back up to a little above 210.

Got back to my house, parked in the driveway, left the jeep idling to see if it would overheat, walked inside and came back out maybe 5 minutes later and looked into the engine compartment to see the exhaust manifold glowing RED hot.

Killed the jeep instantly and then it started boiling over like crazy.

So i'm thinking;

- Exhaust restriction?
- Leaky Intake Manifold gasket?

HudsonN

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

505

Idle air control system.

500

Vehicle speed sensor a.