Irrattic Idle

[Hudson]

99 XJ/4.0/AW4/NP242.

Been tinkering with my red jeep recently trying to get it going right so i can sell it.

Installed; (trying to fix an overheating issue.)

According to the PO a shop in Florida rebuilt the top end of the motor. New head, headgasket, water pump, yata yata yata. (He had receipts for it but didn't know why they did it.)

I have personally replaced;

- New Water Pump
- New Radiator
- New Radiator Cap
- New Thermostat (195)

- Installed the throttle body off my black jeep that was running fine. (Swapped over the IAC, TPS, & MAP (or is that a MAF?) as well when i swapped throttle bodies.

It's having a bunch of weird issues.

First off the idle. Upon starting the jeep it idles at like 1400 RPMS and doesn't ever drop down to a normal idle. If you rev it up to like 3k the idle increases to right around 2600RPMs and then doesn't drop down.

Driving the jeep it's the same thing. If you're going down a dirt road and take your foot off the gas instead of the jeep kind of coasting the high idle pushes you down the road and you're forced to ride the brakes to keep it slow.

I drove the jeep around earlier for 20 minutes seeing if i fixed the overheating issue. 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. Idle was still high.

Any ideas where to start looking? I already sprayed all the vacuum lines coming off the intake with starting fluid and i can't hear the idle changing.

I'm thinking;

- Leaky Intake Manifold Gasket?
- Clogged Cat?
- Throttle Body Manifold Gasket? (I didn't use a new one when i swapped TBs)

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

[Hudson]

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.

Any ideas?

[CumminsJeep]

Sounds like a vacuum leak somewhere to me. Obviously if it's idling high it is going to kick codes for the IAC and TPS. Pull the intake manifold and throttle body off and clean the mating surfaces and change the gaskets. It's running red hot because it's running lean as hell. Since you already swapped a throttle body on I'm going to guess its the manifold gasket. Make sure all of the vacuum hoses are in the correct places too.

[Hudson]

Sounds like a vacuum leak somewhere to me. Obviously if it's idling high it is going to kick codes for the IAC and TPS. Pull the intake manifold and throttle body off and clean the mating surfaces and change the gaskets. It's running red hot because it's running lean as hell. Since you already swapped a throttle body on I'm going to guess its the manifold gasket. Make sure all of the vacuum hoses are in the correct places too.

That's what i was thinking. Was hoping for a miracle but figured it would boil down to this.

Just bought both gaskets. The vacuum lines coming off the intake are super loose (Like the elbows are just old and worn out and sloppy) but spraying starting fluid on them doesn't make a difference.

is a "low input" code related to an intake leak or does that sound more like a faulty wire issue?

[customcreationsllc]

Also Sounds like a vacuum leak to me.
pinch off hoses, plug them up etc until you find the one with the hole.
Usually you can hear which one is leaking.
Brake boosters can leak, and maybe the PCV on this is just an orifice could be broken and sucking in too much.
Or even a broken intake gasket.

Hard to guess without looking at it.

once you get the idle down then worry about cooling.
Is the fan a clutch fan? when it gets hot is the clutch nice and tight?
The electric fan will not turn on until 220 xj's run hot.
But boiling over and melting the exhaust is not normal.

Hot cat sounds like you have a mixture issue which could be because you have a vacuum leak.

[Hydra]

ERRATIC

[Hudson]

Also Sounds like a vacuum leak to me.
pinch off hoses, plug them up etc until you find the one with the hole.
Usually you can hear which one is leaking.
Brake boosters can leak, and maybe the PCV on this is just an orifice could be broken and sucking in too much.
Or even a broken intake gasket.

Hard to guess without looking at it.

once you get the idle down then worry about cooling.
Is the fan a clutch fan? when it gets hot is the clutch nice and tight?
The electric fan will not turn on until 220 xj's run hot.
But boiling over and melting the exhaust is not normal.

Hot cat sounds like you have a mixture issue which could be because you have a vacuum leak.

Pulled the intake and exhaust manifold off. Gasket seemed fine and wasn't visibly messed up. Putting it back on tonight and i'll see if that does anything.

If it doesn't i'm going to really dig into a vacuum leak. The only vacuum lines on a cherokee come off the intake manifold correct? If i dont install any of them and plug them all with my fingers that should rule in or out a vacuum leak correct?

By PCV you mean the elbow on top of the valve cover?

It does have a fan clutch. I haven't looked into that yet.

[CumminsJeep]

Eventhough the gasket looked ok that doesn't mean it was torqued properly. Last time I put an APN header, with a new gasket, on mine I torqued them in order and just figured hand tight would work as the spec isn't very high. However, when I started it, it ran at like 1500rpm and wouldn't come down. I let it cool, retorqued them and it was fine. If it was installed incorrectly after all that head work was done (or maybe the mating surfaces not cleaned up?) it could still do it. If the gasket looks good, that doesn't mean it is either. I like using Permatex Copper Spray-A-Gasket on those too. Just coat the outside, let it tack up, and throw the gasket on. Just a piece-of-mind for me.

Like Justin said, just plug off vacuum lines until it fixes itself. But I would start with redoing the manifold gasket too.