Jeep 4.0 high idle when warm... driving me crazy

[NotMatt]

Ok, so I finally got things all buttoned up on my Jeep and have been able to take it out and do some hot laps, very exciting. However, I'm having one major problem that it never did before I tore it all apart... once the thing warms up and the ECU goes into closed loop, it idles at 1500 rpm.

It's a 91 wrangler with 4.0 high output, 5 speed.

I've checked over and over for vacuum leaks... unhooked and plugged everything except the MAP sensor and FPR, no change. Took the throttle body apart and cleaned the IAC really well, tried a different IAC I had, no change.

What's my next step here? Coolant temp sensor? What range should I be looking for when it's warmed up fully? MAP sensor? What else can cause a high idle like this?

[EDIT]... Here's a loop to ponder... I swapped an earlier RENIX cherokee exhaust manifold on in place of the factory HO exhaust manifold, because it's outlet was better for my exhaust setup. Everything seemed to line up fine and the installation went well, but now I'm second-guessing myself. I eye-balled both manifolds before the swap, and didn't have to modify the earlier one at all to make it fit, it was a bolt-in kind of deal... but is there possibly a way for air to get in between the intake manifold and the head with the older exhaust manifold that I overlooked when I did the swap?

 

[crash2]

Any chance you can get a scanner to see the data? Wondering where the IAC is sitting.

 

[NotMatt]

Any chance you can get a scanner to see the data? Wondering where the IAC is sitting.

All I've got is a cheap obd2 code reader, no good for this one. Fyi, nothing changes if I unplug the iac...

 

[NotMatt]

Also, it ramps up slowly, even if I start it up after its fully warmed up

 

[1982cj7]

The port sizes are different on the heads between the Renix engine and the 91 H.O. head so you might have a vacuum leak in the joints somewhere. also the o-2 position is different between the 2 manifolds, this might not affect it but would be worth checking o-2 operation.

 

[WA-Wheeler]

Sounds like an IAC to me.... generally if you unplug something it should make a difference...... but there could be an ECU problem ect. ect. If it was me i would start with replacing the IAC, or just throwing in one you know is good. Got any buddies with a 90-94 jeep??

Edit:

Didnt see the manifold switch. That might be an issue......not sure though. If it was me i would tear it apart again.

 

[NotMatt]

I've got another throttle body kicking around somewhere that came off a good running rig, so I'll start with swapping that IAC tomorrow. I did change the ECU out because the old one had one of the mounts broken off, so I swapped back to the original earlier tonight with no success.

I did unplug the O2 tonight just for giggles to see what it would do... didn't change a thing. The O2 is mounted in the manifold on the older ones, the newer HO motors have it mounted right below in the B-pipe That could be part of the issue, because the bung on the old manifolds is thicker and the O2 doesn't sit fully in the exhaust stream but recessed a little... doesn't seem like it would cause a problem, but who knows. I'll dig into the wires tomorrow and see what kind of numbers I'm getting out of it at full warm temp and compare.

Last resort I guess is to swap back to the other manifold to see if that fixes the problem... if it does, I have some work to do because the front part of my exhaust will have to be re-done... I used the old manifold because it dumps close to straight down, instead of back right at my driveline like the later ones... I ran the exhaust down and around the front of the oil pan with the old manifold, only way I could do it and clear the driveline because I'm running a narrowed 60 front.

 

[NotMatt]

Changed the IAC and intake air temp sensor tonight with known good ones. Don't have a spare coolant temp sensor so that will have to wait til tomorrow. No change. Also checked and cleaned the block ground at the dipstick tube bolt, and realized I hadn't put a block to frame ground on when I re-did the battery wiring... so I added that with no change. Also swapped the O2 sensor for a good one too, no change.

It's not storing any codes or throwing a CEL, something has to be fooling the air or temperature readings cause it ramps up so slowly once the ECM goes into closed loop.

I'll beat my head on the wall some more tomorrow and hopefully i can get it figured out soon.

 

[whatthe?]

You really need a scan tool. You need to know what the O2 mV are and the block learn value is. Whats your fuel pressure? I would think your IAC is fine, the computer is trying to fix something by having it open. Can you block the IAC port to see if it idles down? I'm going through the same thing with an aftermarket EFI swap onto my 454, idles high in closed loop. It was running lean (high block learn) so I'm having a new PROM made. A scan tool is really needed.

 

[thursday]

can you manually check the codes? you can weed out vacuum leaks via carb cleaner or starting fluid spray on the intake. IDK about jeeps but in older nissan pickups they used a wax iac valve and if you were low on coolant it would result in a hight/lopey idle....something to consider

 

[NotMatt]

Figured it out....

The intake was leaking near the back of the head, just enough to screw things all up. A little carb cleaner uncovered that. Just goes to show ya... don't overlook the simple things first!

I pulled it apart and cleaned everything and inspected the old gasket... there was a big chuck taken out of it where the leak was... guess I screwed it up during the install of the earlier manifold. Put a new one in and torque everything down and it runs great with no leaks.

Also, for future reference for anyone who is wondering... the early exhaust manifold works fine on the late head. The exhaust ports on the manifold flange themselves are identical, so whether the head ports are different or not, you're bolting the same thing up. The design of the manifold might not be as efficient as the later HO exhaust manifolds, but if you need the earlier one to better route around a front driveline, it works.

:awesomework: