TJ high idle

[Nick66]

Good morning Jeep Peeps!

I’ve been struggling with my new LJ. 2005 LJ 4.0L 88k manual transmission.
I am experiencing a high idle, usually at 1500 RPMs. This isn’t my first Jeep and roughly 6th vehicle with the 4.0 (YJ, XJ, WJ, TJ, etc).
The majority of the time the LJ starts and idles right up to 1500 and never comes down warm or cold loaded or unloaded. Every now and then after I have messed with sensors and electrical connectors it will idle at 750, as soon as I do the happy dance and take it for a quick drive it goes right back to 1500 and stays there.
What I’ve done, replaced the IAC (mopar), cleaned throttle body and IAC port, replaced throttle body gasket, checked all vacuum ports for leaks and cracks- found a few (more on that later), currently waiting on replacement (mopar) TPS, MAP, and Air Charge Temp Sensor.
Vacuum- so I found a few cracked rubber fittings. I wanted to identify the fitting that may have caused the high idle while I waited on a mopar replacement rather than duct tape and zip ties (no offense lol!). I pulled each fitting and capped them off with the correct sized vacuum caps. PCV, A/C, emissions, brake booster, and I replaced the fitting to the MAP sensor with fresh vacuum hose temporarily. With all the lines pulled and capped it still idles at 1500.
This is where I’m stumped, high idle usually means IAC or vacuum leak but why am I still seeing no change after an IAC and no open vacuum ports?
Thanks for any help!

 

[CharlesHS]

There is a chance you have a vacuum leak between the head and the aluminum intake manifold. Spray water or WD-40 between the flange for the intake manifold and the head; see if the idle RPMs drop. Also check at the base of the throttle body and around the vacuum fittings.
This will indicate where the vacuum leak is or is not....

 

[williambmac]

Like @CharlesHS says, check for leaks between the head and intake manifold. Also check the throttle body itself. I have seen them leak at the throttle shaft itself which would require replacement of the throttle body.
Do you have access to a scanner? any codes? If you look at engine data what is the Throttle body % @ idle? fuel trims?
Have you tried disconnecting the battery and doing a global reset? Disconnect + and - cables and hold them together for about 30 seconds. Reconnect and see what happens.

 

[Nick66]

Thanks for the quick input! I think I’m going to work at torquing the intake manifold after my son’s baseball game today. Does anyone know the torque spec? I have a basic scanner, no codes. I have looked at live data during idle, the throttle position says 12.5-13.5% at idle. I don’t know which line item fuel trim is, or if my scanner reads that. I have done the computer reset, disconnect, touch positive to negative for 30 sec, key on, lights on then off, key off. If the throttle body shaft is leaking, will WD-40 be enough to identify that leak?
Again, thanks for the help!

 

[williambmac]

you can try wd-40 or brake clean and see if the idle changes.
the throttle cable isn't binding is it? Try disconnecting the throttle cable from the throttle body.
12.5-13.5% sounds about right.
I wouldn't go throwing parts at it, map, tps. etc.

 

[CharlesHS]

Does anyone know the torque spec?

23-24 Ft Lbs
bolts/bolts 1-5 and 8-11 are 24 ft lbs
nuts 6&7 are 23 ft lbs

 

[Jerry Bransford]

Thanks for the quick input! I think I’m going to work at torquing the intake manifold after my son’s baseball game today.

Unless your exhaust manifold and intake manifold have been removed for work I would not torque its nuts. If no work has been done that would have required their removal they are fine. That leaves either a vacuum leak or perhaps a dirty IAC (idle air controller) that is hanging up and letting more air in than it should. The IAC does NOT need to be removed to be cleaned, basically you just spray aerosol throttle body cleaner into the throttle body while it is at idle rpms. That draws the cleaner through the IAC and its orifice it fits into. Don't rev the engine or the cleaner will bypass the IAC and not clean it. The engine will die as the cleaner floods the IAC but that's good, it gives the cleaner more time in the IAC. Just restart the engine and do it again until 1/3 of the can or so has gone through the IAC via being sprayed into the throttle body.

Look for vacuum leaks too as described above.

 

[HickoryBoy]

So did you fix it?