Very high idle

[hear]

Looking at a friends '99. It is idling very high, like 2500rpm. No codes, PCM is brand new from Wranglerfix. Taking the air cleaner off changes nothing, but mostly covering up the IAC port in the throttle body will bring the idle back down to normal. I can 100% control the idle speed by moving my thumb over the port in various amounts. Fully covering it kills the engine.

It seems like obviously there is an IAC problem, but we've tried a known good IAC and gotten the same result. No IAC codes either. What could be wrong that a functioning IAC is making it idle super high, but partially covering the port makes it right? If there was a leak after, covering the port wouldn't make it die. I'm pretty sure. This is a recent phenomenon, and the jeep has not been driven very much over the past 4-6 months.

 

[Gilaguy23]

Did you try the disconnect the battery for hours bit?

 

[Jerry Bransford]

Follow the wiring from the IAC back through the engine compartment and look for insulation that is damaged/worn through.

 

[hear]

Did you try the disconnect the battery for hours bit?

The battery had been disconnected for some time prior to this.

Follow the wiring from the IAC back through the engine compartment and look for insulation that is damaged/worn through.

The entire engine bay harness has been out of the jeep recently, inspected, repaired, & re-loomed. I know it's not foolproof, but I would have expected that sort of thing to induce an IAC code.

 

[Jeepster78]

I had a similar issue with my daughters Jeep went through 2 different brands of IAC and a map and I finally found that the intake manifold had a small leak. You can try spraying carb cleaner around the intake really good and if you get any type of sputter/idle change you will need to check for a cracked intake and/or get another intake gasket. For some reason my daughters Jeep the intake came unseated very slightly causing a high idle. If they took the intake off they could have improperly seated the intake causing a small air leak.

 

[hear]

That was on his list of stuff to do anyway, but what confuses me is that if there is enough air getting through a bad gasket or intake crack, why is it that covering up the idle air port is enough to choke it to the point it dies?

 

[Zorba]

Make sure the rear CCV elbow is the restricted type.

 

[hear]

Make sure the rear CCV elbow is the restricted type.

Every time I think I know a lot about TJ’s I’m knocked back down a peg. Got a pic of what you’re talking about and why restricted matters and anything else relevant on the topic?

 

[Zorba]

Every time I think I know a lot about TJ’s I’m knocked back down a peg. Got a pic of what you’re talking about and why restricted matters and anything else relevant on the topic?

There are numerous threads here.

 

[freedom_in_4low]

Every time I think I know a lot about TJ’s I’m knocked back down a peg. Got a pic of what you’re talking about and why restricted matters and anything else relevant on the topic?

I don't have any photos, but if you pull the elbows and look inside, one is wide open through the elbow, and one has a tiny orifice to regulate the rate of airflow. If you have two wide open elbows there will be a large amount of airflow pulling through the crankcase vent system (intake tube -> valve cover -> intake manifold. It bypasses the throttle body and IAC entirely.

I'm skeptical that you'd be able to stall it covering the IAC port if this was the issue but it's worth a look if these have been replaced or messed with since the last time it ran correctly.

 

[hear]

I don't have any photos, but if you pull the elbows and look inside, one is wide open through the elbow, and one has a tiny orifice to regulate the rate of airflow. If you have two wide open elbows there will be a large amount of airflow pulling through the crankcase vent system (intake tube -> valve cover -> intake manifold. It bypasses the throttle body and IAC entirely.

I'm skeptical that you'd be able to stall it covering the IAC port if this was the issue but it's worth a look if these have been replaced or messed with since the last time it ran correctly.

We helped a friend put a new 4.0 into their daughter's jeep, we ended up scavenging a few things off his jeep, including those elbows. Scientifically, I'm not sure how it could be the problem given the data, but if I knew everything I would never have to ask.