Jeep WJ, Slow, hunting idle and intermittent hard starting.

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Apr 1, 2020
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My daughter's Jeep WJ with the 4.0 has started idling poorly and once in a while it will be hard to start. The idle is just under 500 rpms in gear and surges up and down slightly. It feels like it wants to stall and the IAC valve gives it a goose so it doesn't die. Occasionally it will stall. It doesn't seem to have a miss and no codes are present. Fuel pressure is normal and plugs are fresh and look normal. Long and short term fuel trims look excellent at zero and minus 3% at idle.

The one odd thing I noticed was that ODBC reports 12.4 to 12.5 volts but I measure high thirteens at the battery terminals. I plan to clean the body grounds but visually they look fine.

What say the Jeep gurus?
 
When mine acts like that, the battery is dieing. Voltage doesn't play into it much, somehow a bad battery throws it off. The 500 RPM idle is always the telltale for me
 
What’s throttle position read? Do you have data? They like to fail.

You can set the timing on them as well, they fall out of timing slowly, we used to always set it while doing plugs.

Some had heat soak problems when the intake / exhaust manifolds would heat up the injectors and they wouldn’t function properly.
 
Does the problem persist while in neutral or only in drive after warm up. Incorrect ATF level can cause this issue, or at least that was the culprit on my daughters ZJ, which is the same vehicle.

One common issue with these is the PCM.

Good thing is parts are cheap and plentiful.
 
Does the problem persist while in neutral or only in drive after warm up. Incorrect ATF level can cause this issue, or at least that was the culprit on my daughters ZJ, which is the same vehicle.

One common issue with these is the PCM.

Good thing is parts are cheap and plentiful.
The idle speed is low in neutral, too, cod and hot doesn't seem to matter.
 
What’s throttle position read? Do you have data? They like to fail.

You can set the timing on them as well, they fall out of timing slowly, we used to always set it while doing plugs.

Some had heat soak problems when the intake / exhaust manifolds would heat up the injectors and they wouldn’t function properly.
Thanks, I'll try to test the TPS when I clean the throttle body. I've never had one go bad before.
 
I'm in the middle of pulling of the throttle body for a thorough cleaning. Who else hates Chrysler weather pack connectors? I need a bandaid.
 
The throttle body has an idle air control valve, IACV. It’s a motor that spins a mushroom-shaped pin to allow an adjustable amount of throttle bypass air by. These crud up and get sluggish. Common cleaning item of these as they rack up some miles. BITOG went through a wave of these 10 years ago.
 
The throttle body has an idle air control valve, IACV. It’s a motor that spins a mushroom-shaped pin to allow an adjustable amount of throttle bypass air by. These crud up and get sluggish. Common cleaning item of these as they rack up some miles. BITOG went through a wave of these 10 years ago.
Yeah the IAC valve was pretty dirty. I cleaned it and the channel it travels through with carb cleaner and a tooth brush.

The rest of the valve body was dirty, too, but not to the point of affecting the throttle operation that I could tell. I wondered if maybe the MAP sensor passage could be gunked up, but it was clear.

I dug out an old analog multimeter and tested the TPS, too. It's resistance was nice and smooth throughout it's range and was right on for spec top and bottom.

I'm waiting for a break in the rain to go reinstall the throttle body since I'm doing it in the driveway.
 
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