High idle in gear only - JeepForum.com
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post #1 of 17 Old 03-28-2020, 10:57 PM Thread Starter
Wjar
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High idle in gear only

I have a 2002 grand Cherokee limited 4.7 4x4 it will only idle up to 1000rpms in any gear. Going down hill is the worse but going up hill it will pull itself down. When I come to a stop it will pull rpms down to normal. In neutral and park it’s perfect! I have fixed all the vacuum leaks no check engine lights. I took the throttle body off cleaned it all and even replaced the tps sensor. I put a new fuel filter on and The cruise control works everything seems great but it will idle up in gear. The Jeep dealer in town charged me for a diagnostics fee and said it was a bad purge valve I replaced it and it’s still the same. Some guy told me his high pressure switch on his ac made his Jeep act like that once? I’ll try that next but I am lost. Has anyone seen this or know what else to try? It’s only in gear so I don’t know what would tell it to idle up. When I come to a stop it will idle normal.

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post #2 of 17 Old 03-29-2020, 02:01 AM
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Sorry for long post.
I would ask if the jeep dealer mentioned any codes? As you know the CEL can be off and the system can still have codes. Maybe a scan to know for sure,

In N and park is ok, and "When I come to a stop it will idle normal." So when is idle an issue?

Yes the engine load caused by AC would cause the idle to rise but IIRC only if AC or defrost are selected and the compressor kicks on.

They mentioned PURGE valve, cheek the vac line to purge, be sure the valve is seating not passing vacuum to EVAP system.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 17 Old 03-30-2020, 12:41 AM Thread Starter
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They said there were no codes spent 30 minutes on it and said purge solenoid was bad. It idles perfect when I put it in gear and let off the brake going down hill it will stay idling at 1000rpms or if I’m on level ground it will stay at 1000 rpms until I come to a stop or until I put it in neutral. It does it after it warms up a little. Not so much when it’s cold. If I’m going up hill it won’t idle up at all like if there is a load on the motor. There are no other vacuum leaks I’ve fixed them all on evap and motor side cruise control works everything is perfect until I put it in gear. I can’t figure out what would tell it to idle up just by putting it in gear? It’s weird
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post #4 of 17 Old 04-02-2020, 04:44 PM
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Quote:
Originally Posted by Wjar View Post
They said there were no codes spent 30 minutes on it and said purge solenoid was bad. It idles perfect when I put it in gear and let off the brake going down hill it will stay idling at 1000rpms or if Iím on level ground it will stay at 1000 rpms until I come to a stop or until I put it in neutral. It does it after it warms up a little. Not so much when itís cold. If Iím going up hill it wonít idle up at all like if there is a load on the motor. There are no other vacuum leaks Iíve fixed them all on evap and motor side cruise control works everything is perfect until I put it in gear. I canít figure out what would tell it to idle up just by putting it in gear? Itís weird
Your issues are something I see regularly with faulty pcm drivers for the Idle Air Control stepper motor.

When the IAC drivers in the pcm (or at least one of the four) are failing it takes data from the tcm partition when you shift and falsely commands the IAC motor to ramp up or down out of spec.

If you happen to be local to me get in touch I can give you a loaner pcm to troubleshoot with.
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post #5 of 17 Old 04-03-2020, 06:32 PM Thread Starter
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I live in Arkansas and that sounds like that would be the problem honestly! Where should I get one at?
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post #6 of 17 Old 04-04-2020, 01:36 PM
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Quote:
Originally Posted by Wjar View Post
I live in Arkansas and that sounds like that would be the problem honestly! Where should I get one at?
Salvage yard is your best bet, I can program it for you to be plug and play ready when it comes back. I have 2 of those in back of the shop with bad valve seats that are headed to the scrap yard at some point if you can't locate one I can pull one of those for you just send me a PM if needed.
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post #7 of 17 Old 04-11-2020, 09:10 PM Thread Starter
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Would you happen to know how to test one or have a pin diagram I could read to trace my power? I was trail riding it today with my family and it just pulls so much it’s a pain to drive.
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post #8 of 17 Old 04-19-2020, 03:41 AM Thread Starter
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Bump?
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post #9 of 17 Old 04-19-2020, 02:58 PM
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Would you happen to know how to test one or have a pin diagram I could read to trace my power? I was trail riding it today with my family and it just pulls so much itís a pain to drive.
Where do you want to trace power to / from? I can look it up for you.
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post #10 of 17 Old 04-21-2020, 02:55 AM Thread Starter
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I need to know what in the transmission can give signal to idle up the iac. It doesn’t really do it on a cold motor but when it starts to get up to temp it will do it. So if you could tell me what would effect the iac motor in gear and voltage/ohms to those that would be awesome! When I unplugged the iac with it warmed up and in gear it was perfect no high idle.
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post #11 of 17 Old 04-21-2020, 02:41 PM
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@Wjar
It appears the IAC is controlled strictly by the PCM, based on PCM inputs during certain operating modes. The only connection I see to the transmission is Park/neutral switch (gear indicator signalóauto. trans. only).

A couple of Modes below:

IGNITION SWITCH (KEY-ON) MODE
This is an Open Loop mode. When the fuel system is activated by the ignition switch, the following actions occur:
- The PCM pre-positions the idle air control (IAC) motor.

ENGINE WARM-UP MODE
This is an Open Loop mode. During engine warmup, the PCM receives inputs from:
- Battery voltage
- Crankshaft position sensor
- Engine coolant temperature sensor
- Intake manifold air temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor signal
- Park/neutral switch (gear indicator signalóauto. trans. only)
- Air conditioning select signal (if equipped)
- Air conditioning request signal (if equipped)
Based on these inputs the following occurs:
The PCM adjusts engine idle speed through the idle air control (IAC) motor and adjusts ignition timing.

IDLE MODE
When the engine is at operating temperature, this is a Closed Loop mode. At idle speed, the PCM receives inputs from:
- Air conditioning select signal (if equipped)
- Air conditioning request signal (if equipped)
- Battery voltage
- Crankshaft position sensor
- Engine coolant temperature sensor
- Intake manifold air temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor signal
- Battery voltage
- Park/neutral switch (gear indicator signalóauto. trans. only)
- Oxygen sensors
Based on these inputs, the following occurs:
The PCM adjusts engine idle speed through the idle air control (IAC) motor.
Attached Files
File Type: pdf Idle Air Control.pdf (4.26 MB, 4 views)
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post #12 of 17 Old 04-21-2020, 05:14 PM
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Quote:
Originally Posted by Wjar View Post
I need to know what in the transmission can give signal to idle up the iac. It doesn’t really do it on a cold motor but when it starts to get up to temp it will do it. So if you could tell me what would effect the iac motor in gear and voltage/ohms to those that would be awesome! When I unplugged the iac with it warmed up and in gear it was perfect no high idle.
Transmission has nothing to do with the problem, aside from some neutral switch problems that can cause some drivability issues not specifically idle related.

As I said before your pcm drivers are probably bad, with everything cooled off unplug the connector to the IAC stepper motor and get your DVOM out.

Ground the negative lead, set the meter to Ohm's and probe each of the 4 IAC driver circuits on the connector end and record those.

Let the engine come up to temp, shut it down and repeat the test again lmk what you find.
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post #13 of 17 Old 04-23-2020, 02:13 PM Thread Starter
Wjar
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Nothing was showing any continuity or ohms on any wires doing the ohms test on cold or hot. Ohm meter works I used battery cable as ground.
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post #14 of 17 Old 04-23-2020, 03:55 PM
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Quote:
Originally Posted by Wjar View Post
Nothing was showing any continuity or ohms on any wires doing the ohms test on cold or hot. Ohm meter works I used battery cable as ground.
Best case you will always get something for ohms normal readings right at 116Ω, worst case you will see 100% continuity to ground in the rare event all four wires were shorted at some point.

You could remove the pcm and bench test directly to the pins on the module if you want to try that lmk and I will look up the pin locations on the module for you.
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post #15 of 17 Old 04-23-2020, 05:54 PM Thread Starter
Wjar
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I would like to try bench testing for sure if you could get me the information.
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