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2000 WJ Limited QD 4.7 Idle Issue

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3.7K views 13 replies 4 participants last post by  jagar  
J
#1 ·
New member from PA. First time Jeep owner. Not new to Mopar's. New to the Power Tech 4.7.

Got a really funky idle issue. First I have seen. When cold, it idles low but as it warms up, the idle starts to go up. Complete opposite from whats it suppose to do. When I first took ownership of the Jeep. When cold, it would idle at about 1000 to 1100. Not very steady but as soon the water temp gets warmer, the rpm's would go up gradually. Finally sitting at 1400-1450. Tested the TPS, checked out fine, even when lightly tapping the sensor. It does have 5v supply. I have no idea on how to check the IAC, since its controlled by 4 individual IAC drivers. I also tested the ECT and its getting the proper 5v supply.

Sensors I have replaced, that involves with the idle: ECT, CPS, TPS, MAP and IAC.

I understand you're "not suppose to" mess with the IAC pintle. Bought a new IAC, installed it. Now it idles at 1600, 1700. Long story short. I messed with the pintle and got it to idle lower. Still starts to idle higher as it gets warmer but not as bad when I first got it. It struggles to stay running (at 500 rpm) when cold but doesn't stall, even when you put it into gear. Since i had the battery disconnected, I decided to run it. I have put 40 miles since the last time i had the battery disconnected. I went to go get coffee, drove for a mile. Sat and left the Jeep running. Its idling at 600-650 after I messed with the pintle. As i'm sitting and letting the Jeep idle, the rpm's just gradually went up to 1400-ish and stayed there. Like I couldn't believe it.

I'll check the IAC. There is no vacuum leaks. TB is clean. I installed a new TB gasket. Temp needle, when warm, sits right at 210. I am not sure if that is normal. I know if the ECT picks up that the engine is getting too hot, the engine will idle higher but I honestly don't think its overheating. New fan clutch has been installed as well.

Any help/suggestions is appreciate it.
 
J
#4 ·
Had a P1294. Vacuum leak or IAC, high idle symptom. No vacuum leaks. Bought a IAC from AAP, followed their instructions. Idles too high, can't drive it that way. PCM doesn't close the IAC when the memory is cleared. Set off a P0505. Will be getting a IAC from Mopar. Will disconnect the battery. If no change, i'll be scraping the thing.
 
#5 ·
jagar said:
Had a P1294. Vacuum leak or IAC, high idle symptom. No vacuum leaks. Bought a IAC from AAP, followed their instructions. Idles too high, can't drive it that way. PCM doesn't close the IAC when the memory is cleared. Set off a P0505. Will be getting a IAC from Mopar. Will disconnect the battery. If no change, i'll be scraping the thing.
Click to expand...
when you say no vacuum leaks how did you prove it? By removing and caping every vacuum nipple (yes I said nipple) on the intake and throttle body, Or just spraying and looking around? Plenty on here eventually found the cracked vacuum line or failed whatever months down the road to solve there high idle.
 
J
#6 ·
Every vacuum port was blocked off. I honestly think theres a sensor thats screwing with the IAC drivers. I tried 3 different IAC's and when the PCM looses its memory, its suppose to move the pintle to the close position on Key ON. Its not doing that and its how I am getting my high idle.

I got fed up so I taped off the IAC port to where its idling reasonably so I can drive it. There's 4 wires going to the IAC 2 have 12v and the other is 5. Not sure if thats suppose to be like that.
 
#7 ·
The IAC is a stepper motor. The PCM knows how many steps it takes to do whatever, it's all based on fuzzy logic and adaptive learning. Look it up.

So, at a cold start the PCM knows to go X steps. A hot start is the same. (X steps). This is all based on the adaptive parameters it learn during normal driving. Killing the memory forces it to use the factory defaults for all the above settings. And depending on how worn the engine is depends on if those settings work or not. (Some report after unhooking the battery it will stall at a stop for a few times. ).

So a high idle, it knows to drive the stepper closed. And then tweak ignition advance to control the real time idle rpm. Cause that's what it does. Eventually you will get a code when the "Target idle" doesn't match real idle.

Too much air is getting into the engine. It could be a vacuum leak, a failing IAC, or even a dirty throttle plate preventing the plate from closing to its mechanical stop.

It's not out of the realm of possibility that the PCM is bad. But I've never heard or read anything with regards to a bad PCM causing a high idle...........
 
J
#8 ·
The PCM has been reset several times and per the FSM after being reset. Its suppose to move the pintle to the close position at key on. Its not doing that. Its in a "no load" mode and idles at 1800-2000 rpm. Its not recognizing its idling high, even after closed loop. I get no codes. There is no vacuum leaks. All ports were capped off. I've already ruled that out. I tried 3 different IAC's and none of them move after reconnecting the battery.
 
#13 ·
Have a similar issue on my 04 JGC Overland 4.7 V8 H.O.
Seems to idle fine at normal operating temp at 700 rpm
However once at a stop will dip to 500 and the engine seems to struggle with a slight shake.
This isn't every time but frequent enough
I am going to clean out the IAC and where the valve makes contact
 
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