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Rough idle and running after warm up.

Tiny
GCV8
  • MEMBER
  • 2000 JEEP CHEROKEE
  • 105,000 MILES
Hi,
I've got a Jeep Grand Cherokee 4.7 v8 that has suddenly developed a problem that makes the Jeep idle rough and run bad like it is missing or starved of fuel or something.

On a cold start, where the car has been sat for some time the car will crank and start straight away without any problem. It will rev up on start and settle down as normal. You can rev the engine hard and it is smooth as it should be and you can drive it as normal.

But, (and here is the problem) once the engine starts to warm up a bit and the temperature needle starts to climb (gauge says 40) the revs suddenly drop off to around 500 from around 800 and the engine starts to tick over rough and run rough. Revving the engine hard doesn't clear it.

Once the engine has cooled, if I start it again it will do the same thing, run well until it started to warm and then run rough.

I've changed the air filter to a K&N one, I've changed the spark plugs (although the old ones looked clean and in good order), I've taken the throttle body off and cleaned it out, I've removed and cleaned the Idle air motor and throttle position sensor.

Any ideas?

kind regards,
Gary.
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Wednesday, October 10th, 2012 AT 7:59 PM

9 Replies

Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
The idling speed is too low causing the problem. Cleaning the IAC and throttle body should have resolved the problem of low idling unless the IAC is not functioning correctly.

The ECT is another possible cause of the erratic idling as it is temperature related. Did you clean the MAF?
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Wednesday, October 10th, 2012 AT 8:45 PM
Tiny
GCV8
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Hi,
Ok I'll have a look at the ECT (Engine coolant Temp?) Sensor. I cant find a MAF, googled it and some say the Grand cherokee v8 doesnt have one?
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Wednesday, October 10th, 2012 AT 9:01 PM
Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
Sorry I did not check that out. Did you check for trouble codes?
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Wednesday, October 10th, 2012 AT 9:45 PM
Tiny
GCV8
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Just checked out the error codes (although I've had no check engine light on the dash). Had a whole bunch of codes.
P0201 through to p0208 (injectors 1 to 8)
P0123 (throttle position sensor)
P0152 (O2 Sensor Circuit High Voltage)
P0505 (Idle air control).

I take it (hope) one problem is causing the error codes of the others?
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Wednesday, October 10th, 2012 AT 10:18 PM
Tiny
KHLOW2008
  • MECHANIC
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The codes could have been set when you cleaned the throttle and IAC etc. Clear the codes and go for a test drive for the computer to relearn and see if the problem is resolved.
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Wednesday, October 10th, 2012 AT 10:26 PM
Tiny
GCV8
  • MEMBER
  • 5 POSTS
Hi, just finished a 12 hour shift and thought I'd pull the negative lead off the battery for a while to kill off the codes. Reconnected the battery and checked for codes and they were cleared. I started the engine (which has been sat cold all day), started fine and idled fine at 750 revs. Then again as soon as the temperature his 40 something happens and the revs drops off to 500 for a short while and then picks back up to around 750 but the car is a bit rough and the rev needle jumps around a bit. I didn't have time to take it for a drive, as soon as it started to play up I stopped the engine and checked for codes.
I guess I didnt give it long enough to check all sensors as I only had these this time.
P0202, P0203, P0204, P0206 and P0208. (Injectors)
Although it feels rough the engine will rev up all be it a bit lumpy and will hold a good rev if I rev to say 3000 it will sit there no problem.
Going to be a bit hard finding time to check things out for the next couple of days as Im on shift but will start afresh Sunday.

Regards,
Gary.
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Thursday, October 11th, 2012 AT 8:19 PM
Tiny
KHLOW2008
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First thing you need to check would be the injector wireharness connectors. Ensure they are secure.

Next on list would be the injectors themselves. Ohm them out for continuity and manually test them with battery voltage (monemtarily)to see if they click.
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Thursday, October 11th, 2012 AT 8:27 PM
Tiny
GCV8
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Hi,
Managed to take it out for a quick drive after work today, from cold the revs and drive is smooth. But as always, as soon as the temperature gauge creeps up to the 40 mark the engine does something, (I guess the fuel is adjusted back to normal from the rich cold start mix?) And then what ever looks after the fuel mix is having a hard time trying to manage it. Someone at work asked me if I've noticed any problems shifting gear and it does hold a gear a bit longer than normal and give a bit of a harsh shift. He said it could be the o2 sensor is bad and cant adjust fuel strategy thus giving the rough running.
I gave the injector connectors a push to make sure they are secure then took it for the short drive, after I went through the error codes and I have all 8 injector codes, no o2 or tps this time. I cant believe all 8 would go bad at once, it has to be something that works out the fuel mix.
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Friday, October 12th, 2012 AT 7:45 PM
Tiny
KHLOW2008
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All eight showing misfiring would be easier to handle than one or 2.

This can mean you have fuel starvation or excessively rich fuel supply. It could be due to vacuum leakages or clogged air intake system.
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Friday, October 12th, 2012 AT 8:03 PM

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