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Unread 09-05-2009, 09:37 AM   #1
skinnyjl
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4.0L Rough idle/misfire(?) when engine is warm...

I have a 2004 Grand Cherokee with a 4.0L engine and 99k on the clock. The check engine light has come on and I get an intermittent rough idle (500-600rpm) and misfiring/poor performance at highway speeds and when I put a load on the engine (going up a steep grade, ect..) This seems to happen when the engine is at operating temp (~180-200F). Sometimes the engine light will flash when the engine is running poorly. I have put new plugs and a new coil pack in and my mechanic tracked down 3 vacuum leaks, but the problem still persists. The kicker is, engine runs fine before it gets to operating temp. I really don't want to go the dealership route, as the $100 per hour labor rate is hard on my limited income.

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Unread 09-05-2009, 10:14 AM   #2
SouthSeaPirate
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Flashing engine light is usually a missfire indicator. And if Im not mistaken, the amount of flashes will inticate what cyl it was.

When your running cold your ride is in sort of a limp mode. It wont use such things like O2 sensors and a few other sensors until its warmed up. Could be something along those lines.

An 'early' educated guess would be TPS, CPS or PCM. I would also check the vacuum line to the MAP sensor. But to early to tell anything just yet. So get the codes pulled asap.

Also check the oil for water and water for oil. Any smoke coming out of the tail pipe? I would check these first. Then MAP vacuum and then get the codes pulled.
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Unread 09-05-2009, 10:18 AM   #3
skinnyjl
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No smoke or water and oil in the wrong places. Just the occasional whiff of unburnt gas.
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Unread 09-05-2009, 10:21 AM   #4
SouthSeaPirate
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Good thing, get those codes pulled and back to us.
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Unread 09-05-2009, 12:07 PM   #5
skinnyjl
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I was able to pull the codes using the "key method" and got the following:

P0202 - Injector Circuit Malfunction - Cylinder 2
P0302 - Cylinder 2 Misfire Detected
P0442 - Evaporative Emission Control System Leak Detected (small leak)

Last edited by skinnyjl; 09-05-2009 at 02:25 PM..
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Unread 09-05-2009, 12:41 PM   #6
4Slow9erHybrid
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Quote:
Originally Posted by skinnyjl View Post
I have a 2004 Grand Cherokee with a 4.0L engine and 99k on the clock. The check engine light has come on and I get an intermittent rough idle (500-600rpm) and misfiring/poor performance at highway speeds and when I put a load on the engine (going up a steep grade, ect..) This seems to happen when the engine is at operating temp (~180-200F). Sometimes the engine light will flash when the engine is running poorly. I have put new plugs and a new coil pack in and my mechanic tracked down 3 vacuum leaks, but the problem still persists. The kicker is, engine runs fine before it gets to operating temp. I really don't want to go the dealership route, as the $100 per hour labor rate is hard on my limited income.
Disconnect the upstream O2 sensor. If the problem goes away, you diagnosed a failing O2 sensor. But remember this is just a test, don't drive like this for prolonged time. Replace with genuine Mopar O2 sensor ONLY!!! This is quite a common issue with the 4.0 Grands.
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Unread 09-05-2009, 06:25 PM   #7
skinnyjl
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I pulled the plug on the O2 sensor and it didn't seem to change much.
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Unread 09-06-2009, 07:56 AM   #8
20jeepwj00
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If you have an injector circuit code it could be an injector,when the injector gets hot it could be failing,when the problem is occuring pull off the #2 injector connector and install a noid light an if its flashing the wiring and the controller are ok and its probably a bad injector.
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Unread 09-06-2009, 09:02 PM   #9
skinnyjl
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B..u..m..p
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Unread 09-08-2009, 05:51 PM   #10
JonM81
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Oh my... I may have a bad injector... Got the same thing with all the O2 sensors replaced... I was hoping it was the spark plugs... eeek
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Unread 09-19-2009, 02:01 PM   #11
skinnyjl
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Just a follow up...

After my mechanic decided I had a bad computer, I wasn't convinced. So, I gave in and took it to the $tealer$hip. They determined I had a short in the engine harness that was causing the misfire. I picked it up yesterday and all seems well so far. No CEL or codes and it seems to be running well. I just hate the fact that it cost me $506 for a tech to put shrink insulation around a bare wire. Oh well... I knew I was going to get screwed when the service rep said that it was a 5 hour job before they even did the work. It could have taken them 5 hours or it could have taken them 45 minutes. I'll never know...
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