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Old 05-19-2014, 09:16 PM
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Mystery Misfire

2002 jeep grand cherokee 4.7.

Runs rough - bad mileage, small vibration in engine during idle and acceleration, overheats on occasion, occasional gas smell and smoke from exhaust pipe, ticking sound.

Most recent code: P0307 - misfire on cylinder number 7, possible fault: spark plugs, ignition wire, coil, vacuum leak, fuel injector, high or low fuel pressure

Work and diagnostics already attempted:

replaced and gapped all eight spark plugs.

Checked all eight coils for good spark with spark gap tool - all sparked.

Pulled wires to fuel injectors one by one to see if one was bad. Engine ran worse on each one, ergo nothing is wrong. (If it was the injector, engine performance would not change when disconnected.)


Possibly unrelated recent repairs:

Replaced battery.

Removed, dismantled, cleaned, filed off corrosion to bare metal, reassembled and reinstalled battery connectors to wiring harness.

Replaced oil, oil filter, air filter, topped off coolant, replaced thermostat



Other issues possibly in play:

Starter on its way out.

Engine sometimes overheats - needs topping off regularly

Fuel pump and fuel filter may need replacing. (Planned as part of tune - up)

Vacuum return hose to air box was discovered completely disconnected and lying on top of the engine block. Ran like this for at least a week or so. Derp



I'm out of diagnostic tests to try and track down why I'm getting a misfire. But here's what I'm considering doing next anyway:

vacuum test

Fuel pressure test

Pulling the valve covers to inspect valves


Any further suggestions would be most appreciated. Kudos


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  #2  
Old 05-19-2014, 09:24 PM
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Re: Mystery Misfire

Welcome to the garage.
When you changed the plugs, did you see anything different to plug #7? (maybe coolant going into this cylinder)
Do you have a scanner with live data, so that you can check the upstream O2 sensors. (eventhough i would expect a P0300 fault in that case, from a random misfire)
Take care not to overheat this engine, because it will warp the heads.
Also fallen of rocker arms are known on this engine. Could be due to sticking valves. Sometimes a valve seat drops and can cause all kinds of internal damage. Wont expect all these last possibilities, because other wise removing the injector connector of that respective cylinder would make no difference.
Removing the valve covers could at least show you how healthy the engine is. Sludge formation is also a known problem in these engines.
You could try to put some seafoam in the fuel and oil and see if that makes any difference. Sticking valves, caused by carbon formation on the valve stems, could be cleaned that way.
Also the fuel pressure is a good one to check, as you already mentioned. Or just change the fuel filter/pressure regulator and try.
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Old 06-07-2014, 01:06 PM
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Unhappy Re: Mystery Misfire

Quote:
Originally Posted by UrbanWoodsman81 View Post
2002 jeep grand cherokee 4.7.

Runs rough - bad mileage, small vibration in engine during idle and acceleration, overheats on occasion, occasional gas smell and smoke from exhaust pipe, ticking sound.

Most recent code: P0307 - misfire on cylinder number 7, possible fault: spark plugs, ignition wire, coil, vacuum leak, fuel injector, high or low fuel pressure

Work and diagnostics already attempted:

replaced and gapped all eight spark plugs.

Checked all eight coils for good spark with spark gap tool - all sparked.

Pulled wires to fuel injectors one by one to see if one was bad. Engine ran worse on each one, ergo nothing is wrong. (If it was the injector, engine performance would not change when disconnected.)


Possibly unrelated recent repairs:

Replaced battery.

Removed, dismantled, cleaned, filed off corrosion to bare metal, reassembled and reinstalled battery connectors to wiring harness.

Replaced oil, oil filter, air filter, topped off coolant, replaced thermostat



Other issues possibly in play:

Starter on its way out.

Engine sometimes overheats - needs topping off regularly

Fuel pump and fuel filter may need replacing. (Planned as part of tune - up)

Vacuum return hose to air box was discovered completely disconnected and lying on top of the engine block. Ran like this for at least a week or so. Derp



I'm out of diagnostic tests to try and track down why I'm getting a misfire. But here's what I'm considering doing next anyway:

vacuum test

Fuel pressure test

Pulling the valve covers to inspect valves


Any further suggestions would be most appreciated. Kudos


Sent from my iPhone using JeepGarage


I have a 2005 Jeep Liberty 3.7L V6 Rocky Mountain Edition with the very same issue, only my cylinder in question is #2 (P0302 AND P0300 codes are coming up)


I've put in $$ for replacing all 6 spark plugs (plug on coil, so no wires), had the coils from #2 and #3 swapped, and the same codes came back. I took the Jeep back in to the garage, when they swapped the fuel injectors for #2 and #3 again. After both swaps, they cleared my code, Jeep ran for a couple hundred miles before the light and codes came back. During this time, of course while I was obsessing over every sound and vibration, my Jeep seemed to run fine...initial start up was much smoother, however over time I began to just wait for the light to come back on. My engine would feel like it was stumbling again.

Let me back up a little here, and give some more history. In January or so, possibly February, once I jumped on the highway (after only a short in-town drive from starting point) the Jeep would go for about a mile or so and I would get vibrations from the front. It went in to have the tires balanced and rotated, just in case this was in, and also have the alignment inspected - alignment was fine, and the problem returned after the tire work. (Treads are all evenly worn as well, no cupping or damage there). After the vibrations came back, it went into my mechanic, who could find no fault, and it was chalked up to possibly ice or mud in the wheel well (I live in Maine), where it was sporadic and cannot be duplicated.

Then, in March, my check engine light finally came on. Keep in mind, I've also been driving this vehicle to/from work 5 days a week, at about 90 miles a day round trip. Took it in to get a read, and got the codes P0300 and P0302. Brought to my mechanic, where we decided to have all spark plugs replaced (originals from bought new 2005, it's about time to have that done anyways), code reset...seemed to be ok. Drove for a while, light came back, same codes, brought back in, had the coils swapped, mechanic drove vehicle home and code came back up after startup in the morning. Mechanics swapped fuel injector, reset code, drove home again, and the next morning it seemed to be fine, so I picked it up. Drove again 5 days a week for about 3 weeks, and the light came back on. During the three weeks, my Jeep would get the rumbles and stumbles again here or there, but without a code, I wasn't sure what to bring it back in for, other than more time and $$ in labor. Code finally came back, still the same - nothing jumped to the #3 cylinder. When code came back, my Jeep ran rougher for longer than it had been - normally after warming up some it works itself out. But through three start ups and drives, it was rough.

Two days later of the Jeep sitting in my garage, I took the Jeep out around the driveway to get it washed and cleaned, and it ran even rougher (to the point I'm thinking to get it anywhere I'm going to have it towed), my engine light is now flashing, the exhaust is worse (it smells and sounds like an old truck). And I don't want to suggest it's the catalytic converter, but if that, too, is now breaking down, I did have a subtle sulfur smell in the exhaust as well this time.

Jeep is still sitting in my garage as I debate putting more money into it as I try to figure out exactly how to diagnose it (has roughly 96,000 miles on it, body is in great shape (no rust) and everything else is still working just fine), or just getting a new vehicle if this is going to start costing me more than I'll ever get out of it...

Any help and suggestions would be greatly appreciated. Thanks! Also, my Jeep is serviced at a Dodge/Chrysler/Jeep certified garage.
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Old 06-09-2014, 09:33 PM
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Re: Mystery Misfire

Welcome to the garage.
The flashing check engine light indicates that a situation is present which will harm the catalytic converter. Unburned fuel comes out of one or more of the cylinders and will burn of in the catalytic converter, melting down its internals. Do not continue driving with the light flashing, because then for sure you will have to change the converter.
Are you loosing coolant as well?
The intermittent caracter of the complaint seems to be pointing towards wiring problem or a problem internally to the PCM.
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JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.

VW Beetle 2.0 TSI 2013
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Old 06-22-2014, 04:38 PM
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Re: Mystery Misfire

Quote:
Originally Posted by Frango100 View Post
Welcome to the garage.
The flashing check engine light indicates that a situation is present which will harm the catalytic converter. Unburned fuel comes out of one or more of the cylinders and will burn of in the catalytic converter, melting down its internals. Do not continue driving with the light flashing, because then for sure you will have to change the converter.
Are you loosing coolant as well?
The intermittent caracter of the complaint seems to be pointing towards wiring problem or a problem internally to the PCM.

Thank you for the information. As far as I can tell, I don't have any fluids leaking. Since my light started flashing, my Jeep has sat in my garage while I debate if it's time to get rid of it , or just keep investing time and money to try to figure out the problem.
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Old 06-22-2014, 07:27 PM
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Re: Mystery Misfire

Quote:
Originally Posted by UrbanWoodsman81 View Post
2002 jeep grand cherokee 4.7.

Runs rough - bad mileage, small vibration in engine during idle and acceleration, overheats on occasion, occasional gas smell and smoke from exhaust pipe, ticking sound.

Most recent code: P0307 - misfire on cylinder number 7, possible fault: spark plugs, ignition wire, coil, vacuum leak, fuel injector, high or low fuel pressure

Work and diagnostics already attempted:

replaced and gapped all eight spark plugs.

Checked all eight coils for good spark with spark gap tool - all sparked.

Pulled wires to fuel injectors one by one to see if one was bad. Engine ran worse on each one, ergo nothing is wrong. (If it was the injector, engine performance would not change when disconnected.)


Possibly unrelated recent repairs:

Replaced battery.

Removed, dismantled, cleaned, filed off corrosion to bare metal, reassembled and reinstalled battery connectors to wiring harness.

Replaced oil, oil filter, air filter, topped off coolant, replaced thermostat



Other issues possibly in play:

Starter on its way out.

Engine sometimes overheats - needs topping off regularly

Fuel pump and fuel filter may need replacing. (Planned as part of tune - up)

Vacuum return hose to air box was discovered completely disconnected and lying on top of the engine block. Ran like this for at least a week or so. Derp



I'm out of diagnostic tests to try and track down why I'm getting a misfire. But here's what I'm considering doing next anyway:

vacuum test

Fuel pressure test

Pulling the valve covers to inspect valves


Any further suggestions would be most appreciated. Kudos


Sent from my iPhone using JeepGarage
Have you solved your problem with your Jeep?
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  #7  
Old 06-24-2014, 09:42 AM
Frango100's Avatar
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My Jeep: 2004 4.7L WJ
 
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Posts: 5,207
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Re: Mystery Misfire

Did they already check the fueltrim figures and the O2 sensor volts? A bad O2 sensor can cause a too lean or too rich mixture, resulting in misfires.
If you have an android phone, you could install the free app Torque and just buy a cheap bluetooth dongle . You can then monitor many parameters, check fault codes and reset them yourself.
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Frank

JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.

VW Beetle 2.0 TSI 2013
My new toy
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Old 09-02-2014, 10:55 AM
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Thumbs up Re: Mystery Misfire

Quote:
Originally Posted by Frango100 View Post
Did they already check the fueltrim figures and the O2 sensor volts? A bad O2 sensor can cause a too lean or too rich mixture, resulting in misfires.
If you have an android phone, you could install the free app Torque and just buy a cheap bluetooth dongle . You can then monitor many parameters, check fault codes and reset them yourself.


We had another garage take a look at it, and they found a cracked head gasket. (After all that mystery, time and money in diagnostics...I had a "bang head here" moment ) THEY found the coolant, I think it was, leaking into the cylinder. We ran a radiator sealant through and it seems with have solved the problem.
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