p300 random miss fire
02-16-2012, 10:42 PM
#1
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Year: 1997
Model: Cherokee
Engine: 4.0
p300 random miss fire
Ok so I just got up to speed (65) and my check eng light comes on and starts flashing got to where I was going and scanned it for codes and got the 5 codes random miss fire and it said 4 cylinders to.... I just put plugs and cap and rotor on it a week ago and she ran pretty good all week. Don't seem like theres any vacation leaks. It has just about 198 k on her to and she is a 97 xj with the 4.0. Im a lil lost any help would be great
02-17-2012, 05:46 AM
#2
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Year: 1999
Model: Cherokee
Engine: 4.0
For a P0300, I would look for a major systemic problem, rather than anything specific to a cylinder.
The cause here is often something that upsets the engine's air/fuel mixture,
1. I usually start by performing a comprehensive search for a major vacuum leak.
2. Verify fuel pressure with a gauge; no shortcuts. For a 97, you should have 49 psi, plus or minus 5 psi.
3. The MAP sensor can also be tested with a meter and a manual; be sure the vacuum line leading to the MAP is pulling good vacuum.
BTW, a flashing check engine indicates a serious misfire, which you already know. But continued driving your XJ with that light flashing will result in you destroying your catalytic converter in pretty short order.
Good luck and keep us posted.
The cause here is often something that upsets the engine's air/fuel mixture,
1. I usually start by performing a comprehensive search for a major vacuum leak.
2. Verify fuel pressure with a gauge; no shortcuts. For a 97, you should have 49 psi, plus or minus 5 psi.
3. The MAP sensor can also be tested with a meter and a manual; be sure the vacuum line leading to the MAP is pulling good vacuum.
BTW, a flashing check engine indicates a serious misfire, which you already know. But continued driving your XJ with that light flashing will result in you destroying your catalytic converter in pretty short order.
Good luck and keep us posted.
02-17-2012, 12:41 PM
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Year: 1999
Model: Cherokee
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If it doesn't and it stays solid for one particular cylinder, I approach it in this order.
1. Spark plug
2. Spark plug wire
3. Distributor cap
4. Fuel injector
5. Internal condition of the misfiring cylinder
Good luck and keep us posted...
02-17-2012, 03:49 PM
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Year: 1997
Model: Cherokee
Engine: 4.0
Does anyone know where the compression should be at my number 2 is at about 55 60 and 1 and 3 is at about 100 to 125
02-17-2012, 05:45 PM
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Compression spec for the 4.0 is 120-150 with no more than a 30 psi variation between cylinders.
You should add a bit of motor oil to cylinder #2 and re-do it. If the compression comes up with the addition of the oil, then you most likely have a problem with rings. If it doesn't change with the oil, the primary suspect are the valves.
You should add a bit of motor oil to cylinder #2 and re-do it. If the compression comes up with the addition of the oil, then you most likely have a problem with rings. If it doesn't change with the oil, the primary suspect are the valves.
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03-02-2012, 05:59 PM
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Your compression test data indicates a problem internal to the engine.
Compression spec for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders.
You mention low numbers between cylinders 1 and 2. Could be a head gasket failure.
Compression spec for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders.
You mention low numbers between cylinders 1 and 2. Could be a head gasket failure.
03-02-2012, 06:11 PM
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Year: 1999
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Re-do your compression test on those two cylinders. If they are still low, add a tablespoon of oil and do it again. If the compression comes up, your rings are shot. If the compression remains low with oil, that cylinder head is going to have to come off.
03-03-2012, 07:53 AM
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Year: 1997
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Im thinking its time to just overhaul it! Now that i got my full cluster in the oil pressure is real low
