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autoracer, ASE Certified Technician
Category: Jeep
Satisfied Customers: 60
Experience:  dodge,chrysler,jeep specialist
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2002 Jeep Wrangler sport 4.0L 6 cyl. I have a code P0301.

Customer Question

2002 Jeep Wrangler sport 4.0L 6 cyl. I have a code P0301. Cylinder #1 misfire. I replaced the spark plug just to check and nothing changed. (I replaced all 6 of them almost 2 months ago) I can't do a compression test because the of the coil rail ($125). (no plug wires) What can I do? What would cause this and how can I fix it? I also have a code p0122 thats throttle position sensor A circut low. Should I just replace the sensor $75 sensor or is it something else?
Submitted: 9 years ago.
Category: Jeep
Expert:  Amedee replied 9 years ago.



You may need to replace the tps sensor, however, lets take care of one problem at a time. Seeing that the misfire is causing the engine to run bad, lets start with that.


Lets be sure compression is up to par. To test compression you will have to get a compression gage and pull that spark plug back out again to screw in the compression tester. Be sure the injector is good and the signal to the injector is good. The signal to the injector can be test via noid light while testing the injector by a ohm meter will only give you electrical specifications. You might need to pull the fuel rail up and watch to make sure the injector is spraying correctly when the engine is running.


If all checks out good, I would replace the coil pack assembly. This could cause your misfire. Just be sure there are no vacuum leaks and or any other mechanical problem with your motor.

Customer: replied 9 years ago.
So basically I can test the compression even though I won't have any plug wires on? Remember the coil pack assembly has the boots built right in, and its not flexible, so either all of the plug boots are on or off. I was under the impression that to do a compression test, you had to pull out one plug at a time but leave all of the other ones on.
Expert:  Amedee replied 9 years ago.



The right way to do a compression test is to pull all spark plugs out (obviously this includes the spark plug wires, but in your case it will be the coil pack assembly.) and screw the compression tester in on cylinder at a time and just crank the engine over and take your reading. Oh... and don't forget to hold the throttle wide open.

Customer: replied 9 years ago.
whats normal compression?
Expert:  Amedee replied 9 years ago.

Compression Pressure Range 827 to 1,034 kPa (120 to 150 psi)

Max. Variation Between Cylinders 206 kPa (30 psi)

Customer: replied 9 years ago.
cyl 1 was 65-70 and the rest were 90 and then I tested cyl 1 again and it was 90. Put plugs in again and fired up and I got max oil pressure and a strange noise. then I turned it off and started again and its seeming normal.
Expert:  Amedee replied 9 years ago.

Wow! Compression seems extremely low!


Are you sure your gauge is working correctly? You might want to try a different one.

Also, you can try squirting some oil down the cylinders to seal them up. If gas has washed the cylinders down causing low compression, then the compression should come back up when you do this. If compression does not come back up and your gauge is working correctly (I would try it on another vehicle) than I would be looking at a mechanical issue with your engine.


WHat is engine vacuum when idling?



Expert:  autoracer replied 9 years ago.
a very common problem for the newer 4.0s is carbon builup on the valves causing them not to spin like they are suppossed to.i have worked at a jeep dealer for 8 years and seen this problem numerous times.the only way to check and see if this is your problem is to remove the valve cover and rocker arms on cylinder 1.look at the tops of the valves and see if there is a bullseye mark or straight lines.if there is straight lines go to a jeep dealer and get combustion chamber engine till hot and then empty both cans into throttle body .let soak with hood shut for and hour then drive jeep very hard(run engine above 3500 rpms for 15 seconds and then idle for 15 seconds several times)you may have to do this a couple of times but it should fix your problem.let me know how this works for you