Cylinder three misfire, code P0303

PLAUERMA

MEMBER
2002 JEEP GRAND CHEROKEE
2.2L
V8
4WD
AUTOMATIC
229,000 MILES

I have had the P0303 cylinder three misfire code for at least six months. We repaired the smaller things such as the fuel injector and spark plugs. The car shakes and runs rough, the check engine light has been on. At times, I hear rattling (could be catalytic converter). There was also times where my car has shut off on me while accelerating (we replaced the battery). I had a cylinder compression test done and cylinder three is at 60 when it is supposed to be at 120. Currently, my check engine light has been flashing rather than staying intermittent. The light will flash until I start driving the vehicle and then will become intermittent again. I am assuming that this is the catalytic converter and that it is damaged. Could a bad catalytic converter be the reason or be related to why I have such poor pressure in cylinder three, or should I not get my hopes up and prepare to replace the engine/find a new vehicle?

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi and thanks for using 2CarPros.

The flashing check engine light indicates a misfire is occurring at that time. It may or may not be related to cylinder three. Now, the catalytic converter may have been damaged as a result of the cylinder three compression issue. However, the converter didn't cause the low compression. When pressure is that low, either you have a bad head gasket, burnt valve or worn compression rings. The only way to be sure is to do what is called a wet compression test.

Here is a link that shows how a compression test is done and a couple more that may be of interest to you:

https://www.2carpros.com/articles/how-to-test-engine-compression

https://www.2carpros.com/articles/the-reasons-for-low-compression

https://www.2carpros.com/articles/symptoms-of-low-compression

A wet test is this. First, compression is checked as discussed in the aforementioned link. In this case it is 60psi. Next, about a table spoon of motor oil is placed into the cylinder and then the test is done again. If there is a substantial increase in pressure, the compression rings are bad. If there is no change, then there is either a burnt valve or head gasket issue.

If there is no change, then you would need to check for a head gasket problem. Here is a link that shows how that is done:

https://www.2carpros.com/articles/head-gasket-blown-test

Once that is done, if you find no evidence of a bad gasket or compression rings, then you have to suspect a problem with the valves.

None of the things I mentioned, head gasket, rings, valve, are cheap or easy fixes. The engine has a lot miles on it, so there is a chance of any of the things I mentioned.

I hope this helps. Let me know if you have other questions.

Take care,
Joe