have an 01 jeep cherokee with the I6. i had a cylinder 3 misfire code. so i replaced plugs coil pack the cylinder 3 injector and the heat soak shield thing. still throws po303 code. it runs really bad when hot and check engine light flashes. please help

 

First off, a "flashing" check engine light indicates a serious misfire and if it is not resolved immediately, it will trash your catalytic converter and then you'll have bigger fish to fry.

How long have you owned this 01? Are you having to add coolant to the coolant reservoir? The reason I'm asking is because the 00-01 have the 0331 cylinder head, which has a casting defect between cylinders 3-4. One of the main things you'll notice if your head cracks is that you use coolant (being burned out through the engine) which is very hard on engine internals like bearings.

Another thing you could do is to test compression on all cylinders and see where cylinder #3 tests out at compared to the other cylinders. The compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders. The compression test isn't usually all that helpful in troubleshooting a cracked head, but compression is a basic data point that needs to be ruled in or out.

Good luck and keep us updated!