I have to go get my daughters 2004 Jeep Grand Cherokee and fix the engine. She came home for Xmas and her eng. light was on so I hooked it to the scanner and it said PO301 cyl. #1 missfire so I put a plug in it although the one in it looked OK. It was also a 1/2 gal. low in coolant and I didn't see a leak. She went on home 4rs. away and all seemed OK. Now it is in a shop up there and they say it has a flashing PO303 cyl. #3 missfire. They tried a plug and coil and it did not fix it. They are telling me now it has a lot of valvetrain noise and only 90 psi. comp. on that cyl. Are these engines having problems? Broke spring or rocker? Blown gasket between 1 and 3? I know 5.2's and 5.9's like the back of my hand, but have never even had a valve cover off a 4.7. HELP!!!!