2000 Chevy 2.2L OHV Engine PO303 Code: No Compression in Cylinder #3, Possible Blown Head Gasket?
My son was driving the 2000 Chevrolet 2.2L OHV engine when he reported a popping sound during downshifting from 5th to 4th gear. The vehicle would stall, spit fuel, and fail to stay running. We've already replaced both oxygen sensors, the throttle position sensor (TPS), spark plugs, and ignition wires. Despite these repairs, the PO303 fault code persists. Testing shows no compression in cylinder #3 — we’ve checked the valves and found neither intake nor exhaust valves are bent. The rocker arms were loosened slightly to ensure they weren’t opening the valves improperly. We've also injected oil into cylinder #3 to test for compression, but results remain unchanged. No visible signs of oil in coolant or coolant in oil have been observed. The engine has not overheated during operation. Given this, could a blown head gasket be the root cause? I’ve heard that head gaskets can fail without showing obvious leaks — is this possible in this scenario? Any advice on how to proceed with diagnosis would be greatly appreciated.
Using an OBD2 scanner, you can review the freeze frame data associated with the PO303 code — this will show conditions like RPM, engine temperature, and vehicle load when the fault was set. This helps identify potential triggers. I'm not sure of your technical experience level, so consider checking compression in adjacent cylinders (cylinders #2 or #4) to rule out a broader issue. A compression leak-down test could be very helpful: inject compressed air (from shop air) into cylinder #3 and listen for leaks. This can pinpoint where compression is escaping — whether through valves, blow-by, or into adjacent cylinders. Let me know if you'd like step-by-step guidance on performing this test.