Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

97 Corolla misfire

check engine code P0303 - cylinder 3 misfire. I have replaced the plugs, wires, injector, and coil. I am still getting the code and frequent, but not constant, sputtering or chugging. What else can I check? Sometimes the light goes out and the car runs fine.

Check compression. Low compression on one cylinder could be a burned valve or broken ring.

Another thing that can cause this problem is excessive carbon deposit buildup on the backsides of the intake valves. When this happens, the carbon deposits can absorb some of the fuel that fuel injectors introduce like a sponge. This then causes a lean condition in a cylinder where it causes a misfire. You can take it in and have someone do a decarbonization of the engine to see if it eliminates the problem. Or, you can do this yourself.

Purchase a can of SeaFoam Engine Treatment. Get the engine up to operating temperature. With the engine off, disconnect the brake booster vacuum hose from the brake booster. Adapt a hose that will fit into the end of the brake booster hose and into the can of SeaFoam. Take a pair of pliers and pinch this hose off. Have someone start the engine and bring the speed up to 2,000 RPM’s. Slowly open the pliers so that the SeaFoam begins to be drawn into the engine. It’s here where the throttle must be manipulated and the pliers opened and closed to prevent the engine from stalling. Once all the SeaFoam has been drawn into the engine, shut the engine off and let it sit for a half hour. Reconnect the brake booster vacuum hose. After a half hour has past, restart the engine and bring the speed back up to 2,000 RPM’s until the smoke clears from the exhaust. Take it for a cruise and see if the miss is gone.


There can be other things which can adversely affect the air / fuel mixture, momentarily. The idle air control valve may be sticking, sometimes. The EVAP canister control valve, or the EGR Valve may be opening at the wrong time.
The problem is that nothing is likely to show up, during tests, when the engine is running good. Use a vacuum gauge and take some readings, when the engine is running good. When the engine is running bad, take some vacuum gauge readings. Compare. Here is a tutorial on what to look for during vacuum tests: Check a little green “Scenario” to see what the vacuum gauge should indicate for each condition.