Misfiring P0303 99 Corolla

[shawn99corolla]

Issue with 99 Corolla CE with 147,000 miles....I have a misfiring P0303 Code - Cylinder #3 Misfire. Here is what has been done in effort to remedy the misfire. A major tune up, new spark plugs, new fuel injectors, new spark plug wiring, new coil packs, The car runs ok once warmed up but in the morning the engine vibrates until warm and also shivers a little while the car is stopped at a stop light. I have dropped it off at the mechanics a couple of times now to no avail. Once the code is reset it will not display the CEL for couple of days but the symptoms still remains. The CEL comes back on within 2-3 days with same code. Really frustrating! Well at least this time CEL in not flashing like before it comes on and stays on. Any ideas what can still be causing the misfire. I really appreciate your help. Thank you in advance

[Brian R.]

P0303 Misfiring of cylinder 3 is detected during any particular 200 or 1,000 revolutions
For any particular 200 revolutions for engine, misfiring is detected which can cause catalyst overheating (This causes MIL to blink)

From the FSM: "The ECM uses the crankshaft position sensor and camshaft position sensor to monitor changes in the crankshaft rotation for each cylinder. The ECM counts the number of times the engine speed change rate indicates that misfire has occurred. And when the misfire rate equals or exceeds the count indicating that the engine condition has deteriorated, the MIL lights up. If the misfire rate is high enough and the driving conditions will cause catalyst overheating, the MIL blinks when misfiring occurs."


When the symptoms are predicted to be the worst, disconnect the spark plug connections to all cylinders one at a time. There should be a poor idle when this is done to good cylinders. See if disconnecting the spark to cylinder #3 has much less of an effect on idle as the others. Just to test that it is cylinder #3 is really the problem. It may be a random misfire.

Do a compression check at operating temperature

Check your valve clearances and cam lobes for wear in cylinder #3

Check the wheels which create pulses in the crankshaft position sensor and camshaft position sensor. There may be missing teeth or some kind of damage to the wheel which makes the sensor unresponsive to the condition in cylinder #3.