Originally Posted by scout
But I do not follow the AC belt could you explain the potential problem there.
This is probably unlikely in your situation, but still an issue with 4.7 HO and other engines so I thought I would list it.
The discharge reed valves in the A/C compressors are failing and causing the high side pressure to fluctuate rapidly. The compressor drive belt then has uneven loading on it and the idler pulley jumps up and down. This in turn is transmitted to the crankshaft and the crankshaft speed sensor sees an uneven crankshaft speed. The engine controller interprets this as an engine misfire. The check engine light is then turned on and a P-0300 Multiple Cylinder Misfire code is set. This condition only occurs when the A/C compressor clutch is engaged. The compressor must be replaced to repair the condition.
Here's one more scenerio:
Engine Idling, intermittent misfire, no trouble codes, spark plugs and injectors replaced.
Do a cylinder leakage test.
Listen to the throttle body, exhaust tail pipe and engine oil fill hole for air leaking from the cylinder being tested.
If air is heard at the tail pipe inspect the exhaust valves.
If are is heard at the throttle body inspect the intake valves.
If air is heard at the engine oil fill hole inspect the piston rings.
The valves must be closed in the cylinder being tested. Before charging the cylinder with compressed air, place the cylinder being tested at compression TDC or remove the rocker arms for the cylinder being tested.