QuestionI have a 1999 4.3 Vortec Chevy Astro van with 80K miles that's reported P0300 multiple misfires a few times over the last three years. Used some fuel line cleaner, but don't know if it ever helped due to problem infrequency. Anyway, recently began hearing load tapping coming from engine (sounds like valves). Seems to happen mostly when van is cold on very cold days and eventually sounds normal after warm up. But, have also heard tapping after restarting well warmed van. Used fuel line cleaner and tapping eventually ceased, but weather also got much warmer at same time. Would you suspect valves, EGR system, or other, ... and suggested fix?
AnswerYou may have any one of the following issues:
Misfire may be due to ignition. The vehicle may need new spark plugs, spark plug wires, and (if applicable) distrubutor cap and rotor.
Tapping noise might be low oil level, gummed up (sticking) valve lifter, or camshaft lobe wearing out.
If the tapping noise is pre-ignition, the cause could be a malfunctioning EGR system, low octane fuel, or ignition timing out of adjustment.
I would replace the spark plugs, spark plug wires, and cap and rotor (if your vehicle has them) first. If the problem persists, try some higher octane fuel and either a product called Valve Medic or Rislone in the motor oil.
