Mechanics

CHEVY - RANDOM MISFIRE

1996 Chevrolet Truck • V8 2WD Automatic • 155,000 miles

1996 chevy truck. 305, 5.0, 155k miles. Random misfire, but heavy on cylinder 7. I have replaced coil, plugs, wires, spider injection system, crank positioning sensor. The SES light will flash when idling, but will not even come on when driving, or if I keep the RPM's up by powerbraking it at stop lights. I have had it to 3 mechanics and they all want to rebuild the engine. Could it be a fuel problem? Fuel pressure regulator, fuel pump, bad line/hose? There does not seem to be any vacuum leaks.

Hopefully you can help. I have been messing with this on and off for over a year.
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Wrchambe
August 4, 2011.



And in the off chance that it is a cam, can it just be swapped without causing problems?

Tiny
Wrchambe
Aug 4, 2011.
Has there been any compression, or cylinder leakdown testing on the engine? It sounds like a leaking valve(s).
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Factoryjack
Aug 5, 2011.
Multiple compression checks have been done. The rear cylinders were a little low, but still within specs.

Tiny
Wrchambe
Aug 5, 2011.
How about cylinder leakage, that is going to be your real determining factor. You can have compression differences that would not seem abnormal, but still have a degree of leakdown that can cause a misfire. If you are unfamiliar with leakdown testing, it involves a leakdown tester. It has two gauges, one side attached to compressed air, and when applied to a sealed cylinder(valves closed), the second gauge measures a percentage of loss. From that percentage of loss, if deemed excessive, then you would listen for air leakage through places like the exhaust(exhaust valve), the throttle body(intake valve), crankcase(ring/piston), cooling system(headgasket/crack), etc. If for instance you had a worn cam, you may potentially see higher than normal compression readings as the valve(s) would not be open as long as they should. This would apply more to worn exhaust lobe, as a worn intake lobe will not allow enough air to enter the cylinder in the first place. A vacuum measurement from the manifold when at idle may also show a sign. A good sealing engine should have steady vacuum gauge reading 18-20" H2o. Significant fluctuation or pulsing of the needle indicates a problem.

Factoryjack
Aug 5, 2011.

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