1996 Mystic Cobra 4.6L DOHC Has A P0300 Random Misfire

wheelpro

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Mar 20, 2021
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My Cobra has a P0300 code random misfire and a P1455 EVAP I've replaced spark plugs, EVAP purge Solenoid, fuel filter and it's still running bad. It went from running smooth to running with an extreme mis what's the chance that a bent valve is causing the problem? Or a dirty fuel tank or injectors? I'd appreciate any advice
 
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MustangIIMatt

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what's the chance that a bent valve is causing the problem? Or a dirty fuel tank or injectors? I'd appreciate any advice
Bent valve? Virtually zero. A single bent valve would cause a single cylinder misfire.

Dirty fuel tank? That'd usually lead to a lean code before a misfire code if the car ran at all, what did the fuel filter you took off look like? Could you blow through it? If it wasn't filled with crud and you could blow through it, a dirty tank probably wasn't your problem.

Injectors? Well, possibly, but not likely. All eight failing at the same time wouldn't be likely, and one or two would cause single cylinder misfire codes (P0301-0308).

Where I'd start for the diagnosis of this would be to visually inspect the entire evaporative system. This means checking out the fuel tank grommet on the fuel filler neck, all of the various hoses and tubes running to and from the fuel tank, the charcoal canister, and the tubes that run to the engine itself, as well as all of your vacuum hoses underhood.

I'm surprised that there's not a pair of lean codes stored along with the evap and misfire codes, because a leak in the evap system large enough to lead to misfires would usually cause a lean code on anything other than a BMW.

Another REALLY easy check you can do for your random misfire is to start the car at night and open the hood. See if there's any arcing happening around the ignition coils or plug wires. Old worn out plug wires or coils that have started to fail can arc across, and with that car using twin four-terminal coils, either could cause a random misfire.
 
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