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Mercury 502 Irregular running RPM

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boatglms

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"Hello! I have a 2001 Formula

"Hello! I have a 2001 Formula 400ss with twin Mercury 502 mag MPI's. At idle there is evidence of fuel on the water coming out of the port exhaust and at 4000 rpm there is a 200RPM constant variation on the port engine. Does anyone have any suggestions for diagnosing the fluctuating RPM? Im stumped Thanks
"
 
Sounds like a fuel injector pr

Sounds like a fuel injector problem maybe.
Have you tried some injector cleaner in the gas yet?

Rod
 
Is the engine stock or modifie

Is the engine stock or modified in any way? Does it have oxygen sensors?

Reason that i ask is that any modificaiton to the engine may result in the fuel calibration being off and a potential overfueling situation.

How do you know its fuel you're seeing and not oil?
 
"The engine is completely stoc

"The engine is completely stock, Im not sure if it is oil or gas coming out but I just tried swapping the ecm's, fuel pressure is a constant 39 psi, I checked all of the plug wires, vacuum lines, cap and rotor, and gauges. I also tried fuel injector cleaner. I think tomorrow I will try and change out the cap, rotor, wires and plugs and see if that helps. So far it doesn't seem like fuel after hooking my computer up and comparing both engines. Thanks for all the help!"
 
How many hours on it? Diagnosi

How many hours on it? Diagnosis is better than throwing parts at it as you know. It won't be a vacuum leak as that will cause idle problems and not 4K RPM running issues.

You could be getting slugs of oil in the form of blow by off the crank case PCV vents into the air intake. Any smoke? Any loss of oil?

Does it have O2 sensors?
 
"206 hours, there is no oil lo

"206 hours, there is no oil loss or smoke, no O2 sensors either. I checked the PCV as well today and it is clean as a whistle. I am currently trying to use the data I recorded from my Diacom program to look for sensor readings that may be faulty. Today we replaced the ignition coil, plugs, cap, rotor, and plug wires. The only thing i can think of is to test the wiring harness and the injectors."
 
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