Whats wrong with my 496

[SeaRayCaptain]

My 496s MPI is running perfect on idle, have tested the compression on both engines and they are good. If I cruise slowly on 1000-1500rpm, everything seems normal, good temp and oil pressure.

But then when I start to cruise over 1500rpm, starboard engine tachometer starts to "jump" 4-500rpm back and forth and then the engine stops. No problem to restart and then it runs poorly even on low prm and often stops.

The engine did also backfire at 2000rpm today....

What can be wrong??!, don´t think is the fuelpumps, any suggestions...

Thanks

 

[wvsundancer]

I would see if someone has the computer available to plug into your engines.. this may save you time looking for the problem

 

[becnme]

Check your ignition wires. I had a similar problem last year with those engines and new wires made it run like new. Look very carefully for a light gray dust as a sign they are going to ground instead of the spark plug.

 

[aqua*holic]

Is this a fuel injected engine? When you said the tach jumps, is the motor RPM actually surging with the tach or is the tach just moving on it's own?

If it is FI and the engine is surging, it sounds like the ECM is loosing either crank position or cam position reference. If just the tach is surging, sounds like there is an issue with the RPM feedback system.

 

[SeaRayCaptain]

becnme: Thanks will check the cables.
aqua*holic : Yes these engines are fuel injected, and yes both the engine and the tach is jumping....:/

 

[SeaRayCaptain]

Have pluged in the engines to computers, two times, and no specific alarms second time. The Mercruiser dealer recommended to check fuel or ignition. Common that the fueltanks get dirty?

 

[carterchapman]

How many total hours?

When were the fuel filters, plugs and wires last changed?

 

[PlayDate]

I'm with Mercruiser. It sounds more like a fuel delivery issue than anything else. The components that usually go wrong are the pump and the regulator. You can check the pressure on the fuel rail which will tell you if the pump/regulator are working properly. The regulator bleeds off high pressure to maintain a consistent pressure for the injectors. If the reading is stable and starts dropping as the rpms increase.....the fuel pump is the likely culprit. If the readings flucuate at idle....either the regulator or fuel pump is bad.


-John

 

[SeaRayCaptain]

The engines have 400h on them, the sparks and filter where changed before this season. When I go over 2000rpm the engine start to go bad, and then I have to go down to 900rpm and then slowly increase the throttle so I can continue driving. Almost feels like that something plug the fuelline, therefore I suspect crap in the fuel tanks, but they are not easy to clean...:/