Merc 5,7 EFI knocking/spiking

[HenkaQP8]

My Rinker 232 year 2002 has a Merc 5,7 EFI Bravo 3, engine starts very easy and runs quite smooth and nice at idle, problem begins at about 2000 rpm and above, specially at higher torques, like when you are going up on plane, the engine is spiking/knocking very hard and loud. It sounds really bad. I have a new fuelfilter and new gasoline 95 octane. Anyone who can help me ? /Henka

 

[basspirate]

Re: Merc 5,7 EFI knocking/spiking

you might have too high an octane fuel in the engine. contrary to popular belief higher octane fuels do not clean your engine better or produce more power unless your engine was designed for it. i.e. very high compression ratios like 12-1 or if you are using a supercharger/turbo. higher octane fuels burn slower so at idle it'll run fine cause the fuel has more time to burn. at higher rpms the fuel may not be completely burning. i would try to get some fuel out of the tank and replace it with some lower octane gas, like 87 or if you can find it 85.
the other possibility is you threw a rod bearing. if it's a dull thud thats loud (sounds like tapping a hammer on a steel plate) then it may be time for a new engine. try the gas thing though first.