Random pre ignition and unstable timing Mercruiser 3.0L 2007

[anddah]

I got a Mercruiser 3.0L 2007 that acts funny. If I check the timing advance during 3000+ RPM the timing is very unstable and I can get as much as 12 more degrees BTDC then it should be but many of the cycles are spot on.

If I run the boat I can hear what sound like pre detonation during heavy load and high RPM.

A new symptom now is that my RPM gauge is also acting up. Sometimes it says that the idle RPM is 2000+ but I can hear that it runs ok at around 700-800. If I use the throttle it can be stable around 1200 PRM and when I go back to idle it first jumps up to 2000 RPM and then drops to 700 (the engine drops directly).

Went for a ride yesterday anyway and now blew the head gasket so I must replace it.

I believe that this must be electrical but can it be mechanical? Any ideas

 

[alldodge]

I got a Mercruiser 3.0L 2007 that acts funny. If I check the timing advance during 3000+ RPM the timing is very unstable and I can get as much as 12 more degrees BTDC then it should be but many of the cycles are spot on.

If I run the boat I can hear what sound like pre detonation during heavy load and high RPM.

A new symptom now is that my RPM gauge is also acting up. Sometimes it says that the idle RPM is 2000+ but I can hear that it runs ok at around 700-800. If I use the throttle it can be stable around 1200 PRM and when I go back to idle it first jumps up to 2000 RPM and then drops to 700 (the engine drops directly).

Went for a ride yesterday anyway and now blew the head gasket so I must replace it.

I believe that this must be electrical but can it be mechanical? Any ideas

If you do have detonation going on your engine is destroying it self. If detonation is happening your engine will be trying to reduce the timing to help save it's self. Pull the plugs and tell us what they look like. Why do you think it blew the head gasket?

 

[Bondo]

If you do have detonation going on your engine is destroying it self. If detonation is happening your engine will be trying to reduce the timing to help save it's self. Pull the plugs and tell us what they look like. Why do you think it blew the head gasket?

Ayuh,.... Probably oughta check the condition of the gas, 'n fuel system, Before ya run it again,....