Metallic type rap under load 7.4

[RDP3430]

All,

Have a 1997 7.4. Have a metallic type rap when boat under a heavy load (2000 RPM and up): Runs fine up to about 1900 RPM and oil pressure is 60 plus. Thinking possibly knock sensor ? Running premium fuel and just did a repower not too long ago.. Ideas ?

Thanks,

Rick

 

[fc3]

Knock sensor retards timing when detonation is sensed. Don't know if you have EFI or not. So can't help with that, but the knock sensor doesn't cause knock. Since the boat was repowered without specifics, don't know what generation engine you have. So we could be looking at everything from piston slap to bad connecting rod bearings. Of course, could also be a stern drive problem.

First idea that springs to mind, other than there's no way to diagnose an engine problem from two terse sentences, is that your repower was with a badly rebuilt engine that has problems.

 

[MorichesBayBum]

If you free rev the engine do you have noise? If you put it under a load if its a slow bang is more than likely a drive issue. Take a look at the fluid and look for metal. If the boat was put away with water in the lower unit it may have pitted the bearings causing them to let go. Try runing under a load in reverse, if you dont hear the noise its not the engine. If you still hear it, we are still at square one.
If the cone gear in the drive let go, the reverse side is probably ok. It can be a few other things but you can start there.
Good luck!

If it is a fast bang, it may be

 

[RDP3430]

All,

Here is a bit more detail:

Just did a rebuild on a Gen VI 454. Engine has about 10 hrs on it. Runs fine up to about 1900-2200 RPMs or so.

New mains, rod bearings, rings, and heads redone.

Oil pressure good (40 to 60)
Temp is about 165/170
Not burning oil

The problem is I get a metal rapping/clanging anything above 2200 -2300 especially on hard acceleration and decceleration. It is not a low tump like mains or pinging sound.

Looking for ideas before I tear it down.

Thanks,

Rick

 

[BonBini]

It's difficult to help you with this type of noise not being able to hear it. The first thing I would do is cut open the oil filter for inspection.

As far as the noise: not knowing what the "rebuild" entailed, Maybe you've got a tight wrist pin? ie: Piston comes up, conn rod rocks over center and swings the piston with it. Noise is a hollow knock as the piston skirt slaps the cylinder. This typically would be on a cold engine. As the piston heats up and expands some. the wrist pin can again float as it should.

Just throwing something out to you. Look at that filter first. That will tell you if you've got something real bad.