Mysterious Rattle

[Kmort]

I have a 1995 Mercruiser 4.3L V6 and just recently have developed an engine rattle sound. When cold the sound isn't there until engine has run for about 10 minutes. The rattle does not increase with intensity at higher RPM.

I've heard of engine noises like ticking, clanking, tapping, etc., but I've never had experience with any kind of engine sounds (other than normal). This is what I definitely call a rattle since it is an inconsistent sound. It sounds like a stone bouncing around inside the engine. I can't seem to identify any particular location of it either.

Any ideas would be greatly appreciated.

 

[stonyloam]

Re: Mysterious Rattle

Does it come from the back of the engine? Only thing I can think of is one of the exhaust shutters has broken loose, but that is only a wild guess.

 

[jtmarten]

Re: Mysterious Rattle

Time to diagnose location with a mechanic's stethoscope. We could point at a multitude of components that can cause noise.

 

[Kmort]

Re: Mysterious Rattle

Thanks guys! I just purchased the scope and will report back after I get a chance to use it. The sound (by ear) seems generally the same no matter where you turn an ear.

I've had some think it's the lifters, some say timing chain and another say bad gas from the marina. What puzzles me is that the sound doesn't meet all the criteria of any of them.

 

[Silvertip]

Re: Mysterious Rattle

This is a balance shaft engine and the balance shaft and some of those engines had a rattle that sounded like pre-ignition at certain light load conditions. If this is what you are hearing, this is not an issue and does not affect longevity or performance. It is simply a clearance issue with the bearings. The noise is generally not audible at idle or under load.

 

[Kmort]

Re: Mysterious Rattle

Where would I locate these exhaust shutters Stony?

 

[stonyloam]

Re: Mysterious Rattle

Where would I locate these exhaust shutters Stony?

Right here in the exhaust pipe: #6 http://www.mercruiserparts.com/Show...1.png&inbr=1599&bnbr=200&bdesc=EXHAUST+SYSTEM Just put the scope on the pipe (one on each side) if it should be pretty obvious.

 

[Kmort]

Re: Mysterious Rattle

Thanks Stony!!!! Great link also. Maybe I will become more familiar with my engine with it.

 

[MartyL]

Re: Mysterious Rattle

I had similar issue with my 5.7 recently got some good advice on here checked spark plugs found they were polished clean. Did some further work checking compression came to find had a blown head gasket. Not sure if that helps but I was puzzled with the noise I was hearing as well

 

[Kmort]

Re: Mysterious Rattle

***UPDATE***

I scoped the engine for the rattle sound and came up with nothing except the suspicion of the main pulley. I came as close as I could to the moving part and is the only place I could detect the closeness of the rattle with the scope. The rattle is consistent now but still doesn't change any under RPM variations. Could it be a timing chain or a pulley bearing? The engine does run a little too high at full throttle. 5100 RPM

Also found the oil leak while scoping. The oil sensor! The vacuum lines from the cylinder covers also had light oil on them and smoking a little at the other end near the arrestor. I also drained about a half a quart of oil since the dipstick was showing it overfull. The oil was black!!!! I've only ran the engine 10 hours since the spring oil change.

I know, I know... this all sounds bad, but any ideas and where to begin would be greatly appreciated.

I did take it out for a spin for about 10 mins and it run ok. Temp maintained at 150 and the oil pressure at 40 but occasionally would jump up 5 or down 5

 

[KJSmitty]

Re: Mysterious Rattle

***UPDATE***


I also drained about a half a quart of oil since the dipstick was showing it overfull. The oil was black!!!! I've only ran the engine 10 hours since the spring oil change.
- oil pressure at 40 but occasionally would jump up 5 or down 5

Black oil after 10 hrs, 1/2 qt overfull, somewhat low oil pressure,,,,, Did you smell your oil for fuel? If not fuel in the oil it may be time for a compression/leak down test. As for the rattle...?? If it doesn't change with RPM that makes it quite strange when considering bearing or other internal engine issues

 

[Kmort]

Re: Mysterious Rattle

Yes I did smell for fuel in the oil but didn't notice any. The engine did have a compression test done pre-season and was told it was good. I agree with you about the rattle and believe me it's driving me nuts. I'm still trying find a replacement oil sender for this motor. Where might I find one of those?