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Strange mystery noise in a 5.7

C

CaboJohn

Regular Contributor
Hi Guys,
Arrived in Mexico for the season last week. Boat partner had already splashed the boat, checked fluids and started both engines. (2002 5.7 Captains Choice with around 950 hours each) He was anxious to fish so we went tuna fishing the next day. After several hours we noted a loud roaring (whirring?) noise starting at about 1200 rpm on the port engine. It seemed to go away, or at least diminish so it could not be heard above about 2000 rpm. We got back to port, lifted the hatch and tried to find the noise while at the dock in neutral. It was there, came on suddenly at 1200, and surprisingly loud with the hatch up.

I had brought down a raw water pump (P105) for that side as it had developed a leak last spring and was dripping salt water, so I figured that was perhaps the source of the noise. We suspended fishing and I put on the new pump and changed the belt the next day (it was somewhat worn and getting loose). Started it up and the loud noise came back at 1200 rpm as before. No change at all. To me it sounded like it was from the front part of the engine. I did a scan at idle and found no codes thrown and all readings looked normal. I wanted to scan with the noise happening but turned out not to want to come back.

So I am flummoxed. I have addressed the most likely causes (the failing pump and the belt). The thing is intermittent and goes away entirely at lower rpms and sometimes at all rpms, so it is unlikely to be something mechanical like a valve train failure. I did note an occasional mild "squeek" from the engine when at idle so that is not entirely normal.

But when the new noise happens, it is loud! Has anyone experienced anything like this or might have some guesses as to where to go next?

The boat is pulled right now so next week I plan to loosen my new belt (never easy on the port side) and then carefully hand turn the alternator and circulating pump to see if the bearings feel rough. I will replace if I find them suspect.

Any suggestions or speculations would be welcome. I am sure it will eventually be found (hopefully not 20 miles offshore) so a headstart or road map would be useful.

Thanks for your help.

CaboJohn

PS On a related note, that raw water pump that I took off was installed new in January 2017. The one before it also developed a leak. Since that boat sees less than 6 month a year of use (and certainly not every day), that is a very premature failure of the shaft seal. Anything I could have done in the installation to cause such outcomes? I am always willing to learn!
 
If raw water cooled being run in salt the circulating pump would be a likely cause. They last much longer on a FWC engine. Did you remove the belts one at a time to help isolate the problem? The only thing you could have done to shorten the life of the new pump would be to overtighten the drive belt.
 
Alternator is another possibility.

If the belt removal doesn't help, a stethescope or long screwdriver may help in identifying the source.....
 
I went to the boat yesterday (on the hard) and removed the fan belt. The bearings in the alternator and the circulating water pump turned smoothly with no play so it is doubtful they are the issue. However, I did find the the long bolt that supports the alternator was somewhat loose and the alternator could be made to wobble a little. Of course the tension in the fan belt should prevent that from happening, but.... I am using a cogged fan belt. That's all that is sold down here and it is supposed to be interchangable (a arguably superior) to the traditional solid belt. Still, it seems possible that at some engine speed a harmonic of some sort could make the whole thing vibrate with the observed loud noise. Kind of a stretch, but everything else checks out perfect.

I will splash the boat next week and do a running test to see if tightening that bolt made a differnce. If not, I will find the source with the rubber hose stethescope trick.
 
CaboJohn said:
I went to the boat yesterday (on the hard) and removed the fan belt. The bearings in the alternator and the circulating water pump turned smoothly with no play so it is doubtful they are the issue. However, I did find the the long bolt that supports the alternator was somewhat loose and the alternator could be made to wobble a little. Of course the tension in the fan belt should prevent that from happening, but.... I am using a cogged fan belt. That's all that is sold down here and it is supposed to be interchangable (a arguably superior) to the traditional solid belt. Still, it seems possible that at some engine speed a harmonic of some sort could make the whole thing vibrate with the observed loud noise. Kind of a stretch, but everything else checks out perfect.

I will splash the boat next week and do a running test to see if tightening that bolt made a differnce. If not, I will find the source with the rubber hose stethescope trick.
Click to expand...

You can run the engine for a short time to see if the noise is still there or if goes away. Sounds like you relied on just the hand spinning the components test. May not give you an accurate test. Run engine cold with the belt off for a minute and run up to rpm. Then you should know.
 
It seems I was right about the cogged belt. The somewhat loose horizontal bolt that the alternator hangs from allowed the belt to cause the alternator to vibrate and rattle at certain rpms. Tightened everything up and the noise is gone.

My only concern at this point is that the alternator bracket attached to the block is (I believe) cast and if so is unlikely to be designed to be tightened to such an extent that it can "lock" the alternator down so it cannot vibrate. But it didn't crack so for now, all is good.
 
For the future an easy way to check if the noise is from a belt is while the engine is running carefully put a bar of dry bar soap on the running belt. As the belt coats with dry bar soap the noise goes away if the noise is from a belt.
 
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