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Ongoing problem with serpentine belt.

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poloperson57

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I have a 2020 Mercruiser mag 350 mpi in my 1999 Rinker fiesta Vee 242. I hadn’t had the boat long when I noticed a rattle coming from the engine, and when chugging slowly up the river, it suddenly cut out.

Long story short - water circulating pump bearing failed, so I replaced the pump. Then the serpentine belt kept coming off, so after checking the alignment of all the pulleys I discovered the tensioner pulley was at the max position to get the belt tight enough, so I figured the belt had stretched and had lost its elasticity, so I replaced the belt.

Cautious of over tightening it, I made sure it was tight enough and I could just twist it by 90 degrees with moderate force. Ran the engine for 5 minutes or so, shut it down and then re-tensioned it. Went for test drive and when upping the revs to only around 1500-2000 or so, the belt came off. I guessed I just hadn’t tightened it up enough as it was a new belt. So I put it back on and re-tightened it, ran the engine for 5 mins and tightened it again, but this time higher tension (as tight as I dared).

Off we went again and within a few minutes the engine cuts out. Check the belt and it’s still on ok, but it had loosened. So re-tensioned it again, started the engine ok, and tried again. Same thing. Engine cuts out, and belt has become loose again. Retensioned and I’m back at the maximum position on the tensioning pulley.

So, docked the boat and came home wondering why the belt keeps coming off ‘ getting stretched. My only hypothesis is one of the pulleys is not turning as freely as it should and the belt is slipping on that pulley, causing friction, heating up and stretching? The idle pulleys are spinning fine, as are the water circulating pump and alternator, but tbh I didn’t check the others. My guess is the sea water pump is jamming / jammed and that’s causing the problem.

But…. Any ideas from all you experts are most welcome!
 
poloperson57 said:
So, docked the boat and came home wondering why the belt keeps coming off ‘ getting stretched. My only hypothesis is one of the pulleys is not turning as freely as it should and the belt is slipping on that pulley, causing friction, heating up and stretching? The idle pulleys are spinning fine, as are the water circulating pump and alternator, but tbh I didn’t check the others. My guess is the sea water pump is jamming / jammed and that’s causing the problem.
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Ayuh,..... I agree, check 'em all,....
 
In addition to Bondo's thoughts, I'd add to make sure the pulleys are all in the same plane...if any of the brackets had been removed and not correctly reassembled, the pulleys could be offset which will cause belt throwing issues.

the other is to change your tension checking methodology...don't rotate belts to check tension, use one of the deflection methods...too much tension will damage a belt.
 
makomark said:
In addition to Bondo's thoughts, I'd add to make sure the pulleys are all in the same plane...if any of the brackets had been removed and not correctly reassembled, the pulleys could be offset which will cause belt throwing issues.

the other is to change your tension checking methodology...don't rotate belts to check tension, use one of the deflection methods...too much tension will damage a belt.
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Yeah, this is something I’m really struggling with. The manual says something like “1/4 inch or 6mm deflection in the middle of the longest part of the belt between 2 pulleys, with moderate thumb pressure”. What is moderate thumb pressure? I did try this method and it pretty much correlated to the rotation method used by some, but it’s all so subjective as it relies on your interpretation of moderate pressure. How do you sense the right tension?
 
the belt method for determining 'tight enough' is to use a belt tension gauge but they aren't cheap. if your vehicle has a serpentine belt with a tensioner, you can experiment with that. agreed that 'moderate thumb pressure' is subjective.

as far as the pulleys, check them with the belt off. the idler, alternator and circulating pumps pulleys should spin easily and smoothly (bearing tight and free). If those check out, they all need to aligned in the same plane so the belt will stay on.
 
makomark said:
the belt method for determining 'tight enough' is to use a belt tension gauge but they aren't cheap. if your vehicle has a serpentine belt with a tensioner, you can experiment with that. agreed that 'moderate thumb pressure' is subjective.

as far as the pulleys, check them with the belt off. the idler, alternator and circulating pumps pulleys should spin easily and smoothly (bearing tight and free). If those check out, they all need to aligned in the same plane so the belt will stay on.
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I need to find someone with this engine, and who goes fishing. Then I can ask them to take out their fishing scales, hook it up to the belt, and pull it up by 1/2" and tell me what weight they are reading. That should give me a fairly accurate tension reading, as long as the 1/2" travel is accurate.
 
I doubt its a tension issue. If it were a bit tight it would probably just wear out and squeel/get noisy quickly.

If it were to loose (which you seem to have made sure its not), I would think it would have to be extremely loose to come off as easy as you say.

You should be using a long metal piece with a FLAT edge to confrim all pullies are in alignment. At least the outer edges of the pullies.
Also look very close at crankshaft pulley and the harmonic ballancer behind it to determine if there may be a wobble/alignment issue.

Lastly, confirm all pullies, the grooves are still there. If the are worn down that may be a contributing factor.
 
kghost said:
I doubt its a tension issue. If it were a bit tight it would probably just wear out and squeel/get noisy quickly.

If it were to loose (which you seem to have made sure its not), I would think it would have to be extremely loose to come off as easy as you say.

You should be using a long metal piece with a FLAT edge to confrim all pullies are in alignment. At least the outer edges of the pullies.
Also look very close at crankshaft pulley and the harmonic ballancer behind it to determine if there may be a wobble/alignment issue.

Lastly, confirm all pullies, the grooves are still there. If the are worn down that may be a contributing factor.
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Thanks Jack, I’ll definitely check all that out very carefully next time I’m at the boat.

One thing that’s puzzling me though is why the belt appears to be stretching so much. It’s gone from being difficult to actually get on with the tensioner as loose as it goes, (it was a brand new belt) to only just being able to get it tight with the tensioner full on, in one day of messing about with it. Will the act of it coming off cause it to stretch? Or, if the pulleys are out of alignment and/or the harmonic balancer is wobbling, would this cause the belt to stretch?
 
poloperson57 said:
Thanks Jack, I’ll definitely check all that out very carefully next time I’m at the boat.

One thing that’s puzzling me though is why the belt appears to be stretching so much. It’s gone from being difficult to actually get on with the tensioner as loose as it goes, (it was a brand new belt) to only just being able to get it tight with the tensioner full on, in one day of messing about with it. Will the act of it coming off cause it to stretch? Or, if the pulleys are out of alignment and/or the harmonic balancer is wobbling, would this cause the belt to stretch?
Click to expand...

What do the pulley splines/grooves look ater it come off, If they are all reounded and not very sharp, that may mean one of the pullies is not spinning properly (meaning slow/binding) or one of the pullies is so worn is spining but heating up the belt.

Hard to say... so more investigation is needed
 
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