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Thread: Burning rubber smell

  1. #1
    About a month ago while at Lake Mohave, I noticed a very slight burning rubber smell in our boat. The only time I would notice it would be after running at speed when coming back to idle speed. I'd notice the smell for a while, then it would seem to go away. Boat has a 1995 Merc 350 Mag EFI/MP (version with the tunnel ram style intake) and Bravo 1 with Rex Marine cutouts/thru hull exhaust (y-pipe connected to the cutouts). I checked the belts, exhaust hoses etc. and didn't notice anything out of the ordinary, nor could I isolate the source. I'm suspecting the flappers in the exhaust pipes at the transom to be the culprit, but I'm not sure. Water and oil temps were all normal. Exhaust manifolds and elbows didn't appear to get overheated and there was the normal flow of coolant out the pipes. Is this a sign of impending water pump failure, exhaust flapper failure or are the exhaust manifolds on the fringe of coming apart? Boat has 480 or so hours on it and has always had routine fluid and filter changes. Engine still runs like a champ and starts on cue. Any insight or suggestions welcome.

  2. #2
    How about your belts? Any signs of slippage?

  3. #3
    Tahiti Day Cruiser
    I was at Mojave a month ago when my water pump went out. I experienced the exact same symtoms as you did. We were cruising around by the dam and started smelling this burnt rubber smell. Nothing on the engine looked out of order and the temp was o.k. After driving some more the pump went out and did cause the engine to over heat. I think my pump was on the verge of failing and was probably just pumping enough water to keep the engine cool, plus when your going fast the wind cools the engine and you also have the force of the water pushing water through at least some of the cooling system. I think it would be worth your time to check the water pump out , even a couple of burnt fins on the water impeller will considerably lower it's pumping power. If you have a bravo-1 then the pump is located in the engine is fairly easy to change.
    Good luck

  4. #4
    78E, I did check the belts and they all looked okay. Tahiti, thanks for the input. I'll break out the manual and check the pump out. I'd hate to overheat and crack the exhaust manifolds or start a fire. BTW, any chance of the manifolds causing this problem, i.e. blocked water passages etc?

  5. #5
    Tahiti Day Cruiser
    More than likely if it was your manifold it would cause other kind of problems, Like water getting back into the engine. I'm not sure why the manifold would smell liked burnt rubber unless something in the exhasut passage is melting which I think is unlikely. It could be something else but I have heard that those water impellers are notorius for going bad at the worst time. Sometimes they just get old and rot out,or if you ever run your engine for even a minute without water, this causes major damage to the water impeller.

  6. #6
    where does you valve cover breathers go to? It's pssible you could be smelling a little "blow by" after a hard run if you breathers don't go to the carb or recirculate somehow.

  7. #7
    My guess is that your coupler is going bad (it mounts on your flywheel) and the shaft from your lower unit goes into it, should this be your problem the motor will have to be pulled to replace it.

  8. #8
    Had a similar event, slight smell of burning rubber was from the battery wire to the starter, it was off its hanger and was touching part of the exhaust manifold.

  9. #9
    Thanks for all the input. I don't believe the smell is caused by the engine coupler. I just to make that much power and the smell is noticed with the engine hatch down, and only for a brief period. It also does not appear to be the EGR hose or electrical wiring near the exhaust manifold. That area is pretty clear. I'm guessing the water pump is on the verge of failure, and provides minimal coolant flow when coming off plane as the rpms drop. This is probably reducing the water flow to the exhaust manifold briefly which in turn allows hot/un-cooled exhaust gases to come in complete contact with the rubber exhaust hose and flappers in the tailpipes, or even maybe the rubber connector hose to the y-pipe.
    Anyone know how much the pump costs? If the pump can be taken apart, is the impeller damage obvious?

  10. #10
    Havasu Hangin'
    Originally posted by Trash:
    Anyone know how much the pump costs? If the pump can be taken apart, is the impeller damage obvious?
    They sell rebuild kits...I put a new impeller on almost every insurance.
    If the impeller is bad, it will be missing veins (you need to get them out) or the veins will be cracked at the base...pretty easily seen...

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