So last season i replaced my old 454 Mercruiser with a CPO Mercruiser 496Mag, closed fresh water cooled with the existing thru-hull exhaust. Problem is it's been blowing holes in the 18" rubber exhaust tubes that go from the headers to the thru-hull. Engine never overheats, sea pump has been replaced at just 25hours, rubber exhaust tubes don't even seem hot....I'm lost and frustrated so any help is greatly appreciated. Not sure if it's pressure or temp related or what. Thanks so much in advance, Pls feel free to email me: [email protected]
PS: I've also just recently changed out the thru hull water pickup hose that runs thru the drive and transom so that's all new as well. Thanks for the thoughts and ideas...i'm leaning towards the angle of the tubes now. I can shine a flashlight in the thru-hull and see the burns on just the top of the tubes and see the angle is rough from the header. Thanks!!!

Did you plug the prop exhaust,are there baffles in your tailpipes?It may be to much back pressure. Are you using the correct hose?

Could be plugged risers or a riser set that is dumping the water low, or not holding enough water allowing it to properly buffer the hot exiting exhaust.. Also may have not enough water flow in general, but not enough for the engine to over heat yet.

I would guess the angle of the exhaust..? Are the holes blowing on the top side of the rubber exhaust tubes?

I had a similar problem, the angle of the rubber was too steep and the water was not getting to the top section of the rubber tubing and the hot exhaust gas was burning a hole through......I ended up adding a small section of stainless exhaust pipe cut and welded at a slight down angle, good luck with sorting it out.....

Thanks, did you ever get a 2 beep alarm every couple mins during warmup or idle?

Nope......I think with your set up the alarm would be gear oil level if you have the resorvoir with the sender, engine/water temp or faulty alarm...? I don't believe it would have anything to do with the exhaust pipes getting holes blown in them?

Where are the holes happening....??

I tell my customers its allot easier to understand if they send me a PIC. In this case can you post a pic?

Hi, I had the same problems with a 496mag, on the exhaust it is as FogduckerIII stated in his reply, it is the angle and his answer to correct the problem is as I did. On the 2 beeps, I had to get a digital diagnostic tool and the beeps were a bad sensor. Problem was I had one after the other go and the engine would only run to a safe rpm, I had so many problems with sensors I swapped out to a different big block and sold the 496 motor, could not keep up to the sensor issues, I just had a bad lemon drop 496, I know others with 0 issues on this engine.

Thanks so much for all that replied. I ended up having some fabrication done and placed ss covers over the rubber hose and the boat ran fine for over an hour without ever heating up. Looks as if the exhaust angels from the 454 to 496 were just varied enough for the exhausts team to burn the tops of the hoses. Thanks again all!!!