Strange noise help needed

jg216ssi · 03-26-2007, 03:09 PM

I have a 2001 Chaparral 216ssi with a Volvo Penta I/O 5.7 GSI injected engine and I have a loud knocking noise from what sounds like behind the engine. On the advice of a few local boat shops, I have replaced the gimble bearing, rear drive coupler, and have gone through the outdrive. I have inspected the flywheel and have checked for excessive endplay in the crankshaft and have found little if any wear on anything. The boat only has 260 hrs. and has been overmaintained. The noise only happens at and above 1800 rpm under load only. It won't happen running on the trailer and the engine runs very smooth even when the noise occurs. I am looking for any advice on what else it could be. I feel a possible slight vibration when the noise is occurring but it could be that I am hypersensitive to the noise. Does anyone know if there is some sort of baffle flap in the exhaust that could make this noise and where is it located? Any input would be greatly appriceiated.

sixpack324 · 03-26-2007, 06:37 PM

I once drifted around the bay, listening for an intermittent engine noise that also seems to "maybe" have a slight vibration. I narrowed it down to only happening when I was running against the current (which was slightly swift), which made me think engine load was causing the problem, right around 2000 RPM's, then once in a while at higher and lower RPM's......then I finally found and fixed my engine noise.....

I pulled ALL of the fenders in.......

mikie · 03-26-2007, 07:11 PM

Don't know about the Volvo, but Mercruisers had an exhaust shutter in the pipe just below the exhaust bellows to stop water incursion, I believe.

Local Motion · 03-26-2007, 07:24 PM

Did you check your motor alignment? This is important.

Also u-joints ok? I/O's make a horrendous noise if the trim limit is exceeded (limit broken or out of adjustment) and the motor is started. This puts the u-joints at a real bad angle. If someone did this in the past to the boat, the u-joints may be damaged.

There are usually flapper valves (I also am only familiar with merc) in the y-pipes to prevent water from flooding the engine. These generally do not 'knock' though and you usually hear them at idle speeds when you do hear them flapping and it is an easy noise to locate. At higher rpm they won't be flapping at all.