2GM20F Exhaust Question

Benny17441 · Jan 11, 2016

It is normal to get a gradual build up of salt and minerals in the hoses, heat exchanger and mixing elbow. This build up can create restrictions in the water flow. It seems that as you increase RPM it increases the pressure which restores the volume of water flow to the exhaust sound that your ear is attuned for. It may not be a problem now but it will gradually continue to deteriorate until it will start causing the engine to overheat. This is a slow process but it tends to speed up in the latter stages. I would check the hoses, heat exchanger and the mixing elbow. Always start by checking the impeller and making sure the pump is performing adequately. While you are at it check for water leaks and inspect the exhaust hose.

markwbird · Jan 11, 2016

Check raw water pump. That is the sound of a dry or nearly dry exhaust. I had this very symptom on my boat and it was the raw water pump belt. It was loose so it was still turning intermittently and occasionally spitting water out but not enough. It got gradually worse until the engine overheated.

kloudie1 · Jan 11, 2016

Exactly.. hard to tell from the water burps and spitting exactly how much water is being pumped. points to raw water pump not flowing as much as it should.. belt tightness and impeller condition would be the prime suspects. There have been reports of some Yanmar impellers where the rubber was not solidly attached to the inner drive, causing them to slip on the shaft.. Yours sounds like a loose drive belt or a vane or two missing from impeller.

Dan_Y · Jan 12, 2016

When I replaced the mixing elbow on our 2gm20f it gave a different, quieter sound since the coked up exhaust restriction was gone.

whatfiero · Jan 18, 2016

I have to clean my exhaust mixing elbow every other year and replace every five years I use a wire brush on drill

NYSail · Jan 18, 2016

I changed my mixing elbow when I bought my boat 9 years ago.... This past summer, had a problem. Water was flowing fine, engine was cool and mixing elbow could be touched under long runs..... However noticed smoke for first time and engine sounded a little different. Figured I would replace elbow.... turns out water passage was 100% open however exhaust section was 97% blocked. Replaced..... No smoke and engine sounds fine.

Nelsonhow · Feb 12, 2016

I had a similar problem on my old 2 gm.... After 2 seasons and lots of angst...it turned out to be a bad cover plate or impeller on the pump.combinet with a partial blocked hose. I replaced both the place and impeller and cleaned out the hoses and valve and "VIOLA" all fixed.......

whatfiero · Feb 18, 2016

Not to hijack this thread but Sailm8 mentioned the muffler what does an inline muffler do other than reduce the sound is there a back flow preventer in there mine froze and cracked and I don't want to replace just to have to do again every year

druid · Feb 19, 2016

Of all the old engines I've pulled out, I've NEVER seen significant buildup in the exhaust system (Water system in RWC is a different story - some of the holes in the 40-yr-old Volvo were plugged completely). Maybe it's cuz here on the West Coast we often motor all day, which allows the engine to heat up nicely and "burn out" any carbon...

More on the hijack: All the exhaust websites I've looked at show a muffler, but I've never seen one installed. Do they actually make a difference? I would think not: most of the noise comes from the engine itself, not from the exhaust.

druid

DaveJ · Feb 19, 2016

More on the hijacked muffler link....my last boat, a 1983 Catalina 30 with a Westerbeke diesel, did not have a muffler. It had a metal backflow preventer (like a check valve) to prevent water from backflowing into the engine. Did not seem to make any more noise than any other similar engine, except for a little bit of clanging as it opened and closed.

dj