Joel, I have had some experience with a Yanmar 2QM15 which was in my old C&C 29 mk1. The 2QM15 manual states that at 2000RPM, the raw water pump should deliver 400 l/hr (which is about 1.75 US Gallons/minute). It should be easy to bypass the raw water just before it enters the mixing elbow and measure the flow rate. If the flow rate is fine, then the problem may be a clogged mixing elbow or even a bad thermostat(see comment below). If the flow rate is low, then check for free flow of the input line to the water pump. That is, with engine off, disconnect input hose from pump, lift the end above the waterline, open seacock, and lower hose into a bucket to let the water flow in.
If the input flow rate & pump impeller are fine whereas the engine output flow is low, then you can look into pickling the engine as suggested. But to be honest, pickling the engine with vinegar works well for yearly maintenance, but likely won’t remove serious engine scale. If you suspect a serious scale problem, then use something like Barnacle Buster. It is best to set up a circulation loop: put 1 gallon BB in a 5gal bucket, use a cheap Walmart-like bilge pump to pump the BB into the engine (after raw water pump), take a hose from the engine output back to the bucket, and the let it run run run. Note that a bad thermostat can cause the engine to overheat. These can be easily removed and tested. DO NOT run the engine without a thermostat as the thermostat not only allows engine water to flow when open, it also closes the raw water engine bypass as it opens. Also, be sure that the engine is actually over heating. I once spent some time tracking down an overheating alarm problem when in the end it was a bad temperature sender unit. Scale can be a real problem. Here is a link to Sailnet post I wrote some time back regarding rebuilding a 2QM15. http://www.sailnet.com/forums/gear-maintenance/90991-yanmar-2qm15-rebuild.html Here is a direct link to photos. It was amazing how much scale was in the engine. Ssee photos 12 &13 which shows the inside of the block after removing the cylinder sleeves. It a big pile of crud. https://www.flickr.com/photos/dreuge/sets/72157631067396554/ - Paul E. 1981 C&C 38 Landfall S/V Johanna Rose http://svjohannarose.blogspot.com/ > On Jun 19, 2017, at 11:24 AM, cnc-list-requ...@cnc-list.com wrote: > > From: CnC-List [mailto:cnc-list-boun...@cnc-list.com > <mailto:cnc-list-boun...@cnc-list.com>] On Behalf Of Joel Aronson via CnC-List > Sent: Monday, June 19, 2017 8:25 AM > To: cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> > Cc: Joel Aronson <joel.aron...@gmail.com <mailto:joel.aron...@gmail.com>> > Subject: Stus-List 2GM15 salt water cooled engine overheating > > All, > > The J30 I race on is overheating. It's not the impeller. The engine is 35 > years old and salt water cooled. I pulled the hose off the elbow and blew > into it. There did not seem to be any restriction. (Does that mean > anything?). What's next? Thermostat? Clean heat exchanger? Elbow? > > Thanks. > > Joel > -- > Joel > 301 541 8551
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