1981 Yanmar Diesel, hull #267. The engine temperature light/warning noise comes on regularly after about the first half hour of operation (strangely enough, it'll NEVER go on when motoring out, only when motoring back in). Raw water strainer is clean. New thermostat installed in spring of 2002 but problem has continued. Does this sound like an impeller problem or should I be looking elsewhere?

Is there water spitting out the back? If there is none, check the water pump impeller. Also, I've had problems with my water strainer (Perko?) lid not sealing tighly and letting air into the system.
If there is very little or no water, check the fitting where the water goes into the exhaust elbow (a cast upside down U shaped pipe). The fitting I have bends 90 degrees and has clogged twice over the last 8 years. The first time I had to replace the exhaust elbow, the next time I managed to clean the fitting out with a stiff wire.

Ed Austin
Chinook

Try changing your oil and replacing it with a single weight marine oil. If your oil has failed the engine will not cool properly. Has your oil pressure changed? If the oil has broken down you pressure will be lower especially when the engine is hot. Good Luck.

Very common problem. The impeller is the first thing you'll want to check. The impeller may look perfect, but I've actually had one where the brass hub came unglued from the rubber impeller so the hub spun but the impeller didn't move. Be aware that the wear inside the water pump housing can reduce the pump's efficiency, so you may still have water spitting out the back but just not enough. Check to see that the walls inside the pump are not scored or worn. Inside the housing there is a small replaceable cam that compresses the impeller blades, if that is worn down you lose efficiency. Another big thing to check is the sea water strainer. If it is not air tight, it will leak air into the system. The old Perko strainer fastens with wing nuts on two sides, if you get it tightned down at an angle, it'll break vacuum and pull air. And check the gasket in the strainer cap. If its cracked, you'll lose the vacuum and pull in air. Finally, pull off some water hoses and make sure the water passages are not plugged with scale.

I replaced the sea water strainer and the water pump. Problem fixed.

If you do pull apart the water strainer, be prepared to replace it. Ours was fine until
we checked it (while sorting out a failed
engine seacock) and then basically all the various bits which are supposed to form a seal
stopped doing so. It was impressive. The gasket had clearly been compressed, but cracked, and once uncompressed, came apart. It went downhill from there. We quickly decided to simply replace it. 20 year old metallic bits which are in a salt environment can easily be dodgy.

- David

This summer I had a totally random series of times that the engine overheated - sometimes just after long hours of satisfactory operation. I had replaced the cam last year and the impeller several times, even though it did not appear to be defective. The mixing elbow is only two years old. I finally caught up with a very good Yanmar parts distributor at Orrs/Bailey Island in Casco Bay. He cautioned me that any scratch big enough to catch a fingernail in the pump housing or pump cover could create these failures. I removed the cover, which did have some scratches inside, polished up the outside and reinstalled it inside out. End of problem. It was clear that the pump started pushing more water out the exhaust, and the problem has never returned.