Engine over temp at idle
 

Went out for a sail Saturday and when returning to the slip I left the engine running at idle until I had the boat secured. (This is something I always do) While idling the temperature started to rise above normal and quickly went back to normal when I increased the RPM. Not sure what to look for. I have not changed the impeller yet this winter. I have cleaned the sea water intake filter. Engine is a Universal 12hp diesel. Any recommendations?

Re: Engine over temp at idle
 

Must change impeller, whether the culprit or not. Is your seawater pump driven by a belt? Worn or loose belt will dramatically reduce flow rate. Overly tight belt will wear too quickly.

Re: Engine over temp at idle
 

Water flow may be restricted at the mixing elbow too, but by all means change the impeller first. If you find missing/broken impeller bits, be sure to track them down. If it's FWC they'll likely be at the inlet side of the H/E..

Re: Engine over temp at idle
 

Was about to suggest an airlock in the primary circuit, but if you have a 5411, you don't have a primary coolant circuit. Raw water cooled only.

Re: Engine over temp at idle
 

Along the lines of what MarkSF is suggesting, there are a few conditions where you will get virtually no water flow at low speed but some at higher speed. This can be caused by an air lock. Additionally, your water pump can put out no flow at low rpm if the impeller is very leaky. Raising the rpm will build pressure enough to get flow.

Did you have water coming out the exhaust when it overheated?

Re: Engine over temp at idle
 

The exhaust water looked to be about normal when I looked. I am not sure that was not after I added some RPM. I always check at start up to insure it is OK. I had planned to change the impeller this winter but the weather has been good enough to sail so when I went to the marina I went sailing instead of doing it. I will change it this Friday.

Re: Engine over temp at idle
 

Some what similar problem, but I get no flow:

Did some maintenace work on the engine, everything was running fine before I did the following items on Yanmar 3GMD (direct salt water cooled):
1. Adjusted valves.
2. Replaced three engine head/block zincs (more than 50% so cleaned and replaced).
3. Replaced raw water pump impeller.

Went to start engine, started up fine. About a gallon of water came out of exhaust and then nothing- higher or lower rpm on engine still no cooling water. Thought maybe the cooling water seacock was clogged so I attached a bucket (that I use to flush the system with- it puts a head of water to the pump inlet- so it primes the inlet) of freshwater and started the engine. Still no water. I ran out of time, will try to trouble shoot a little later today. Never had a problem with air lock or pump priming (after impeller change) so wonder if when I pulled out the zinc, debris clogged the outlet water passages? Where the thermostat is is very small areas for water passage. Or maybe the debris clogged my mix elblow. Still suprised what ever happened stopped all the flow. The pump belt is good.

Any ideas? I can trace the system, it just takes time.

Will post what I find.
Regards

Re: Engine over temp at idle
 

Casey, I've been there and it has always turned out to be the impeller. The first gush of water was probably what was sitting in the exhaust. Pull the impeller and get a new one. It sounds as if you've unintentionally run it dry for a bit now. Clean out the interior housing of the pump with a scotch brite pad and load up the new impeller with glycerin. Overload it! You may also want to insure that your seawater strainer doesn't have an air leak.

Re: Engine over temp at idle
 

Did not have time to work on engine yesterday (but I did get the AC to work on my 87 bmw 325e after many hours of trying to figure out the wiring when using 5 different wiring diagrams that are all supposed to be for the same car- turned out to be a bad unloader relay).

I will work on the yanmar on Friday evening. Plan to first make sure all hoses and passages are clear and then will move to the impeller. Have heard stories the impeller (even a new one) can spin on the shaft. I did load up the impeller with silicone grease prior to install. I had greased the one I removed with a water proof grease and it looked almost new when I removed it (been in service for 2 years). FWIW I had accidentaly run the engine a couple times without the seawater valve opened for several minutes about a year ago and about 6 months ago, and it apparently did not hurt the impeller- I try not to do this but mistakes happen. I use genuine yanmar impellers. I will stick to the water proof grease as this does seem to help save an impeller.

Regards

Re: Engine over temp at idle
 

If my silicon grease, you mean the sticky stuff in a tube, that may not be a great idea. It should certainly help prime the pump, but I would worry what happens to it as globs move through the heat exchanger or block. Glycerin or dish soap, which are less viscous, are the norms.

Running the impeller dry will do damage, even if you can't see it with the naked eye. The vanes could let go on the next run. Best to always replace an impeller that you know you ran dry for more than a few seconds to prime.

Could be a blocked intake, but it would be very unusual. Blockages in the exchanger are more common. Or, as I said, if the seawater strainer has a leak, it may not allow for the suction necessary to draw the raw water.

Good luck.