Yanmar Diesel overheating puzzle

[Mariellec]

I have a Yanmar 3GMD (raw water cooled) with 3700 hours on it. For some reason today, it has been persistently overheating. I have done the following:
- checked the strainer (three times) - it is sparkling clean, not even any slime - the water here in Alaska is cold.
- took the hose off the strainer and, with the seacock open, blew back into it to make sure nothing was stuck on it from the outside. I heard my bubbles against the hull and my blowing went through.
- watched the exhaust - water is coming out, although I cannot tell if it is as much as it used to be.
- took out the thermostat and tested it by heating it in a pan of water. It seemed to work just fine.
- Ran the engine with the thermostat out and it still overheated. so it is not the thermostat.
- Took apart the water pump (twice) - it looked fine, but I did replace the impeller just in case. the belt tension is fine, and the pump is turning. Water comes out of the hose that leads from the pump...I left it unhooked and started the engine to check that.
- took off the hose that leads from the exhaust manifold to the exhaust/mufflet and blew thru it - totally clear.

The engine starts and runs fine, and if I idle along, it does not seem to overheat unless I rev up and try to go more than 4 knots. Then it overheats.

Is it possible that there is a problem with the sending unit??? Do these things go bad?
Nigel Calder's book says to run some oxalic acid or phosphoric acid through the block to flush it - thoughts on this?
Finally, someone suggested it could be a head gasket (please say it ain't so)...how would I test for that?

Would so appreciate any help!!!

[Fence Man]

Sending units can go bad. You can double check it with an infrared thermometer. Leaking head gasket will generally, but not always, show water in the oil. I have a Yanmar in my primary post driver. It has, in the past, run on the warm side while under load. It has always been a slipping belt on my machine. Good luck with your repairs.

[Vasco]

Check your mixing elbow. It might be clogged up.

[DeepFrz]

When you had the exhaust hose off did you check the mixing elbow to make sure there is nothing in it restricting water flow?

[hugosalt]

No diesel mechanic here, but have maintained my 3GMC raw water cooled (C= saildrive)
for 31 years.
Checking with a IR thermometer good idea, raw water motor should run around 140 degrees. I take readings at exhaust manifold.
Raw water out put is rated at 4 gallons per minute @1000 RPM's and 8 gallons per
minute @2000 RPM's.
I never was able to get that out put but @ 3 gals. and 5 gals. respectively I would run all day with no problem. Put a bucket at water exhaust exit and measure.
Earlier this month I noticed raw water out put was low, I changed exhaust elbow
(and removed exhaust manifold and cleaned out some carbon build up)
Water out put back up to (my) normal.
I also flush motor every 5 years or so with Rydlyme (see detailed steps from Mainesail
here on CF or sailnet and follow). Important as when saltwater overheats creates salt deposits in small passages. The Rydlyme flush will clear up.
If an exhaust elbow is in your future, nice stainless steel elbows for Yanmars from
exhaustelbow.com at reasonable cost.
Hope this helps,
Hugo

[hugosalt]

Just realized to ask...how long have you owned boat,
(as others above have asked about) exhaust elbows need to be changed out as often as every 4-6 years. They often times rust internally or have carbon build up causing
overheating and sometime catastrophic engine failure. (see thread started by
1 boat2many today.)

[Captsteve53]

Run the product SALTAWAY thru the system, but money on the riser being corroded/blocked by build ups, as above replace 3700 hours means it's due!!,

Careful with just removing thermo stats to resolve issues as many are what call "double acting" which means they open to allow water to flow one way but in this action they close another gallery thus forcing water through the system, if this closed gallery is not plugged then you dont solve any issues!

Being raw water cooled your thermostat may not be a double acting type but pay to check and just for forum advice

Cheers Steve

[atkinsoa]

Had a similar problem with my Yanmar 4JH2e. After replacing the raw water
strainer, cleaning the impeller parts out of the whole system, replacing the exhaust mixer drum (what's it) and re-routing the hoses it still overheated, so
I replaced the raw water pump body, and it has worked fine from then on.
The other parts needed replacing, but were not the problem.

[DeepFrz]

Originally Posted by atkinsoa

Had a similar problem with my Yanmar 4JH2e. After replacing the raw water
strainer, cleaning the impeller parts out of the whole system, replacing the exhaust mixer drum (what's it) and re-routing the hoses it still overheated, so
I replaced the raw water pump body, and it has worked fine from then on.
The other parts needed replacing, but were not the problem.

Yes, the raw water pump is a very good bet. Its also possible that the impeller installed was not quite the correct one but pump wear could be the problem.

[Captndave1]

I had the same problem with my 3GM raw water cooled engine. I removed the exhaust manifold and removed the cover to find the water passages almost completely blocked, cleaned all the crud out and reassembled. all is good.

[Scaramanga F25]

I did have a similar problem, no overheating at lower rpm and overheating at higher rpm. Small air leak between gate valve and water pump. I my case bad cork gasket on Groco filter.

[mikedefieslife]

I has a similar problem recently. Turned out the lid on the strainer wasn't tight enough to allow the water pump to do its job. i.e it was sucking in air from around the lid.

It was held on by a wing nut, that of course fell into the bottomless pit that is the engine bilge. Bought another two, which was just as well because one of those ended up down there too.

[IslandHops]

I'd bet it's your mixing elbow. There are two types that were prevalent on the 3gm series, an upside down "U" and an angled tube. Exhaust can crud up where the raw water flows through. They can be cleaned by removing and soaking in an acid mix (you'll need to look that up), but it's easier to replace the unit and gasket.

It's also possible that your raw water impeller deteriorated and shed some rubber which is clogging the tubes in your heat exchanger, and sometimes even these can build up a layer of scale, reducing flow.

But I'd still put money on the mixing elbow.

[wesevans]

you have received many items to check. All are valid but if none fix the problem check the freshwater side you may need to flush the fresh water side and engine block. Coolant needs to be changed and the system flushed every couple of years.

[DeepFrz]

Originally Posted by wesevans

you have received many items to check. All are valid but if none fix the problem check the freshwater side you may need to flush the fresh water side and engine block. Coolant needs to be changed and the system flushed every couple of years.

Hard to do when he doesn't have a fresh water side.