yanmar 3gmf overheat and low rpm

[Dan Johnson]

Claude...

I picked up a gasket rebuild kit at WM for mine last fall when I cleaned out the strainer and couldn't get the clean-out cap to reseal without dripping. Seems to me that the cost for the kit was under $20.

Replaced the compressed and deformed gasket with the new rubber-like one that came in the kit and have had no problem ever since. Still have the two gaskets for the top and bottom of the strainer glass, but may not replace them as it's not leaking (sleeping dogs, etc.).

 

[Carl A]

I had the same problem

One thing to look at is the raw water intake on the outside od the hull. Mine wouldn't overheat until the ocean temps got to the high 60's ... The raw water intake hole was
half the diameter it should have been because of all the paint jobs over the years. So it
couldn't get enought water thru the opening when it needed it at higher rpm's. I scraped
out the old paint.. along with new exhausted elbow and impeller. No more overheated unless
there is seaweed in the strainer.

 

[rkssail]

I had the same problem

I had much the same issue with the engine overheating at speed (approx 2800 rpm) this past Sunday. After dropping her down to idle for a few minutes the alarm stopped. I checked everything on the sea water side (raw water strainer, water pump, hoses, belts etc.) and all looked good. She ran fine at lower speeds as I incrementally pushed her higher at 20 min. intervals, but once she was back to 2800 rpm for about 10 min. the alarm went off again. I was able to bring her home (about a 15nm trip dead into the wind and tide) at about 2200 rpm without setting off the alarm again. I didn't want to check the freshwater side until she cooled down and so I had a chance to go back to the boat today and check her out.
What I found when I took the pressure cap off was that she deifinitely looks like she needs more water/coolant. However, the overflow sub-tank was filled well beyond the full line. Shouldn't that water flow back into the main water tank? According to the Yanmar mannual, the sub tank should be positioned level with or up to 5cm below the main tank, but just eyeballing mine it looks to be slightly higher than the main tank. Could this be causing the problem or does the pick up tube coming down from the sub tank cap negate that? The boat is a 1986 and I've had her for 5 years and so far as I can tell it has always been positioned there and this is the first time I've had the problem. Any thoughts?

 

[kloudie1]

rksail.. The dip tube being below the surface is supposed to keep the heatex casing full.. If the overflow tank is full and the heatex is uncovered, you have an air leak somewhere between the casing and the overflow tank.. Could be the outer gasket of the radiator cap or a bad cap or a bad tube connection to the overflow or a bad tube itself..

 

[Don12364]

Symptoms may point to the Cause

Engine should turn at 3600 when on full load - presumably a little more when not in gear. You only get 3400 not in gear - maybe tacho error but maybe not.
Can you check this with an external tachometer as Vinny and others have suggested?

Raw Water:-
Engine raw water flow appears sufficient but engine now overheats at full throttle (but previously it was okay?)
The raw water flow should be several gallons per minute, perhaps 8 gpm at 3600 rpm. Can you check this? Simple to hang a bucket on the transom but not possible if the exhaust outlet is under the hull with the boat afloat. You have replaced the mixing elbow so it should not be the cause but perhaps pulling off the raw water feed to the mixing elbow and putting an extension tube into a bucket; then start and gun the engine and time to see how long it takes to fill to a 1 gallon mark previously put on the bucket. This should only be a few seconds and so be quick enough to not overheat the muffler.

If flow is okay forget about raw water pumping.

Fresh Water or Long Life Coolant:-
Presumably the overheat alarm is okay? Take it off and put it in a pan of water on the cooker and, using a simple thermometer and electrical meter, measure the temp it goes short circuit at. Should be just below boiling temperature.

Header tank gets full and does not empty back into the heat exch when the engine cools. Possibly a blown head gasket? Can you run engine with cap off and look for bubbles in the coolant. It needs to be topped up to quite near the top to see these. Give the engine a quick rev up to make it develop max cylinder pressure.
Arguments about whether the header tank is an issue should not affect the overheat alarm temperature or the engine running temperature because loss of pressure will, if anything, lower the temp it runs at. However a significant loss of coolant back into the header tank would.

If you have a calorifier for the domestic hot water check the piping to this is not kinked or otherwise obstructed thereby reducing the fresh water flow round the engine. Simply disconnecting both pipes at the engine and blowing through by mouth might be illuminating. You may also have an air lock. Try simply bypassing the hot tank coil and tubing by a short loop of pipe connecting input and output ports on the engine.

Valves, Timing, Head Gasket and Pistons:-
Engine will not reach full rpm when boat not moored to dock and boat speed is also down? Any defects of prop and/or transmission could cause this.
But if the engine was laboring we would expect black smoke - and the overheating which you do get - but the exhaust is clean?
Perhaps insufficient fuel is being supplied? However insufficient fuel is hardly likely to cause overheating.
This one is a mystery. It can be either one or other but unlikely to be both.

However it could be burned or maladjusted valves or their timing preventing the engine from reaching full power so the rack goes fully open to try to achieve full speed. Valve timing does not alter unless the engine has been opened up.
Just maybe the problem is not (yet) so bad as to cause a dirty exhaust.

You have tried just about everything else already so now, and in view of the possibility of a blown gasket or burned valves, I think it might be time to carry out a compression test and know once and for all if the mechanical parts of the engine are operating correctly.

I wish you luck with this most difficult problem.

 

[Lee Urbani]

Same Engine, same problem; twice

I have the same engine and same problem twice, both overheating causes were baffling to me and my mechanic.

The first time was a true overheating problem. The seawater pickup hose's interior lining came loose and restricted the flow. But here's the problem when the hose was disconnected from the impeller housing the water flowed without any issue. Once the hose was re-connected and under a little negative pressure from the impeller suction the lining collapsed and restricted the flow. It took me days to figure that one out.

The second overheating issue happened four years latter and was not a true overheating problem but was just as nerve racking. At higher RPM usually when I throttled up to get through the Kent Narrows draw bridge the alarm would sound. After passing through this narrow channel and back into the river, I would reduce the RPM and the alarm would stop. No other symptoms, we ran it in gear in the slip with mechanics on board, no alarm, I even purchased a laser thermometer so when it would happen I would go below and "shoot" different spots on the motor to see if I could spot the failure; all to no avail. In addition, we changed everything that one could change, even sent the heat exchanger out and had it "boiled" to remove any scaling, we did it all. This went on for months. Sitting on board one Friday night with an adult beverage, I realized that the only thing I had not changed was the sea water strainer. I thought perhaps the bronze casting might have developed a hole and therefore maybe some air was getting into the system; I told you I was desperate and drinking. The next day I went below and started to remove the existing strainer to see if in the light of day I could see any issue. As I was removing the strainer the overheating alarm went off, it scared the crap out of me until I realized the engine was off. As I lay there in the heat, sweating like a long distance runner, I reached into the engine compartment and touched the wiring harness in several places until I narrowed it down to a plug connecting the engine harness to the alarm system. Low and behold I could make the engine overheat without even running! It seemed that at a given RPM the vibration frequency would cause the offending connection/plug to short out and sent the overheating alarm a false signal. I changed out the plug and no more overheating.

All of the advice I have received and I have read is sound advice but if all else fails and you don't have obvious overheating problems look for one of these issues.

Lee