Yanmar 2QM15 Overheating Problem
 

Okay here is my problem... After cold starting my 2QM15 everything runs fine for five minutes. Then the dreaded temperature alarm sounds and it is time to shut the motor off. I have taken several troubleshooting steps as discussed here on other threads. First I back flushed the intake side of the water pump inlet. Still overheated. Next I pulled the water pump outlet pipe to verify flow. It has good pressure and just as much water is exiting the rear of the boat as it did when it ran perfectly fine 6 months ago. I filled the cooling system with 3 gallons of strong oxalic acid, waited ten minutes and flushed it with raw sea water.Still overheated. After that failed I removed the thermostat and verified that it is opening and closing at the right temp (It does). I replaced the housing minus the thermostat and pulled both zincs (front and back) dipped them in Oxalic acid for ten minutes and replaced them. Both zincs were about 75% intact. Started the motor up and within five minutes it overheated yet again. Ran a second three gallon flush and rinse as I did above and it still overheats. Not sure what else could be the problem. Any ideas guys?:confused:

Have you tried back-flushing the engine? Could be there are blockages and the water is flow is reduced through the cooling passages and is bypassing the engine. The Universall 5411 in my previous boat had this problem and back-flushing would usually fix the problem.

Have you ever put your hand on the engine block when the high temp alarm goes off? The block should be very warm but not so hot you can't put your hand on it for awhile. My point being, could your alarm be malfunctioning indicating trouble when there isn't an issue?

Does it overheat at idle after just five minutes or when in gear under a load?

Thanks guys
 

I have not tried this because another guy did that on this same moter and got water in the mixing elbow, which in turn got the injectors wet. I agree this may be the problem given my previous troubleshoting steps.

FarCry, I have tried leaving the slip and it seems to overheat even more rapidly.

Thank you both for your input. :)

Do you have waterflow through the engine or all water goes through the bypass line?
Rubber hose in front of the engine, from front zinc housing to collector is a bypass line. Plug it somehow, remove the thermostat and try to run the engine. Most likely you will get no or very little water. If so, your engine is blocked by buildup. If water is running freely, you need a new thermostat.

More than likely the water passages in your exhaust manifold are plugged. If the manifold hasn't been pulled and cleaned in a few years, it is most certainly plugged. You will have to pull it to unplug it.

When the engine is first started, and the engine is cold, much water is by-passed by the cooling passages of the engine and the exhaust manifold through the bypass line as noted by CrazyRu above. The thermostat should be closed during this time. This allows the engine to warm up faster.

After the engine is warmed up, the thermostat opens to allow cooling water to enter the cooling passages in the exhaust manifold, cooling the manifold, exhaust gases, and the engine. If the exhaust manifold is plugged, cooling flow is EXACTLY the same as if the thermostat didn't open.

You will need a thermostat housing gasket, a gasket for the exhaust manifold to engine connection, a spacer that goes between the manifold and the engine where the two aft-most studs for the manifold are, a gasket for the blanking plate on the forward end of the manifold, and a gasket for the flange (plate) on the aft end of the manifold where the mixing elbow attaches. If you have the 45 degree mixing elbow, and it is good shape, but it hasn't been off for a while, do not try to disconnect the elbow from the flange on the aft end of the manifold. That flange wasn't available as a spare last time I checked, and if you bugger it up, you may have to repair it or have a substitute made. Pull the manifold and mixing elbow off the engine, then disconnect the mixing elbow and its flange at the manifold connection, pull the blanking plate off the forward end of the manifold, and start digging out the corrosion with tools that won't damage your manifold. Replacements aren't available. Inspect it carefully before you start. They are prone to crack with age and the thermal stresses of their life, especially if they have been overheated and then cooled rapidly while still hot. Cracks can be welded or solder repaired.

The hardest part of this job the first time I did it was getting the mixing elbow and exhaust hose connection apart. I tried to take the hose off the elbow as the first step. Next time I did it, I pulled the manifold and exhaust elbow off the engine so I could pull it forward off the hose. MUCH easier.

Hope this helps.

Mystery solved with the help of board members
 

I did as you suggested and plugged the hose going into the upstream side of the thermosat housing with a three inch long piece of 1/2 inch dowel and hose clamped it in place. Started up the motor and behold no water comes out of the exhaust. Either the exhaust manifold, three way joint, or the block is stoped up by buildup. I placed a call to Pacwest Marine and Industry Here in San Diego and a tech offered some suggestions, first off removing the three way joint that contains the foreward zinc and secondly removing the exhaust manifold for cleaning of the passages. He stated that the exhaust manifold is the most commonly blocked. I am trying to figgure out how to take the hose fittings off the 3 way joint as they must be removed prior to unscrewing the joint itself from the block. It almost looks like a special tool is required to do so. The tips have notches on them. It looked like I could almost use a 3/8 flat blade screwdriver but the blade is not wide enough by about 1/8 of an inch(any ideas on that one?). I will be taking the exhaust manifold off tomorrow morning.
Many thanks to everyone for their help. Without you this would have taken me longer to figgure out.

I got the hose fittings off by using a $5 peso piece held in a pair of Robo grip channel lock pliers. The three way joint is sitting in a jar of oxalic acid. :)

Today I pulled the Exhaust Manifold with the mixing elbow still attached. Cleaned her out real good with a phillips head and oxalic acid. The whole thing is a bit rusted externally so I coated it with POR15 to prevent further rusting. Tomorrow I intend to put on another 2 or three coats and then re-install it. Can I use RTV or is it better to buy gaskets?

Here is the parts list I intend to order... I have a U shaped elbow not the 45 degree version. Do I need to clean the mixing elbow too or just the manifold?
Should I use a product like Carbon off to clean out the soot in the manifold?

124770-13171 aft exhaust flange $10.07
124770-13181 forward flange $10.06
124701-11911 Manifold Gasket $10.29
124770-13131 Mini 2bolt Gasket $2.05
27210-200300 Zinc $4.05
23414-250000 Zinc thread packing $0.96
104211-49160 Thermostat gasket $1.53
27210-200300 Aft Cylinder Head zinc $4.05
23414-250000 Aft Cylinder Head zinc thread packing $0.96