overheating

[Ed Shankle]

Ok mechanics out there, I need some suggestions.
I took the heat exchanger out for a cleaning. Repalced the hoses While I was at it. After reinstalling, the engine overheated. No surprise. So next I pumped the fresh water side with the mini Jabsco pump, as per the instructions in the FAQ's. Still overheated. Next I replaced the thermastat. Here's where it gets interesting. On the mooring with no engine load, the temp got up to 165 - 170. Usually with no load it wouldn't get past 130 - 140. Normally it doesn't get higher than 165 running at 2500 rpm under load. But at least it wasn't overheating. Kept checking it, and it stayed steady for about 45 minutes. Then when I wasn't checking so frequently, the engine alarm goes off. It was overheating.
The hoses that should have felt warm were warm. The raw water out the exhaust seemed ok, but I didn't notice it after the alarm went off, I just shut the engine down right away.
It's a m25xp.
What do I try next?

thanks,
Ed

[vmenasce]

There are two sides to the heat exchanger, fresh water and anti-freeze. It sounds like there is air or a blockage in the cooling circuit that runs from the engine to the heat exchanger. That could be difficult to debug.

The first thing I would check is the thermostat. It may have a problem. Try removing it and letting the engine run "cold" without the thermostat. If that solves the problem, then you could try replacing the thermostat with a new one. You could theoretically run the engine without a thermostat entirely. It will not overheat. However, it will run much cooler than recommended.

Good luck.

Victor.  :confused:

[Stu Jackson]

Ed

If you've removed and replaced your HX, you know how to do it "quickly."

Are you sure that you completely cleaned the inside of the rw and fresh water inlets inside the HX?  

Most people clean the "innerds," but may neglect the connections, much like electrical problems.

My experience, posted earlier on the 'site, is that there could be a "crud buildup" inside the 90 degreee elbows into the HXs.

Make sure, before replacing the HX, that the elbows on the HX are clean INSIDE, too.

Good luck.

[Ed Shankle]

Guess I got my terms a little mixed up. I did use the Jabsco pump between the housing inlet and the coolant reservoir. Seemed like a good flow. I checked the raw water (salt) impeller today, and it was ok. Of course that created another problem. A screw into the Sherwood pump stripped. It's a brass housing, so I guess that is not surprising. This is the 2nd screw that has happened with. Luckily, a slightly longer screw took care of the problem. Anyway, back to the overheating; I ran the engine today for about 1 hr at 1500 rmp, occasionally up to 25 to try and push any air pockets through. It stayed between 165 and 170 degrees, but that still is too high for for neutral, no load. So something still isn't right. I'll try taking the thermastat out and if that doesn't change things, I guess it's out with HX again.

[karista]

I am having a very similar overheating problem, have done virtually all that has been mentioned including replacing the fresh water recirc. pump, but its still running at about 180 at load (2500 rpm). I wonder whether the warm gulf waters (84 degrees) is unable to cool the engine to its normal 160 degrees! Any thoughts?
Bernd Mueller

[Ken Juul]

Don't forget to burb the coolant lines to the hot water heater.  With both the enter and exit hoses relatively high on the engine it is easy to get a bubble in that part of the system.

[SEACRECY]

"Normal" is 160 degrees? I know mine runs warm- 180 degrees at a constant 2000rpm and creeps toward 200 at higher RPMs. At what point does the engine alarm go off?

[jentine]

Diesel engines reach their greatest thermal effeciency at 180 degrees F.  Lower or higher temperatures affect the maximum horsepower and torque.  My M35 runs at 170 at cruising and 190-200 at full throttle.  I would be concerned if the temperature did not approach the norm under load.
Jim Kane

[dsavary]

The temperture should run a the thermostat setting, unless the water temp is so warm that it can't cool the fresh water fast enough. I am running a M35 with 170 being the operating temperture, I have had problems attending this point after I added a larger raw water strainer from Groco. I found that if I lower the strainer by 2", replace the 90 degree elbow that had a 5/8 to 1/2 reducer between the threads and hose barb and lower the hose height loop going to the Sherwood pump I eliminated by elevated temp during 2200 rpms or higher. SHANA #1138
dsavary@verizon.net