MD7A cylinder block temperature. Is the thermostat working?

[romain]

Hello everyone,
I am trying to figure out if the temperature of the front of the cylinder block that I measure with an infrared thermometer is too high when the engine has been running for approximately 1 hour at normal cruising speed.
The engine is a Volvo Md7A in a cape dory 28; always been in freshwater; the temperature gauge stays in the green zone and the exhaust water is just warm to the touch.
I measure the temperature of the front of the cylinder block to range between 180 and 200 degrees, and I am wondering if it is expected, or if the temperature of the cylinder block should stay around 160 degrees (the opening temperature of the thermostat) if the thermostat is working or the circulation of water around the engine is not decreased by a blockage in the circuit.
I have included some pictures to show the temperature of the front of the engine block at different points.

[Caral]

It’s been years since my CD 28/MD7A, but I think you’re good to go. Diesel engines are healthier when running hard and hot. I’m surprised your thermostat is 160*...I would’ve expected 180*. When I had a vane from the impeller blocking a water passage the exhaust sound was distinctly hollow, and the exit water steamy. The data you are reporting sounds healthy to me.

[ghockaday]

I agree, 160 seems pretty cool for a diesel. But you may be seeing a spot where the jacket is not getting a good flow. The Thermostat is in the water which would be kind of an average of what is passing though. I would not think 200 F was horrible either. dee

[sgbernd]

First comment, the bypass which holds the thermostat stays cool as some water bypasses the engine. So the places to check are the head and block. Second comment, the emissivity of the paint is less than 1, which means your thermal gun will read a lower value than it should. An easy way to get an accurate reading is to put some black electrical tape on the engine where you want to read and then read the temperature of the tape. Or, if you can find a dirty rusty spot, rust works very well to give an accurate reading.

I'd bet on blockage is restricting the flow to part of the block. On my MD7B, I had a situation where the gauge and Theromostat were fine but I could hear it boiling when I shut it down after running hard. Cast iron is an excellent conductor, both water and oil conduct heat well, and these blocks are thick so the whole chunk of metal should be roughly the same temperature. In my case, large flakes of rust had blocked some of the water passages completely, creating hot spots.

That said, for a 40 yr old engine, I'd not worry about it if you are below boiling, and even if it starts to boil occasionally, it doesn't really hurt it because the water pump is on the cold side and can't vapor lock. I took my engine apart, cleaned out the corrosion from all the passages, and put it back together, which worked great for a few more years, but I'd not recommend that approach. Rather, I'd keep it running as long as it remains reliable. The problem with cleaning it out is that the corrosion is already inside and just clogs the passages up again after two or three years. So it is lots of work but not a permanent solution.

Also, it depends on the type of sailing you do. For day sailing, the engine never works very hard for very long so it can tolerate reduced cooling. Cruising on a schedule where you need cruising power for 20 hrs straight, is a different situation. In my case, I could keep it cool by reducing the rpms but as the situation degraded, so did the useful power.