What would cause a diesel engine to run cool?

[Valiente]

Nine posts since 2000? This is not a person who asks for help often!

Another point to add to your mulling: Today I ran my old boat from one yacht club to another to do a repair. The Atomic 4 has the fresh water thermostat rated at 180F (good for the engine, but too hot for salt water, where it throws a deposit, and where the 'stat is rated 140F). Anyway, I was cruising at five knots for an hour, and I couldn't see the engine temp get above 145 F.

Then it struck me: The water temperature in Lake Ontario in May is about 9 C/ 45 F. That's pretty cold water going through the block. By July, it will be 21 C/70 F and I'll be getting readings of 175 F and presumably, the thermostat will be mostly open.

Could it be that the raw water cooling circuit is quite cold at the moment, meaning the engine coolant can't warm up unless that circuit is restricted?

The hot water circuit might be at issue, but it really depends on how it's plumbed. Mine not yet connected into the engine, which is coming out for inspection, but I am assuming that it is the raw water circuit that is heated by the coolant (in turn heated by engine heat) which in turn transfers heat to the potable water in the hot water tank. Otherwise, you'd run the coolant itself through the coils in the hot water tank, and that would be a little dangerous. A failure of the raw water circuit...post-heat exchanger...would be noticeable by the lack of water at the exhaust, the change in engine note, and water in the bilge....nasty but fixable if noted before things post mixing elbow start melting.

[MedSailor]

So I'm not an expert, but I'll throw a few things into the mix:

The thermostat's job is to restrict cooling water for a while until the engine warms up, after that it opens and allows maximal cooling. This is to speed warm up and no thermostat is designed to close off at cruising or max rpm it's function should not affect your max operating temp.

If you do want to check the thermostat remove it, put in in a pot of water on the stove and turn it on. Have a thermometer in the pot and record the temp when it pops open.

But, the thermostat isn't the problem is is it? Are you reaching your rated RPM? If you're lugging your engine (running it under rated rpm) you will run cold and have all the problems already listed. Currently we aren't getting up to rated rpm and this summer we're going to troubleshoot this very problem.

Any of these problems can cause you not to reach RMP:
If your fuel is old or restricted
If your throttle cable is not at full travel
If your prop is too large or has too much pitch
If your racor filter element is too fine

Poor engine/prop selection is often the cause. Even the Dashews recommend a large prop turning slowly for "fuel economy". I've know people who repowered to get this effect at the expense of their engine.

Medsailor

[julied]

Thanks for all the great suggestions! A couple of things: the water heater is not new, nor has it been changed. It's just the first thing that we noticed was wrong. I think we are not getting efficient water heating because the engine is running so cool.

I bought a new thermostat from Westerbeke. Used the one called out in the manual (180 degree operating temp). We replaced it one Friday and today tested the old one. It opened at 180 degrees, just as it is rated. So I guess I've got a spare!

Wasn't positive that I had a real problem be am obviously concerned. I'm going to take Camaraderies advice and call a mechanic tomorrow. Hope my bank account can handle it...

Hopefully getting the temp up to an appropriate temp will take care of my inefficient water heater too! (fingers crossed)

[MedSailor]

Let me know what you find out. I have a similar issue.
MedSailor

[Rockter]

I reckon the thermostat is stuck open. That would do it.

[timebandit]

Originally Posted by MedSailor

So I'm not an expert, but I'll throw a few things into the mix:

The thermostat's job is to restrict cooling water for a while until the engine warms up, after that it opens and allows maximal cooling. This is to speed warm up and no thermostat is designed to close off at cruising or max rpm it's function should not affect your max operating temp.



Medsailor

The job of the thermostat is to open and close constantly to change the water flow to keep the engine at a cirtain temp.
If the engine is running hard and creating lots of heat it may open max and then close to a mim at an idle in neututral.