What would cause a diesel engine to run cool?

[julied]

I have a Westerbeke 70 hp diesel that should be running between 170 and 190 degrees but instead is only getting up to 150 degrees. What would cause a diesel to run cool? I changed the thermostat (operating temperature is 180), I checked the coolant level and even checked to make sure that the temperature gauge was working properly. No problems with any of them.

What are the consequenses of a cool engine? More fuel being consumed?

[buddabelly]

I can make mine run cool or hot by adjusting the ball valve in the recirculating line on my 5411. On mine the water recirculates until the thermostat opens then sends it out the exhaust. When valve is all the way open the water takes the path of least resistance and continues to recirculate and very little goes out the exhaust. As I close off the valve the water is less able to recirculate and more water goes out the exhaust and the motor runs cooler. I need to adjust the valve through out the year as the water temp increases.

[erps]

Sounds like julied has fresh water cooling, as it was mentioned that the coolant level was checked. How was the accuracy of the temperature guage checked? Also, maybe the thermostat isn't sealing well so enough water leaks by to keep it from warming up? I frankly don't think that running at 150 degrees vs. 170 degrees will make that much of a difference, but I'm a backyard mechanic, not a diesel expert.

[julied]

We checked the temperature guage by dipping an engine thermometer through the radiator cap and then warming the engine up. We checked the temp of the coolant and the temp on the engines temp guage every 5 minutes for about 1/2 and hour. They only read about 5 degrees different which I understand is okay since the coolant temp will be different in different areas of the engine.

We changed the thermostat and the gasket yesterday. We've got a good seal.

I'm not sure that I actually do have a problem but my engine manual says to make sure it runs between 170 and 190. I didn't even realize that there might be a problem until tracing a problem with our hot water heater which is not heating efficiently off the engine.

[halekai36]

There is no better device for troubleshooting engine cooling issues than an infrared thermometer. Simply aim and pull the trigger and it tells you the temp.. Get one and you'll be amazed at all the uses you find for it. Ignore the price at that link, it's the first one I came across, as you can pay a LOT less money for them than $99.00...

Infrared Thermometer

[Stillraining]

What was the rating on the new thermostat?

[camaraderie]

You may have a loss of compression causing it not to come up to full temp.
Cool temps can be the equivilent of idling the diesel for extended periods which can be damaging as follows:

This is because the engine is not allowed to reach operating temperature, where harmful combustion by-products are eliminated. This acidic by-product accumulates in the oil, where it chemically attacks the engine bearings.

Here's what happens when a diesel engine is idled:

1. Timing gear backlash [gear rattle] is maximized, leading to increased wear
2. The cylinders get glazed, leading to low compression
3. The engine is run at below operating temp, leading to carbon accumulation in the combustion chamber, injector tips, piston rings, piston crown, valves heads, seats, and stems, exhaust manifold, turbocharger, etc. Carbon on these parts will lead to accelerated wear, and possible piston ring sticking and low engine compression.
4. Also, the injected fuel gets past the piston rings, because the engine is not at the correct temperature, and dilutes the lube oil. This compromised oil is sent to ALL the engine bearings, where it increases bearing wear.
   

There's more, but isn't this enough to convince one that engine idling should be avoided?

Yanmar [for one] says not to idle their engines for more than 5 minutes. Bottom line on engine idling - it's great for the engine repair business.

If you've checked out the obvious issues...get it to a mechanic before you really end up with a big bill.

[erps]

The fact that you've introduced a hot water heater to the cooling circut may have changed things. Depending on where you tapped into the fresh water cooling circut, you may have taken the thermostat out of the loop inadvertantly. So even though the thermostat is closed, limiting the amount of water going to the heat exchanger, you may be sending it to the heat exchanger in your hot water heater instead. You could troubleshoot that by pinching/closing off the loop to the hot water heater and see if you get up to operating temperature then.

Also, the temperature of the coolant in the heat exchanger should be lower than the coolant temperature in the engine, because it's in contact with the cold raw water in the heat exchanger tubes. That's what the heat exchanger does, cools the fresh water down. I'd bet a buck that your hot water heater is probably involved.

[Stillraining]

Good point Ray...I didnt think of that..

[timebandit]

Pull the thermostat and place in a pan with water and use your thermometer to see where it is opening. The thermostat is what controls the temp of the motor. When you have it out look at the bottom of the pill and the temp should be stamped there.
If the water is very cold and the flange on the thermostat has a small bleeder hole that may be enuff to keep the enngine from comming up to temp.
If the test show the thermostat is opening ok then pru a pop rivet in the hole.