Volvo D4-300 - Coolant temperature too low

[aculverwell]

Help, I need the Ranger Tugs Brain Trust!

Several times recently, the coolant temperature on my D4-300 dropped from the normal operating temp of 185 down to 60-70 degrees and stayed there (fluctuating a small amount) while underway. When I restart the engine later it seems to work normally and goes back up to 185, only to sometimes drop back down to an unrealistic temperature again. This has happened 3-4 times.

My best guess is that this is due to a failing/faulty thermostat, or possibly a faulty sensor. I have attempted to replace the thermostat, but ran into some challenges getting the *^&$#&$ thing out. It looks like I need to remove the entire oil cooler to access the thermostat, which sounds like a messy hassle. Before I jump off that cliff, I'd like to find out if the thermostat is actually the most likely cause of the issue.

Has anyone seen this before? Any suggestions?

 

[Full_O_Fish]

Volvo D4 and D6 both have dedicated ECT (engine coolant temperature) senders separate from the thermostats. Not sure where it's located on the D4, but on the D6 it's underneath (need to remove) the black engine cover plate near the coolant fill. It's a simple two wire/pin plug to the sender (thermistor).

Corrosion on the plug or a failing sender could cause the reading you're setting.

 

[S@LTD]

I had the same issue and yep it was the coolant temperature sending unit that went bad as mentioned by Full of fish. I purchased a new one separately and had Bellingham yachts change it for me on my way up to Canada.