I had put a new factory 87 deg thermostat in approx. 6 months ago which fixed a cold running problem. Yesterday, I was idling for a few minutes in 75deg weather when I heard the aux fan kick on. I looked at the temp gauge and it was climbing toward the red. I shut down the car to look for a loss of coolant and I found nothing. I had the car flatbeded to my house. The coolant level is fine, waterpump is about a year old...no previous hot running problems noticed... Question...has anyone had a thermostat fail this quickly. The upper rad hose to thermo housing was not terribly hot. I squeezed it to see if maybe air was trapped. Should it be too hot to handle at operating temps? I think I will take the thermostat out and watch it open in hot water on the stove. Any ideas? Thanks I searched the forums first!
Eric

 

I have now tested the thermostat in boiling water and it seems to open and close properly. I will try refilling the system with coolant and see waht happens....!

 

Hello Eric,

Greetings from Chicagoland.

You seem quite knowledgeable, so my reply would be only a thought.

There is a coolant return line, running from the thermostat housing to the expansion reservoir. The inside diameter of this hose is relatively small, and you can check to see if it is blocked by starting the engine with the cap off and verifying that the coolant is running into the reservoir. This indicates normal operation.

There are also some small-diameter coolant lines, especially behind the engine block, passenger side. I do not know how to test these lines/hoses without physically removing them.

Ignition timing used to be a cause for overheating, until the advent of electronically-controlled ignition system, so this probably isn't the case for 850 as the ignition timing is not adjustable.

The upper hose should get really hot if the cooling system is operating properly.

When you install the thermostat back in, the notch on the top of thermostat must face forward, according to Haynes manual.

I too, would drain and refill the system. I might also drain the engine block, there is a drain bolt behind the engine block, a bit difficult to reach. I believe you'll need 18mm or 19mm deep socket.

Other than that, it could have been a fickle coolant temp sensor/gauge. I would disconnect the coupler, spray with WD-40, then wipe off excess (air duster works well) and re-connect.

Good luck,


JPN

 

The coolant temp sensor might be bad Giving a false reading.

 

Here is my diary/log of the procedures that I used to replace thermostat and engine coolant temp sensor.
https://volvoforums.com/m_16392/tm.htm

 

+2
Coolant temp sensor is probably bad. It's located right below the thermostat housing to the right when looking at it standing in front of the car facing the engine.

 

The drain bolt was actually 13mm. Sorry for confusion.


JPN