Bad Thermostat or Bad Fan Clutch?

[tbryanh]

The engine temp in my 6.9 only warms up to ~140 instead of 190 like it used to.

It seems the problem could be either the thermostat or the fan clutch.

Any way to determine which it is?


Side note: I have a new Stewart Warner mechanical temp gage, so I am pretty sure it is reading accurately, but I do plan to pick up a laser thermometer to verify its accuracy.

 

[Trying my best]

Maybe an open thermostat? I wonder if you pull the rad cap off and see if the coolant is circulating when the engine is cool would that tell you if the thermostat is stuck open? Just a guess, not sure if this is a good way to test or not.

 

[Kevin 007]

Probably a bad t-stat. Replace it regardless just for peace of mind. Use only a Motorcraft of IH t-stat.

 

[tbryanh]

Yeah. It is probably a bad thermostat, but it is easier to change the fan clutch that the thermostat, so I would like to be able to find out which is bad. Maybe I will be lucky and it will be the fan clutch.

 

[The Warden]

Yeah. It is probably a bad thermostat, but it is easier to change the fan clutch that the thermostat, so I would like to be able to find out which is bad. Maybe I will be lucky and it will be the fan clutch.

Are you hearing the fan going at all times?

The fans on these engines are unmistakable...when the fan spools up, it'll drown out the engine and almost sound like a jet engine on start-up If your fan clutch is stuck ON, I think you'd know it.

Furthermore, these fan clutches don't fail ON very often...usually, when they fail, they won't engage until a very hot temperature or won't engage at all.

Based on that, I have a rather strong feeling that you need a thermostat...

Also, finding a replacement fan clutch that's any good is not an easy proposition...unless you definitively have the fan clutch singled out as a problem, I would refrain from replacing it for now.

 

[1986IDI6.9L]

yeah sounds like your t-stat is stuck wide open

 

[jaluhn83]

Concur with a bad t-stat. With the fan clutch stuck on you'd for sure notice the sound difference, and I really can't see the fan being on causing that low of a temp. The thermostat would just stay closed, maybe at idle there's be enough airflow cooling to keep temps down, but not once you started driving.

 

[Kevin 007]

You won't be "lucky" if its your fan clutch...they are $300!!!!!!! You could try running with the fan off just to ease your mind,