Discovery: Overheating at start up 08 - 40x Discovery

[Bob Bowker]

My 08 Discovery 40x w/ 350HP Cummins is getting hot when i first start out on a journey.
It will do this every time, but only when it starts off from cold. It will go to 215 to 220 and will start to go into shutdown mode, BUT if i stop and shut it off for 3 to 5 minutes and fire it back up, it will be fine all day & night. 100's of miles, doesn't matter, stays at 185 to 195.
Coolant expansion tank is full and there aren't any leaks.
My first though was thermostat, but last time it did it, i opened the side door and the fan appeared to be spinning kind of slow.
Anyone else have this issue?
Also, if its the thermostat, how hard is that to get to?
Thanks in advance

[wolfe10]

My first thought would be the thermostat starting to stick in the closed position.


Replace it!


Yes, it could be the fan control (assuming yours is clutched), but a thermostat mechanically sticking is still more likely and certainly easier/less $$ to replace.

[RichKimball]

My thought is that the Horton fan clutch is failing. The symptom you describe seems synonymous with failures I and others have had. It gets worse from here....sorry.

There is a vast amount of posts regarding this, including installing a Horton Guard Dog device inline at the clutch connector. Please do a search for this info here on iRV2. And best of luck.

[Lt Dan]

The fan should spin slower until the coolant temps reach about 210 then the fan should lock into high speed mode. If yours is not going into high speed, then the clutch is probably failing. There is a viscous liquid in the clutch that may have leaked out not locking it into high speed mode.

[RichKimball]

The Guard Dog prevents the fan from slowing too much so I’ve been told.
I know my temps rarely waver more than 10 degrees with it installed.
Also told that installing it can prevent clutch failure.