Engine running cold

[Sclerotic Hen]

Over the last couple of days I noticed engine was running cold. Temp gauge hardly rises while driving although it does creep up while vehicle is stationary with the engine running (but it never reaches normal running temperature). This seems to stop the car engaging overdrive as the engine (or temperature reading) isn't warm enough.
So that I know what to listen out for from the mechanic when I drop the car off at the garage does anyone know what might be causing this. Thermostat? Temperature sensors? Loose wire?
The vehicle is a 3rd gen diesel (intercooled).

[Sir Plug]

Over the last couple of days I noticed engine was running cold. Temp gauge hardly rises while driving although it does creep up while vehicle is stationary with the engine running (but it never reaches normal running temperature). This seems to stop the car engaging overdrive as the engine (or temperature reading) isn't warm enough.
So that I know what to listen out for from the mechanic when I drop the car off at the garage does anyone know what might be causing this. Thermostat? Temperature sensors? Loose wire?
The vehicle is a 3rd gen diesel (intercooled).

Check the fan isn't siezed. If it doesn't move when the engine is off, then it's siezed. When the engine is hot, the fan should be tight, when the engine is cold, the fan should be loose (spin). Another quick check is to get the car as warm as you can (take it for a 10 minute drive) stop, turn off and check the temp of the top and bottom hoses. If both are warm but not hot, then the 'stat is probably stuck open. If both are hot, then the fault is with the sender or gauge. Normally, you'll be looking for a new 'stat. Get a lower temp Prado stat fitted as these perform better with towing, hill climbs, etc......

[Sclerotic Hen]

Thanks for that Sir Plug. Just checked the fan with engine off and cool and it moves freely. The vents in the cab were only blowing very slightly warm air last night after a ten mile run when they would normally blow out quite hot with the heater cranked up. So is this looking like a thermostat problem?

[TonyN]

normally diesels run cool when idling, and get hot as you speed up. If the stat is jammed open then it will still heat up after 10mins at motorway speeds, but will cool off as you slow down.

I'd check your fan as said above, but also check the coolant temp sensor. Try the fault codes as if the ECU thinks the motor is cold it will alter how the gearbox changes and as you say won't let O/D select.

[Sclerotic Hen]

Thanks Tony. I was thinking, as the heater wasn't particularly blowing hot that it was the thermostat stuck open. I don't get to get the truck up to overdrive speeds on the island often (200m blast on the duel carriage way going to work is about it) so it wouldn't be an immediate issue if I didn't have to come to the mainland on Tuesday and up the motorway. Just have to see how friendly the garage is feeling on Monday morning.

[Sclerotic Hen]

Problem solved Got the garage to replace the thermostat and give the car a good service while they were about it. Comes up to correct running temperature now and engages overdrive ( in the two places on the Island that you get up to that speed). Thanks for the advice Sir Plug and Tony.