Technical X250 - engine temperature falls on descend -> cabin goes cold

[subjectx]

Greetings,

So, I have noticed, that when outside temperatures are around 0C°, whenever there is downhill, temperature gauge show temperature decrease for at least one bar (one eighth) and at the same time cabin, although I have max heat and vent on 4, goes quickly cold, really cold. Then when downhill ends, it takes about a km for engine temp to go back to the middle of gauge.

Otherwise no other problems noticed around temperature or consumption..


What gives? Is there a way to diagnose this without removing thermostat? I saw that can be pain in the ass..

Engine is 3.0 Diesel, 2010.

Regards

 

[Anthony489]

Hi subjectx

Under normal conditions the thermostat will hold the temperature stable, if necessary by closing off the main radiator and leaving only the heater. However, you have rather extreme conditions, going downhill the fuel is cut off and no waste heat is being produced by the engine whilst at the same time it is being chilled by a blast of freezing air. It's possible your thermostat isn't closing properly, but it would be a nuisance to pull it out and find that it's fine. Does the van warm up in a reasonable time ?

You could try blanking off part of the radiator grille whilst the weather is very cold, but watch for overheating if you have a long climb or motorway journey.

 

[subjectx]

Hmm, how to define reasonable time? I would guess it heats up in a app. 15km but never really checked..

Is there a way to measure temp diff on both sides of thermostat to check when it triggers?

 

[oigle]

Hmm, how to define reasonable time? I would guess it heats up in a app. 15km but never really checked..

Is there a way to measure temp diff on both sides of thermostat to check when it triggers?

If you allow motor to run at high idle, keep your hand on the hose from thermostat to radiator and you will notice a sudden rise in temp when the thermostat opens. Check your gauge to see what that temp is.
Ian

 

[nigelvan]

Older topic, but since I logged my last drive I could add some data:
X250 3.0 2007
Did a longer drive today and started with a long downhill from the mountains, so a lot of engine braking.
Took 25 min for the coolant to reach 80 C. Ambient temperature outside was somewhere between 15-20C.
After that a flat drive doing between 70-90 km/h mostly in 5th gear, temperature held around 80C.
Left it idling a few minutes before switching the engine off, then the temp rose to about 92C.
Most of the time the temp gauge stays just below the middle. I don't know if that's normal.
It only rose above the middle once when stuck in traffic on a hot summer day in France.