Hi

i have a 165 sport diesel and my presumed problem is the main radiator does not seem to get hot even after 15 mins of running. I do do a lot of short journeys but was just checking it the other day.

It just seems to start getting warm on the turbo side of the rad where the two hoses go in. Across the main rad and the top left hose stays cool for what seems like forever while the header tank is toasty and does seem to push the water to the brink of coming out if you take the lid off. Cabin heater seems good.

Is this the beginning of a problem? the temp gauge whizzes up to half way after a mile or two and stays steady, just unnerves me for long journeys.

Dave

A diesel has a lot more metal in the engine than a petrol, and the fuel burns at a lower temperature. Any diesel will take significantly longer to warm up than a petrol.

During warm-up, coolant will circulate through the internal heater, and possibly the header tank. Once the heat in the engine is near its ideal, temp, the thermostat opens allowing coolant to flow to the radiator. Although we have had some nice weather, the nights have still been cool, and 15 minutes may not be enough to trouble the thermostat much. Such short journeys are bad for any engine.

Once the coolant nears 100 degrees, the system will start to pressurise, which raises the boiling point of the coolant. If you release the cap, you release any pressure, so dropping the boiling point to 100 degrees. As parts of the engine are hotter than this, coolant there will instantly boil, causing the level in the header to rise, and potentially steam to be ejected over you. Never remove the cap when the engine is hot, you may injure yourself, and may cause damage internally to the engine.

If you have aircon, make sure it is the coolant rad you are feeling, not the intercooler or aircon condenser, which is usually in front of the rad.

If the thermostat is failing, you can get a good idea without dismantling.
From an overnight rest, start the engine and allow to tickover.
The top hose at the engine should be at the thermostat. Keep checking this hose as the engine warms. There will be some warming due to conduction, then as it gets to temp, the stat opens and the hose gets hot immediately. With colder coolant now entering the engine from the bottom hose, the stat will shut again quite quickly. Only a small amount of coolant gets to the radiator, which cools quickly. It will take some time for the radiator to feel warm all the way down.

Keep hands away from any moving parts, aux belt and especially the radiator fan, as these start up without warning.

Lazy thermostats tend to fail partly open. If this is the case, the top hose gets warm gently as the engine warms, but this should not give you a hot header tank.

What does the temp gauge say?
Why were you looking at this - what problem did you perceive before starting to diagnose. If there's more detail, share it for a better prognosis.