Is it actually doing anything wrong. Diesel engines are quite efficient and if you're going down a long hill in cold weather with the heater on aren't you going to cool the engine?

I know mine has always behaved in a similar fashion, though not so extreme - but maybe it has the same fault?

Thanks for your comments. ideally the engine thermostat should keep the engine under any circumstances to or close to its operation temp. I must be missing something...
Gregory

Well, there's no thermostat on the heater circuit, so if you have the heater on full and are coasting down hill you will be cooling the engine even though the thermostat may be closed, and with my car when it is cold it does just that. Not much the thermostat can do about it, simple laws of thermodynamics.

But as stated, I don't know if my car is typical or has a problem.

Will investigate further and revert./Thanks

When I had my gearbox changed the company that fitted it also fitted an external gearbox oil cooler, they bypassed the oil cooler by joining the redundant water coolant pipes togeather, effectively bypassing the main engine thermostat.
This alone, even though the bore of the pipe is only about 1/2 inch, was enough to over cool the engine.

Plugging the ends of the coolant pipes instead of joining them returned everything to normal.

So if you've had any work carried out on your car recently, check that something similar hasn't happened.

That might be an area to look into as well. Have fitted a new/original gearbox cooler as well on this car. Supposed to have its own thermostat inside to cater for just that. will try and block the return pipe and see it it changes anything although not a good practice as these gearboxes 5GM don't like running cooled.

Just trying to tackle the same issue on my car. I have replaced the EGR valve and EGR thermostat and but the temperature gauge still drops to just over blue on motorway journeys.

The main thermostat valve is the next step (lots of videos on youtube) but if that does not solve it how easy is it to access the oil thermostat valve? Tom

Current Drive
2011 4.4 TDV8 Autobiography - Santorini Black - Ivory Leather

Previous Drives
2004 Model Vogue Td6 (Touchscreen) - Java Black - Parchment Leather
1994 RR Classic 3.9 V8 Soft dash - Niagara Grey - Grey Leather
1972 Series III SWB Safari - Green (Hand Painted) - Black Plastic

Last edited by Flashman on 9th Apr 2018 2:19pm. Edited 1 time in total

Flashman, it's almost certainly the EGR thermostat. That was the problem with mine - not the egr valve, but the egr thermostat. It's simple and cheap to replace, so I would do that as the next step.

Changing the regular thermostat is a pig of a job, so I would do that last.

Thanks Kurt, already done it. Whilst the main thermostat is the next one to tackle, some on here have had issues with the oil one. I just wanted to know how easy that one is to tackle as I would ideally like to do it myself if needed. Tom

Current Drive
2011 4.4 TDV8 Autobiography - Santorini Black - Ivory Leather

Previous Drives
2004 Model Vogue Td6 (Touchscreen) - Java Black - Parchment Leather
1994 RR Classic 3.9 V8 Soft dash - Niagara Grey - Grey Leather
1972 Series III SWB Safari - Green (Hand Painted) - Black Plastic

Sorry, can't help with the oil thermostat, didn't even realise it had one - just a cooler!. I had similar probs, EGR thermostat was fine and I cured it with the main water thermostat.

Would be surprised if the water one was working correctly that the oil one would reduce temps that much though so would defo go water thermostat first.

Wasn't that bad a job tbh, just lots of bits to take off.

Cheers,

Paul. 2011 TDV8 AB
2006 TD6 AB (Gone to Wales)