I have the same problem in my 2007 CDTi. I noticed the bottom radiator hose is permanently cold, so I think I'm going to change the thermostat. I hope this fixes it

 

Your symptoms suggest head gasket but that's quite catastrophic. I'm guessing you have a coolant leak and recheck your exhaust smoke may be normal. I have the same problem but I'm pointing my finger at thermostat as lower radiator hose is always cold

 

Maybe it's just me, but I don't see how a thermostat could cause a cold lower radiator hose.

 

Im no mechanic, so I hope I’m right. The thermostat remains closed stopping the coolant from circulating fully. The lower half of the radiator remains cold.

 

If that was the case then your engine would overheat, big time! That and your heater would blow LOTS of hot air.

 

perhaps you are right, but can you offer any explanation for a cold lower rad pipe plus no hot air from heater? My thinking was that with a permanently closed thermostat there would be no hot water circulation in the heater matrix or lower rad. I’m doing a Sherlock Holmes diagnosis here
. I should add I’m in Ireland. It’s quite cold already. I need more than heated seat set at 11

 

I believe your lack of heat and your cold lower hose are completely unrelated. The cold lower hose is often normal in a cold environment; the lack of heat is typically either your coolant level is low or your heater core is clogged (fairly unlikely if you've maintained your cooling system and used an approved coolant).

 

Yes. I’m afraid I’ve given myself a difficult job for nothing. I have the radiator out and the thermostat housing opened. The new thermostat seems not to seat properly. Does the thermostat go into the housing first then the rubber seal after? Oops. It was all quite mashed up when I disassembled. I acquired the car a month ago with a lot of wee jobs needed done. Perhaps I need go a flush to unclog the heater matrix? Everything else seems normal, working valve etc.

i just realised the rubber seal sits around the edge of thermostat. Thanks for the inspiration 😁

so finally reassembled and as you predicted no heat. Btw Coolant was my first check, the open close valve my second. So flush the heater matrix now?

 

At least flow water through the heater core to determine if it's blocked or not. Possibly save any chemical cleaning for after you've determined that it's blocked.

You're in Ireland, so I'm not sure what parts of the heater/ventilation system are the same as USA. There's probably an air-blend door in the heater duct, so check whether that is operating correctly. Maybe the heater core IS getting hot, but the blend-door isn't taking any air through the core? I'm also guessing those are motorized (like ours) so you need the key "ON" (not necessarily engine running) to operate the blend doors.

 

As a suggestion, take your car out for a drive and let it get good and warmed up and set the heater controls to all of the way hot. When you get done with your drive, check the temperature of the two heater hoses, if both are hot, then coolant is flowing through the core and your blend door is probably the issue; if one or both hoses are barely warm, then you have a heater core issue.