Chilly 300 tdi

So my 110 300 tdi with autobox has started playing up. For years its been on the middle line of the temp gauge when warmed up. For the last two weeks or so its barely got above the quarter mark after a run. Blower is blowing heat but feels not as hot as before. Put in new thermostat and filled with new anti freeze in the correct order but no change. Top hose is warm but not boiling and radiator is all cold. New water pump about 4 months ago as last one's bearing went. New radiator a couple of years ago as well. Now its either a blockage, new water pump has sheared internally so pump isn't spinning or its way too cold and viscous fan is bust on all the time. Any ideas? Ta.

Did you test the new stat a few times before you put it in to make sure it was opening and closing?

No. Its a QH so hopefully of good spec. Anyhow i doubt the old stat AND the new one both would be failed open.

Bled properly? Blocked matrix? I had same on one of my motors and sent a hose through matrix pipes to cure years ago

Its been bled properly. Doubt heater matrix blockage would cause this as main flow is through the rad.

Don.t bet on it, test the stat or you may waste a lot of time.

Mingus said:

No. Its a QH so hopefully of good spec. Anyhow i doubt the old stat AND the new one both would be failed open.

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Was the old one stuck open when you took it out?

Mingus said:

So my 110 300 tdi with autobox has started playing up. For years its been on the middle line of the temp gauge when warmed up. For the last two weeks or so its barely got above the quarter mark after a run. Blower is blowing heat but feels not as hot as before. Put in new thermostat and filled with new anti freeze in the correct order but no change. Top hose is warm but not boiling and radiator is all cold. New water pump about 4 months ago as last one's bearing went. New radiator a couple of years ago as well. Now its either a blockage, new water pump has sheared internally so pump isn't spinning or its way too cold and viscous fan is bust on all the time. Any ideas? Ta.

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Might be an idea to check the securing clips for the control cables on the bulkhead. If the cables are swinging free it could affect what goes on in the box.
Mine rarely registers middle gauge unless I've been thrashing it or hauling a heavy load but it does throw out adequate heat but perhaps I have a high pain threshold.
Anyway, hope you get it sorted as its Baltic where I am today!

Sounds very much like a cooling issue rather than a specific heater one, if the engine isn't getting up to temp then the heater won't have the heat to chuck out.
Does the engine get to temp after a good run (sat idling on the driveway is no good as it is unlikely ever to get up to operating temp)?
The fan shouldn't make a difference as until the thermostat operates hot coolant won't be directed through the rad anyway although of course it does make the whole underbonnet area colder (especially in the current conditions). If you are certain that the system is bled correctly (and I know the 300TDi can be awkward in this) then I would suggest removing the fan (the work of a couple of minutes as long as it isn't seized on) and taking it for a good blast. If it still isn't at correct operating temp after 5+miles (I mean actual coolant temperature, not just relying on the gauge) then the new thermostat has to be suspect.