PipH

I would get the compression checked on all 4 cylinders as a start. Also get any stored codes read, then clear them & see if anything returns. But don't think low compression will bring up any codes.

I don't hhink the injector cleaner will cause any problems other than perhaps a bit more smoke. Water in the inlet can cause major problems, especially in a high compression diesel engine. Water isn't compressible & can cause con rods to get bent if it enters the combustion chamber. This will result in low compression on one or more cylinders. Low compression will give rise to white smoke ( unburnt diesel ) which may clear as the engine warms up. You will have to hope no water went into the cylinders.

Yes, I like to understand how systems work, makes fault finding 10 times easier. I can't understand why any heater wouldn't work when installed in the position shown in the Golf diagram. Coolant would circulate just as it does in a cold engine. Would heat the heater matrix then start to warm the block/ head. So you would get instant heat & better MPG from cold. Best of luck finding your coolant leak. Immediately above the pool on the floor is the obvious starting point & may save you climbing underneath. But the under covers can divert the leak & mislead.

Thanks for the link to the diagram snowathlete, although it's not the correct car it's probably typical for this engine. It doesn't show a DSG stat, but that must be in the pipe between 6 & 7. So as I understand it, & I stand to be corrected by anyone who knows better, but the coolant flow would be as follows :- The pump 2 circulates coolant around the block/cylinder head 3, heated coolant exits from the horizontal pipe at the right end of the block. Assuming the engine is cold, coolant won't be able to pass the thermostat 10. The first vertical pipe going down from the group of 4 at the right end of the block must be a small bore pipe going back to the thermostat located in 10. I assume this is to sense the coolant temp coming from the head so that the thermostat opens at the preset temperature of the stat. The main coolant flow with the stat closed will be the extreem right pipe of the 4 coming from the head, this will go straight to the heater matrix 5, as most cars won't have 11 & 12 installed. It looses its heat from the matrix & returns to the bottom of the engine via 10. It circulates like that until the engine gets up to temperature when the thermostat in 10 opens. When the stat opens coolant is alowed to circulate down the second vertical pipe of the 4, this will be a large bore pipe going to the top of the rad, through the radiator 9 & back to the bottom of the engine via 10. Coolant is also still flowing through the heater matrix which is effectively in parallel with the main rad. There is also another parallel coolant flow connected to that large bore pipe, this goes via the EGR cooler 4 & electric pump 8. I think this pump only runs at certain times, certainly not during a regen, but that is just a guess. The stat in 10 regulates the main flow depending on the temperature of the coolant being sensed. If the DSG stat was open, coolant flows via that single central pipe at the bottom of the block, via 6, DSG stat & 7, back to the bottom of the engine, so short circuits the whole system. So I was wrong in that it doesn't go directly via the rad as per my previous post, so it wont be easy to detect that it has failed open. Checking the rad as I suggested will only be checking the main thermostat which rarely seem to go wrong. If you look half way down this post there is a picture of the DSG stat & its location under the air filter. https://www.pistonheads.com/gassing/topic.asp?h=0&f=66&t=1610899 You would have to expose these pipes then start the engine from cold. These pipes should remain cold while the engine warms up, but a bit of a faf to check.

I didn't realise the 4x4 was part time. So as you say, won't have as much impact on MPG. I might be wrong re the gearing, I think my engine revs at about 2100 @ 70 on the spedo which equates to 70 on cruise, probably a true 67. I havn't seen a cooling system diagram for a Superb, but heat from the DSG has to be dissipated so must go through the rad.

If you had reset the computer just before the motorway journey when the engine was already up to temperature, I think your MPG is a bit low. I have a 2l 140 Passat Estate & would expect to see over 60mpg at 70. Mine is not 4x4 so that will obviously take its toll. ( perhaps 10% ) The DSG should give better MPG on the motorway, higher top gear than manual box I belive. Most common cause of poor consumption is the coolant thermostat either opening too early or stuck partially open. If you've got a few spare minutes on a journey from cold, stop the car when the temp guage is starting to move up from 50c. Choose a safe place & pop the bonnet, feel the top of the radiator. It should be stone cold. If it's warm then water is circulating when it shouldn't & loosing you MPG. The DSG engine has 2 thermostats, one for the engine & one for the DSG. It seems the DSG stat is a fairly common failure.

The thermostat is there to maintain the engine temperature. If it is dropping after being up to temperature, I can't see it being anything other than the thermostat stuck open. If the timing belt were out it would make the engine run less efficiently so produce more heat. One check you can do is start the engine from cold. When the temp guage is around 50 - 60c stop the car somewhere safely & open the bonnet. Feel the top of the radiator, it should not even be warm. If it is warm then water is circulating when it shouldn't. The only thing stopping that is the thermostat.