Turbo Diesel getting hot very quickly when driving

[cobblers]

A mate of mine has recently bought a Karmann Gypsy. It came with a 1.6TD in and he's just swapped it for a 1.9td out of a passat that was running perfectly.
It had this exact problem before and after the engine swap, and the van shows signs of people chasing "hot" issues in the past (cooling fan bodged on a manual switch etc)

it's had a new radiator/waterpump/stat/temp sensor and has been bled up and worked on by ex-vw mechanics who worked on these when they were current.

Symptoms are that it would idle happily all day - fan cuts in and out etc, temp needle is 2/3rds on the gauge so a bit hot but not horrific. Temp sender resistance after 90minutes idling was 85-90 ohms, so ~100c coolant temp.

Within 2 miles of driving (even from stone cold) the needle is off the top of the scale and soon after this, the warning light flashes, but he's driven it for weeks like this . It doesn't boil over (although it did once on his drive home when he first bought it).

I've bench tested his clocks and they are fine and read exactly the same needle position as the 3 other sets I have here (120 ohms is 50%), and I've checked his vehicle wiring and replaced earths etc. The temp sensor is new. Coolant level relay (43) in or out makes no difference.

The only thing I haven't managed to do is completely isolate the temp sensor and measure the resistance when it's driving to be 100% sure that it's reading very very hot water.

My suspicion is a blocked (it still has the original plastic pipes and I suspect one of the steel inserts is working it's way round the system) or maybe a misrouted cooling hose.

Any ideas?

[Stesaw]

Well does the Rad get warm all through? I assume as the fan comes on it does. Does his heater come on warm? Does it have that secondary electric pump that keeps the coolant circulating to cool the engine down after switch off? can they run in reverse if wired wrong? If it's all different temp sensors and such from the 1.9 engine that were OK when in the passat then its gotta be pipework or dodgy wiring related, perhaps or an airlock..possibly or blocked pipe or routed wrong as you say.

If I were getting issues like that I'd just drop the coolant, take the pipes off the engine side that go to the rad and stick a hose in one and see what came out the other one.

[cobblers]

Yup, rad seems to get warm all over, heater works great. secondary pump is unplugged!

I'm giving "Phone advice" to non internet types here so I've told them to physically make sure all the coolant pipes are unblocked (send a garden hose all the way down the plastic pipes to make sure there's no obstructions etc) and that they go to the right place, armed with some photos I got off the net of the TD engine bay.

[davidoft1]

Pull the front grilles off, let the engine run for an hour , check that all parts of the radiator are hot enough to only be able to touch momentarily, check the thermostat is opening by holding the pipes in the engine bay , the one from the head will get hit fairly quickly the other will be cold until the thermostat opens , gauge should be in
The middle and it should noticeably drop when it opens .

Then check by hand all along the long pipes for temperature, get a laser temp sensor to help check.

What temp thermostat is in it
What temp rad fan switch
Which rad fan is it running
What thickness radiator