[DIZZYT5]

Got a right problem with my 2.5 bnz its had a new engine Fitted but I’m having a cooling issue, heaters heat up nice when getting up to tem, when it his 90 the heat drops off and goes colder, apply the revs and I have heat back, drover 38 mile today and the heaters were amazing, no overheating no coolant loss temp stayed at 90, got home
left it running while I opened the garage door and made some space went Back to the van and it’s blowing cold again, as soon as I pulled away and the revs were applied the heat was back

is this a faulty Water pump or air in the system?

 

[dossbag]

Faulty thermostat may also be a possibility, can have a similar effect. 'fraid I can't add anymore than that

 

[T5 TDI]

Does sound like a new thermostat is a good start. But a 'new' engine with a stuck open thermostat doesn't sound very hopeful. Does it have invoices for thermostat or water pump? The 2.5's aren't known for slipping impellers like the cam belt driven VW's but unless you have history of replacements I would change both.

Your little 'flag' icon claims you are in the UK. If you are using a VPN or other internet security and are really somewhere else with temps well below freezing most of the day then the rules are different with a 2.5.

 

[DIZZYT5]

I would assume with a faulty thermostat it wouldn’t be getting warm or if it was the other way it would overheat? It’s a brand new genuine thermostat, brand new water pump but that is aftermarket, the thing is the pumps pumping as I can see it flowing back into the header tank.
im thinking it’s air in the system still?

 

[T5 TDI]

The 5 cylinder engines aren't known for air locks. Usually once the thermostat is open that sorts them out. If you can get access to VCDS you can log group 062 which shows temperatures. There are two engine temp sensors (G62 and G83), one measures the coolant temp straight out of the top of the engine and the other records what temperature the coolant is at coming out of the bottom of the radiator after cooling. The ambient air temp and intake air are shown in that group too.

For comparison with yours this is my AXE 5 cyl logged and graphed from cold (it was summer though) on a 20 mile trip. The fans did come on when I got stuck in some traffic at one point and I did put the AC on a couple of times. Unfortunately I knocked the VCDS cable out for a while and missed a chunk of the middle of the log but the right hand side is after the thermostat is open.