Puzzling coolant temperature problem (not just a sender problem)

[bartonthesparta]

1998 A4 1.8SE
My coolant temperature guage has recently stopped moving, stays around 60 or on the stop, The cars also started running very rich, both signs pointing to the stat being knackered or temp sender. Iv checked the coolant temperature reading on the climate control, this seems spot on, after starting in stays low for a bit then quickly climbs to 90C and stays exactly there - would this mean the stat is actually ok? I realise the temp sender is actally two senders in one unit, one for ecu and one for the guage, I changed this but its made no difference to either the reading for the ecu or the gauge, also Im still getting fault code 16500. I cant see how it can be down to the stat when the climate control gives perfect readings, so I decided to check wiring between the sender and ecu and sender to instrument panel. All the wiring tested out fine including the connections. After checking the wiring and putting it all back together I left the engine running for 20-30 mins, after which the coolant temp guage also shows 90C and the rich running stops, when I went for a drive the temperature dropped back down to nothing again as if the stat isn't closing, yet the climate control still shows 90C until the engine is stopped. Am I overlooking something obvious that would cause this? Does the climate control take its coolant feed from a different point etc?
Any ideas would be much appreciated
Thanks

[muddy]

I'm not familiar with where the climate control picks up 90 degrees from ... but your car shows all the classic Passat /A4 signs of a dead thermostat :-

" ... left the engine running for 20-30 mins, after which the coolant temp guage also shows 90C and the rich running stops, when I went for a drive the temperature dropped back down to nothing again ..."


Just changed one on a '99 1.8 and have had to do the same job on '98 1.8T.
Also had to do this on two previous model 2.0 litre Passats.
ALL showed the gauge readings you are describing.

Forget about climate control. Change the thermostat first.

PS Have a read through post just lower down . "temp problem"