About early Dec my MK4 TDi (130bhp) temp gauge started doing odd things. It wouldn't get above 75 degrees, even after 30 miles. Temp outside was cold, about 2 degrees but the temp outside has never made any difference to my temp gauge before. Anyways, under load or off throttle, it made no difference atall.
I've had the car 7 years and I did have to purchase a new temp sensor (green) about 6 years ago as the temp gauge was just dead, it didn't move. Thinking that may be faulty and it was the cheapest thing to replace, along with losing the least anti-freeze, I set about changing it but got a Febi part for about £10 as VW wanted £38.57 plus VAT for this part. When I replaced this originally with a VW part it was less than £10 at the stealer. That didn't solve the issue and temp remained at 75.
I then decided to change the thermostat, stealer wanted £26.57 for the thermostat and £3.97 for the o-ring, both plus VAT! So I got a Meyle part, I have got the correct part and it opens at 87c. I filled the anti-freeze back up with ready mix G12+. I did have to put nearly a full 5 litres in it after draining it.
Unfortunately, replacing the theromstat has made the car even worse, after 15 miles the temp gauge didn't move! Over the next 15 miles it slowly crept up to the 3rd of the first 3 white marks that are close together at the bottom of the temp gauge. So nowhere near the 75 that it was before!
The hose that has a plastic union which bolts onto the thermostat housing has no temperature to it after 30 miles. Top hose of rad is warm but not hot.
Car heater is barely warm, it was getting warmer before I changed the thermostat. I can confirm there are no air locks as I bled the system and have topped up. Since the 30 mile run, the level of coolant has remained constant.
The car has done 138,000 miles and doesn't use or leak anti freeze.
I am guessing its either a genuine stat now or maybe the water pump?
I've been running the car at about 75 degrees since early December and am now facing next week with next to zero temps with the car not warming up atall. It hasn't warmed up in 30 miles today and its only 8 miles to work! Will this damage the engine atall, should I take it off the road?
Any advice appreciated.