PD130 with dead temp gauge, poor MPG and odd readings.

[thai-wronghorse]

Technical advice required - my PD130 wagon has had an issue since i bought it recently where the temperature gauge would not raise what so ever regardless of how long it had been driven.
My immediate thoughts led to the commonly diagnosed coolant temperature sensor but after swapping this over for a reputably branded item there was absolutely no change to the gauge read out or to the abysmal MPG!
Around town it's giving mid 20's and on a run it's nearer to 40mpg, this said quite often the first drive of the day will return extra (normal MPG readings?) in the region of 50mpg which is baffling to say the least.
Obviously the next step would be to swap the thermostat but from experiences of sticky 'stats is that the gauge would still move even if it was just slightly or inconsistently?...as mentioned I have nowt.
Further research today has lead me to discover that Climatronic equipped cars have a 'hidden diagnostics' built in which i was eager to try out.
Option '7' on this gives a live coolant temperature read out and was saying mine was 22degrees when cold then after driving about 20minutes had only gone up to 33ish degrees....seriously that can't be right when these engines are designed to run at approximately 90degrees.
Even with a stuck open stat id expect around 60-70 degrees???

Any ideas what i should be checking next people?

[Bandit127]

If your aircon is on then the radiator fan (the aircon one) will be running constantly. This could pull the temp down from 50s on a diesel to 33.

I would expect 60-70 on a petrol with a stuck open stat. Diesels are more thermally efficient - I was lucky to see 60 on my 2.5 TDi A4 which had a stuck open stat. The stat on that engine is a bastard to get to so I bodged it with a thermostat stuck in the rad hose.

A stuck open stat is at least one of your problems. 

[thai-wronghorse]

Yeah makes sense especially after the hours of reading about dervs and thermal efficiency last night.

I haven't noticed the fan running constantly but this crop up in a few threads i read and some mentioned a faulty fan control module as been a common issue on these which puts the engine into a state of ''emergency cooling'', so i'll need to to investigate this area further.

First of all though is to get this thermostat swapped over, coolant flushed through and an oil change and go from there, fingers crossed. 

[topher]

i wouldn't rely on the climate display, get vagcom on it and check the measuring blocks for coolant temp in both engine and instrument modules. After 20 minutes driving I can't imagine its at 33 degrees, the coolant pipes would only feel luke warm. Follow the wiring for the temp sensor it may have broke where it joins the rest of the harness.

[thai-wronghorse]

If I get the chance and the rain stops il have another butchers at the wiring as this does seem very plausible.

[Bellend]

Also you say reputable brand, what brand?

Unless it's OEM in my fairly vast experience probs changed about 20-30 now tbh, I won't go near anything but OEM, even a £22 quid sensor caused me a headache until I swapped it for a known working OEM one.

Injector looms can give these engines poor MPG and might be worth whipping the manifold and EGR off and cleaning it out, shot my GF's father inlaws Passat from 33 to 52 MPG but it was the worse blocked one I'd seen.

[thai-wronghorse]

The temp sensor was changed a couple of weeks ago for a Febi branded item.

I've already made plans to remove the EGR and intake manifold for cleaning in a couple of weekends time along with a long list of other bits and pieces that need addressing.

I've read a few comments in regards of the injector looms, are they a particularly common failure point then? Surely if an injector wasn't operating it would be running like a sack of sh!t and mine isn't in the slightest.
Infact even with everything mentioned in this thread it still drives really well and pulls strongly in all gears - It's just sh!t on fuel!