Temperature problem??

[denzelburner]

Hopefuly someone can suggest something for this:
I bought my car (B5 2.5tdi), last year, and it used to sit at bang on 90c but now, 30,000 miles later, it won't really go above 75c (even in summer). I was wondering if this is a blockage.
Also, this is probably just me imagining this, but my mpg has also gone down over this time from 540miles per tank to 480miles per tank. Can this be linked? I have had the car serviced by a reputable audi specialist at the correct milage (currently done 104,000 miles). Am I being paranoid, or is this usual for this age of vehicle?

Thanks for your help in advance

 

[Macduff]

Temp sensor or thermostat problem. MPG drop is probably down to the car running richer as it believes it's not up to temp.

Temp sensor should show up with VAGCOM. Thermostat may need to be removed to check it's function.

A thorough cooling system flush and refill with new fluid may be a cheap first step although I'd expect the car to run hotter if there was any sort of blockage.

 

[moschino]

The thermostat would be the first thing to look at. Sounds like it is locked open.
Diesel engines generate less heat than petrol so for the car to operate at 90Deg C the thermostat has to restrict the flow of water through the rad.
One test you could do is to leave your engine running without moving to see if the temp reaches 90Deg. That way you know if the temp sender is working OK. It might take about 20 -30 mins to reach 90Deg.
Hope this helps.
Jim.

 

[fjtwelve]

My 2.5TDI has nearly always run below 90 since i got it ay 109000 miles, the only time it has got hot was in the Alps when something jammed the fan when we were climbing a steep high (2000m) pass. The thermostat has never shown a problem, the fan interlock was changed at the recent engine rebuild without significant change to the running temperature.

 

[denzelburner]

Thanks for everyones responses

fjtwelve, what kind of milage are you getting to a tanks worth, as my usual is about 480 miles, and that isn't with a heavy right foot, or much town driving?

 

[denzelburner]

Oh, and I tried running it, and left it for thirty mins. It still didn't run above 75deg.C

 

[fjtwelve]

My average over the last 67000 miles is 33mpg, normally filling up between 500 and 530 miles (i run it almost dry generally). Its now dropped to about 30 as i am pootling around cornwall. It does better economy at 90 on the motorway than dodging around the local population who appear to have a different sense of urgency

 

[fjtwelve]

Watched the gauge on a 450 mile run yesterday. Running at 100+ for a while brings the gauge up to 90C, slow down and it drops back to 70-75C. Presumably the system is designed for tooling along autobahns.

 

[golisago]

I think this is most probably down to a bad "coolant-temp-sensor" which operates in two ways , 1st one for the gauge reading and 2nd for the glow plug operation via the ecu.

Is your car hard to start first thing in the morning these days........and was it harder to start when the weather was wormer after it had already been sarted and driven that day.......

 

[denzelburner]

The plugs take longer in the morning to warm up, or after a long while when having sat there, which is to be expected. After the first run it starts up straight away first time every time. It smokes a fair bit when cold on start up and also under accelaration (can usually see it in the headlights of the car behind, but got used to thinking it would be dust, ha ha). When I swap between normal derv and Bio diesel the temp has gone up, but then goes down to around 70oC and stays there...

 

[golisago]

Quote..
The plugs take longer in the morning to warm up, or after a long while when having sat there, which is to be expected.

The don't take any longer to get too there operating temp in any weather.....and stay on longer in the winter than the summer or should do......What colour is the smoke at first start up and the 70c does sound like the temp sensor is faulty it should stay at 90c what ever fuel!!

What mods have you got!!

 

[denzelburner]

It stays on for about 5 seconds when first starting up in the morning, then for the rest of the day, or for other trips close together it is pretty much instantanious. I get white smoke on starting up from cold, and then non for the rest of the time either.

I have no engine mods at the moment