Fluctuating temp guage, AFN 1.9tdi......

[S4_dan]

So I had my first long run out on the Derv Shed last night. On the quiet country roads the temp got to 90 doing around 40mph. Once I got onto the motorway this dropped to 65-75 depending on what I was following. When I arrived at my destination I turned the car off and did collected my goods then returned ten mins later.

Turned the ignition on and temp gauge showed 85 degrees. Once I started the car and drove for a few hundred metres I watched the gauge drop back down to 65 degrees.

I can't stop the viscous fan with a rolled up paper from cold so I'm assuming that's running all the time and hence faulty.

When I gave it some beans up Sutton Bank (One of the steepest in the UK allegedly) The gauge shot up to almost 90, but once I got to the top and travelled a little while it was back down to 65/70.

So is it likely to be the stat or the temp sensor? Does anyone have the correct part numbers please?

On my B6 2.5 it turned out to be the sensor.

Also this morning before setting off for work I thought I'd check the coolant level and the header tank was still under a lot of pressure. Is this normal? My B6 doesn't have any pressure when left to go cold......

Cheers.

 

[aragorn]

Thermostat.

If you look at what you've typed, it basically amounts to the temperature rising when the engines under load, and falling when its not under load or when the airflow substantially increases (motorway). This happens because the thermostat is staying open when it shouldnt be, and seems to be the standard failure method for modern thermostats.

The rising after being parked is normal. Heat soaks into the coolant in the cylinder head and the temp rises due to no circulation. You get in and restart the engine and the coolant starts pumping around again the temp drops off.

The crucial thing here, is the Audi guage will display "90c" at any temperature between about 75c and 105c. So even when its showing 70 its likely quite a bit cooler than that, due to the "curve" on the guage.

If you have VCDS, you can compare the readings in the engine controller to those in Instruments. If they are both reading the same low temperature, then its pretty certain its the thermostat. The coolant sensor has two seperate sensing elements, one fed to the ECU and one to the dash cluster. Its unlikely they'd both fail, and especially unlikely for both to fail in the same way!

 

[S4_dan]

My 2.5 B6 did this but it was the sensor that was at fault, it was supposed to be a genuine item from Poland. I fitted the original sensor in and all was good again. I noticed in VCDS that the ECU side was reading correctly but the gauge wasn't.

With the B5 though, after doing the timing belt etc and just before the MOT I checked the pump timing and the two temps were even, that was last week.

Do you happen to know the part numbers? ECP list a Circoli 'stat, are these any good?
Would it be worth changing the Blue CTS sensor anyway? I seem to recall changing it for a green one on my B5 S4 some years ago.

Cheers.

 

[aragorn]

I've used Gates and Topran thermostats before without issue.

I wouldnt change the sensor unless its broken. They're £40quid, and only very rarely go wrong. All by B5's are on their original sensors.

 

[Mike B]

It might well be the stat but don't buy ECP they are useless.Does the heating work well?if not that will be the stat stuck open.However I would start with the fan which isn't helping if its seized.The pressure when cold is very odd cos the steam should have condensed.

 

[jbh]

long shot but could the cap maybe faulty?

 

[S4_dan]

Yeah there is heat in the cabin. Not burning heat but still a good amount of heat.

So the Circoli stat is no good then?

 

[Mike B]

If you get the heat early the stat could be OK.I have heard that 1..9s are slower to warm up than 2.5s.If you feel the rad and its still cold when the heaters working good then the stat isn't opening too soon. Once fitted an ECP stat and nothing and had to swop for genuine .They ignored my complaints.Try fixing the fan and see what diff that makes

 

[aragorn]

ive had numerous faulty stats and the heater has always worked perfectly fine.

Get onto Jason at All German, he'll sort you out with a decent stat.

 

[frazgsi]

Ive started a thread last night with pretty much the same problem. Only difference is Ive just changed my coolant temp sensor (blue type boght on ebay for 4.99), now my gauge only reads 70, where as before it would sit at 90 all day long. ????????

 

[S4_dan]

In an ideal world, you should only use genuine sensors. Most pattern ones cause all sorts of issues.

I'm going to change the viscous fan before doing the 'stat and/or sensor.

 

[bgale74]

+1 with Aragorn and Mike B.... it's the stat. My 1.8T was doing this and changed the stat twice to fix it. ECP sell Circoli and Behr....... avoid Circoli at ALL costs. New stat then stuck open. Replaced it with one from Andrew Page...... 4.99 all in. Gauge is bang on now and heaters hotter than Megan Fox walking across coals.

 

[aragorn]

I'm going to change the viscous fan before doing the 'stat and/or sensor.

You could blow a 400mph wind thru the radiator and it shouldnt make any difference to the temperature, as the thermostat should simply close. Your problem is happening because the thermostat is not closing. Fixing the fan might mask the issue, but its the thermostat that needs fixed.

 

[S4_dan]

New 'Stat it is then. Will get one ordered through TPS.

Cheers Kev.

 

[aragorn]

If your car has the electric fan for the AC (and it works!) you could just remove the fan blades off the viscous fan. I ran my 1.8T like that for 3 years with no cooling issues.

 

[S4_dan]

My 110 has an electric fan but no AC. I'm lucky (or unlucky, whichever way you look at it) to have a sunroof, to which I prefer over AC any day.

Could I not fit a temp sensor into the lower rad hose to power the fan on when it needs it? Or will the original electric fan work even though I have no aircon?

 

[aragorn]

The temp sensor should already be there if it has an electric fan. Its on the radiator itself on early models, just above the bottom rad connector.

 

[S4_dan]

Yes then I have it. I remember removing and fitting wires when taking off the front end.