A4 B6 Avant 1.8T Coolant temp gauge issus- advice needed

[CLOUD1]

I would value greatly some help getting to the bottom of this problem. My car coolant temp gauge read cold most of the time. It will stay at 50-55 whilst on motorways and will get up to about 70-80 deg if idling in traffic locally. The radiator fan kicks in at start up and will stop once the engine is warm- though the gauge still shows the coolant as cool. The engine has been a little more rough and having done the research online I thought it was the green coolant temp sensor- on the back of the engine. I have replaced this- with an OEM part and not Audi, and the issue it hasn't made any difference. I have heat in the car no problem. There was one episode of overheating and the engine cutting out when in heavy traffic but nothing like that since. I am not loosing coolant. I do not have VCDS/VAG COM and cant afford the dealer price of £102 just to run the diagnostics- and from what people say there is no guarantee they know what they are doing even after running diagnostics. I have ordered an aftermarket cable and will be scanning with VCDS lite.My question is does this point to the grey sensor in the radiator hose, the thermostat, the waterpump or what, as people seem to report different fixes all for the same problem. I am not a mechanic but doent mind getting my hands dirty for simple DIYs. If the grey sensor was faulty, why would the gauge not show a higher temp as I understand that is controlled by the green 4 pin sensor?Or is the engine genuinely not heating up due to the fan over running? ANY advice greatly appreciated

[Sam]

Replacing the CTS with a genuine CTS is the first step.

If that has no effect, then the thermostat should be changed.

Are you getting a rough/hunting idle? Any difficulty in starting? Are the hoses into and out of the rad both hot after a spirited drive?

[CLOUD1]

Thanks for the quick reply

I will bite my lip and order the £40 Audi green CTS
Does that mean not to bother changing the grey 2 pin sensor on the radiator hose?

there is a rough idle from startup- revs at idle hit around 1200, within a minute or so it quietens down itself- regardless of whether I drive the car or leave it stationery-- what does this tell us?

There was one day when it cranked but didn't start- turned key to 0 and after a coupel of attempts started- fine since


I will check the radiator hoses later today and post back- I have been avoiding spirited driving as I read it can overheat the engine( happened once in traffic) if coolant is being 'sensed' as cold by ECU??

[Doctle Odd]

As above, don't be tempted by ebay/europarts/gsf sensors

[Novica]

This summer I had problem with fan. The car (my is 1.9 TDI) had rough idle (very often it was going from 890rpm to almost 1000 rpm than fall down below normal and stable it on normal 880 rpm). Not to longer the story, the problem was that electric fan (I have only that fan) was dead as well as his controller and as soon as a/c compressor turn on and off engine rpm starts fluctuating, if I turn off A/C completely everything is ok. But the temperature was all the time at 90C even if the fan did not work. I replaced fan and it's controller and everything is OK.

[Sam]

There's nothing to say the current CTS is at fault, so if you'd rather, you could jump straight to the 'stat.

I'd recommend a full scan first though, just to rule out anything bigger, we're not made of money

The hunting/fluctuating idle is usually down to a faulty CTS - what you're seeing is normal though, remember the old choke days?

Every issue is different, but like you, I also chased a temp gauge issue for a few months, VCDS scan showed no errors at all. I got fed up and ended up just changing the CTS and the 'stat. The issue vanished.

[CLOUD1]

Is my logic correct in thinking that if the climate control will display a ECU coolant temp reading close to where it is expected- ie 80-90 deg when warm, that would mean definitely the CTS, and if the climate trick shows a low reading then that means CTS fine, and maybe move to thermostat or there is another problem??

[CLOUD1]

After a good run I have checked both radiator hoses and they are hot. Top more so than bottom.

[fourringsrus]

The temp sensor in the bottom hose controls your gauge and fans and is a simple and cheap job.
You are right to say the sensor on the back of the cylinder head is the usual culprit.
When the two sensors are confirmed as good they will tell the ecu the correct temp which in turn will instruct your stat to open/close, assuming you have the avj 1.8t with electronic stat.
Take a peep at my threads dated 2/2/13 and 17/4/13 ref avj coolant temp sensors and issues to see if anything rings a bell

Stuart

[CLOUD1]

Ok, so maybe I made the mistake of putting in an aftermarket CTS.

I'm having the car scanned tomorrow, and if no nasty surprises will get the Audi CTS ordered and update as appropriate.