Unexplained fans running...

[Jemster]

I got out of the car last night after a 10 minute drive home to hear the fans running at full speed from the front of the car.

I'd noticed this before, and always put it down to DPF regen and didn't worry too much about it, but what other causes are there?

I'm asking because VAG DPF is saying my DPF is only at 15g and regen'd 250 miles ago. It's been hitting regen at about 500 miles the last couple of times, so I'm very doubtful it's a regen. The other obvious culprit is overheating, but the engine hadn't even got up to temperature yet (from past experience I'd reckon coolant was about 70 degrees but I didn't actually think of restarting and checking)

Could this be a dodgy thermostat / sensor somewhere? What triggers the fans? I'll read codes, but as I've experienced this before and not found any codes at all, I'm doubtful it's going to show me much.

Thanks for any help!

 

[Neil Wilkinson]

I find if I use AC the fans come on, could it be that?

 

[Jemster]

I find if I use AC the fans come on, could it be that?

Do they stay on after you've stopped the engine and removed the key though? I've read AC puts the fans on at low speed but assumed this was just while it was running.

I've also read of this happening to cool turbos etc... but, again, surely that should only happen on a fully warmed up engine?

After a 10 minute drive back to the office this morning, the fans weren't running when I got out. So it's not an "every time" kinda thing either.

Still trying to work out why my temperature doesn't come up properly and why my economy is rubbish. Yesterday I drove 8 miles, approx. 20 minutes. The oil temperature only just made it up to 73 degrees. Had a half-hour appointment, drove another 10 miles (around 20 minutes again) and by the time I got in to the office, the temperature was up to 82. I got an indicated 35mpg (so about 32 in reality) over the 18 miles in the 45-60mph range.

I have no idea if the two things are linked, but I found it curious that the fan would be running when the car isn't fully warmed up and wondered if it could be a dodgy temperature sensor causing the fans to kick in periodically making it even harder for the engine to get up to temperature.

 

[Neil Wilkinson]

Yes the fans continue to run after knocking engine off when using AC. So firstly I guess were you using the AC?
As you say they fans also cool the turbo, or it could be a faulty sensor.
Really need more input from others

 

[Jemster]

Yes the fans continue to run after knocking engine off when using AC. So firstly I guess were you using the AC?
As you say they fans also cool the turbo, or it could be a faulty sensor.
Really need more input from others

Ahhh interesting. I always (usually) have the AC on out of habit, I'll try running with it off and see if it makes a difference.

I had read that the AC should only cause the fan to use the lowest setting. These were definitely running at full speed, ready for take-off!

Just ordered up an OBDEleven, maybe it'll let me monitor fan utilization (or something!) but I'll throw Carista at it tonight and see if there's an error code (unlikely).

 

[Jemster]

Ok, just back home after another 10 minute drive... AC turned off. Fans blasting again. So I stumped up for the full VAG DPF app and took it for another drive. Mystery solved. Regen was running!

Mind you, that leads me to wonder why it’s decided to regen after only 250 miles and with such a low soot level. But at least it’s not a sensor issue. And the dpf is plenty healthy down at 6g soot and 0.04% ash.

I guess I’m still grasping at straws over the poor fuel economy and in the back of my mind I can’t help thinking it’s down to the huge amount of time it takes to warm the engine up. Darn it, thought I was on to something there

 

[Neil Wilkinson]

Glad you got it sorted mate, not owning a diesel I have no clue about DPF’s or regen

 

[Skull666]

Ac on will drop your mpg too

 

[Jemster]

@Skull666 Yep, but minimally. I've been running with it off as an experiment (for the first time in one of my cars in probably 15 years...) and it's made barely any difference to the "economy".

It's getting very low 30's and the book is 44 Urban - 60 Highway. I'm missing the supposed urban figure by a good 30% - Even on a warmed up engine. When it's cold I'm looking at 25mpg. On a 100 mile cruise at 65-70, I saw 36-38. That's 6 more than my '05 BMW M3 was managing with a 340hp 3.2l petrol lump on the same journey. With the Aircon on.

Before people jump in going "are you thick? You know those figures are unachievable, right?" I can compare with my colleagues Volkswagen CC with the 140hp 2.0tdi. She gets 45-55 all day, every day. She doesn't hang around and it's a similarly weighted / engined vehicle, that should do 56-62 according to the book. It's not unreasonable to think VAG group cars with similar engines should produce figures that vary in a similar manner to the published spec.

Anyway, going to take a look at a 2014 A5 Sportback Black Edition TFSI quattro later in the week... I know I'll take a hit on this one (and it may prove to be far too much to swallow) but having had it 4 months, I'm done banging on about economy and not finding a decent reason. It'll go up by 3-4mpg in the warmer weather from what I gather, but that's still very low. I may as well get 30mpg and enjoy it as fuel economy (and limited availability of well spec'd petrol cars) was the chief reason I went to the dark side Chalk it up to experience I guess.

 

[SRRAE]

I recently found out the water pump or some pump randomly still comes on for a good 30 mins after the engine has stopped.

I realised this having the engine on for about 10 mins, I then removed the top radiator hose to get better access to something. Every couple of mins there was an electrical whine from the front of the engine, which only lasted for a second or 2, but water would come gushing out the top hose when it did it.

This will cause the engine to cool better when the engine is off.

As for the MPG, I'm like you, but I don't have a Quattro. Motorway I can get 55-60+ taking it easy around 60mph. Around town I must be getting low 20s.

 

[Jemster]

They're smart these days . No doubt that'll be to circulate water into the turbo cooling jacket.

I think if I hit 55-60mpg on the motorway, my economy would balance out to a healthy 45-ish which is about what I was expecting. That doesn't seem to happen. If I'm watching the (albeit inaccurate) DIS, the only time I see 55-60 is on an absolutely flat, or slightly downhill stretch of motorway. As soon as I get to anything resembling the slightest of incline (not even what anybody would call a hill!), it'll drop away to the mid-20's again pulling the average down. I'm just about to refill at 330 miles out of this tank with an indicated 20 to go. That's about where I'd have filled up my old '98 323i.

I can't help feeling something's wrong, but the lack of fault codes, and nothing found by the dealer, leaves me without much in the way of clues.