overheating and fan running full speed

[ggenovez]

Hi all.

I have a 2003 A4 B6 1.8T and it recently started running the fan full speed for over 20 minutes after it's been parked. Yesterday while driving home she started to overheat. Coolant is full and thermostat is 3 years old.

Anybody run into this issue before?

I'm thinking it might be the fan relay. any idea where that is located?

Thanks

[egovreau]

The fact your fans come on indicates that the relay is functioning. The fans are coming on due to the overheating condition.

It's possible that your water pump has failed. It's not beyond the realm of possibility that the impeller has separated from the shaft. This is more common with pumps that have a plastic impeller.

I think you can check it by removing the thermostat and trying to spin the impeller by hand. If it turns, then the pump will need to be replaced.

Another possibility is a cracked head/failed head gasket allowing exhaust gases to be injected into the coolant. There's a test kit for that found at any hardware store.

There are probably other causes that escape me at the moment. So just troubleshoot the issue.

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[ggenovez]

@egovreau,

thanks for the info.

I have some more info. I also have the fan switching on and off even when it's hot. It tends to over heat in traffic or when going through a drive through or even at a red light. Last week at a red light it started to over heat, just barely past the line and when the light went green it kept the temperature at the same level, but this was within the city so no highway speeds. I'm now thinking it might be the module.

What are your thoughts?

[egovreau]

There is a temperature sensor at the outlet of the radiator (lower hose, driver's side) that will send a signal to the FCM to turn the fans on if the radiator isn't cooling enough; like when stopped in traffic with little to no air flow through the radiator.

So my guess is that the sensor is going bad or the FCM.

That sensor is a two wire, as opposed to the four wire on the coolant flange, and are relatively inexpensive ($15-20 last I looked) vs the FCM which is not.

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[ggenovez]

That's what my mechanic thought as well and he replaced it. same problem.

[Gunnarrrrr]

If you're not blowing white smoke occasionally out of your exhaust, then you could skip the combustion gas coolant test, or at least save it for last.
You never mentioned if you have heat, that would indicate if your thermostat is opening or not. If your thermostat is stuck closed, you'll overheat pretty quickly.
+1 on the water pump check.

Since you have new working temp sensors, personally I would pop your alternator and intake manifold off and swap the thermostat and j-plug IF those 3 year old parts are anything other than OEM or OE Mahle, for anything else its very likely it could've failed within 3 years. I've seen a bad aftermarket one fail in 3 months. (Mahle only for the thermostat($66 rockauto, TM3100), OEM ONLY for the j-plug and hard pipe to thermostat o ring($16 with coupon code genuineaudiparts.com, 06B103033A & N90365302) and while its out, stick your finger in there and check the pump impeller, and give it a good look with an endoscope.

As for info about he temp sensors, the lower radiator hose one is what the ECU uses, and the rear coolant flange sensor is just directly connected to the dash cluster to give you a reading.

[Mike Tries]

Hey, so maybe the car really is overheating.

I think one place we should be looking is at the radiator itself. How old is the unit that's in there? If it's clogged or scaled up, then it might not be operating efficiently anymore. Your symptoms of overheating while not moving, even with the fans cycling on, might indicate a problem like that. If it was a faulty sensor, then you'd be just as likely to see the temp gauge climb while you're coasting down a hill on a highway at 70 mph as you would be sitting in a drive through behind a drunk college kid while he argues for 10 minutes with the unfortunate Wendy's drive-through person because he wants a "Big Mac", and he knows he's at McDonalds.

Let's talk coolant, too. Do you use the appropriate coolant that matches the Audi specifications? Preferably coolant sold by Audi? If not, then we need to look at the radiator even more closely, because you could have mixed incompatible coolants in there. There's some sciency stuff that happens when you mix the wrong coolants, and I don't fully understand it--I just know not to do it. Maybe they react and form solids that block coolant passages?

Is there air in the cooling system? I had this issue on my own car. I ran it on the dyno, and it overheated almost immediately. I drained and then vacuumed the system and refilled it. No issues after that.

How's the thermostat? Is it new? Regardless of age, let's make sure it's not getting stuck closed. If you're removing it to check it, you might as well just toss it in the trash and put a new one on.

egovreau made some very good points--check to see that the impeller is still firmly attached to the water pump, and check to make sure you're not dealing with a cracked cylinder head. The fact that the car cools off while you're moving suggests to me that the impeller is at least moving water (i.e. not completely detached), but it could be slipping, or broken, or something. Check on the J-plug to make sure it didn't disintegrate into that cavity and damage the impeller (I seem to remember that the J-plug is like... right there behind the pump.) My own J-plug came out of the engine when I rebuilt it in pieces. It literally crumbled into pieces.

If you really are experiencing overheating, and not just erroneous temperature readings, then it's best to figure that out as soon as you can so that you can avoid damaging the car. I had an overheating issue when I drove my car on the race track early on. To be honest, I'm not exactly sure what caused it, because once it ruined two track days, it had lost me enough money to warrant ripping the whole system out and throwing it away. New radiator, thermostat, even the oil cooler. Something fixed the issue, because it doesn't overheat anymore.

Good luck. Keep us informed.

Edit: I reread and saw that the thermostat is 3 years old. I also saw that Gunnnarrrr posted some good stuff that wasn't there as I was writing this. Good advice there. I agree that doomsday cracked head should be last on the list of things to worry about--go after the low-hanging fruit first.

[EuroxS4]

It needs a water pump.