Overheating (but only if going faster than 40mph)

[Raging Insanity]

Hi,

I am hoping for some good advice on here prior to spending serious mula somewhere based on some other imbiciles guess work. I have asked for general advice from two separate Audi dealers, an independant specialist, an ex-VAG serviceman through a friend and all come up with different answers - very frustrating!!!

My local Audi say the water pump has gone and want £500+ just to look at it.
An Audi dealership where I used to live believe the impeller is gon and want around £500 to fix it (if it is that).
An independant specialist thinks it's probably the viscous fan - I will be seeing if it kicks in properly a bit later today.
An ex-VAG serviceman (who didn't know the full facts) reckons its the impeller as they always go.

My own deduction is: the Multi-Function Temperature Sensor has failed.

Car:
Year 2000 Audi A6 1.8T SE 20-valve "V5 Turbo"

Symptoms:
Sit car idle all day: will NOT overheat or cause overheat/low-coolant alarm to be raised
Drive 40mph or less: car temperature will stabilise at normal (90C)
Drive 40mph-60mph: car temperature will slowly climb beyond 90C until overheat alarm
Drive 60mph+: car temp will rise rapidly until overheat alarm
If the car is driven between 40-60mph and the temperature remains 110C or less, then resumption of driving at 40mph or less will see the temperature stabilise back to 90C, as will letting it idle at the road side.

** Coolant level is NOT LOW and I cannot find any signs of leakage anywhere**

I take it from this, that a certain degree of cooling is taking place, but not when the car is driven at speed. It would seem to me that Audi are wrong as I can drive all day at 40mph, or let it idle all day and not have it overheat - something you cannot do if the coolant doesn't circulate when the pump fails. I do not want to fork out for an expensive job (or be charged for an expensive job) which is really just an electrical sensor or viscous fan problem of some kind. I am happy to do this myself if any exhorbitant dealer cost is due to a large labour charge - thing is, with my car seemingly overheating, getting to a decent service garage requires travelling on roads where 40mph will have everybody upset and the police pulling me over and asking me if I have consumed any illegal substances! ("No officer, I just saw the end-of-the-world when Audi proposed their labour charge for investigative work - more than your yearly spend on donuts!")

Has anyone seen this combination of symptoms before? Surely it cannot be the water pump or impeller?

Thanks,
Raging Insanity

[quattronutter]

it may be the water pump,or a blockage,its been known for the impeller to slip and not turn inside causing no circulation,only way to find out is take it off and check...any half decent garage should be able to do this for way less than 500 quid..it may be something else,only way to know is go through all the cooling system,id check the pump first though..

[HPmuirt]

I am no mechanic, hence always asking rather than aanswering questions here, but....

Logic suggests that the reason that you are overheating at higher speeds is that you have restricted water flow.

At idle or slow driving speeds, what little flow you have is sufficient to balance the heat output of the engine, whilst at higher speeds, the heat output suppasses the cooling capacity of the slow circulation, causing the overheating.

This suggests that the pump is working, but you either have a blockage or a slipping impeller. I would first try a back flush on the cooling system to see if that helps. I would also check the hoses to see if the whole system feels pressurised (might point to a possible blockage or more likely at the impeller).

Hope you fix it at minimal cost dude.

Cheers n

Si

P.S. The above is the random drivvel of a self confessed non-mechanic and should be ignored at all costs

[s4_rider]

I had a similar issue, but also my fan didn't switch ON.
Cause: The radiator was blocked (at switch side).

I had my radiator replaced after doing some experiments with another low temp switch and low temp thermostath (VR6).
This finally solved my overheating problem.

[s2avant]

If it's an AWT engine then you can pretty much guarantee it's the water pump. They use a plastic impeller glued on to the pump shaft and they always fall off. Same design as most other new VAG engines.

The reason you only get overheating at higher engine loads is that the impeller rotates with the shaft upto a certain speed. Once the engine speed is high enough the impeller can't move through the coolant so stalls while the shaft keeps spinning.

Ben

[Raging Insanity]

Woah.... Everybody has a slightly different take. I think overheating can be a bit of a 'mare on Audi's and I have only had mine a few months. Thanks folks for the replies so far.

Looks like I will have to get it looked at for a fair chunk of cash.

I did the viscous fan check though and it doesn't appear to be running, which may be a sensor problem. Many people think its the impeller. I have an APU engine code if that's of any help to anybody.

Does anybody know a UK source for printed Bentley manuals?

[CoupeWannabeS2]

Good luck with the A6 overheating issues - £500 is a lot of wedge to look at a car.

Was wondering if anyone had suggestions for a similar problem on my 2L coupe. It also seems to get hotter the faster the car goes.

At the moment I can keep the temp on 90c with the use of the hot air blowers. On a run in summer it would sit at about 100c despite using the blowers to full effect. Does anyone know if this is risky with the 2L 8V engine ? Its a 1995 car.

The first time the car overheated was at 80 mph. It has been losing a slight amount of coolant - but there is a residue under the bonnet above the expansion tank. Am I right in thinking that the car can boil off water if it gets too hot ???

I have flushed the system which hasnt helped. Also going to get the thermostat changed soon - can these partially open and reduce the cooling efficency ???

Also one part of the radiator felt cooler than the other side, but a local garage tell me that the problem could be a blocked rad, but could also be the water pump or stat. Gonna leave the car at the garage tomorrow to see what they diagnose but wondered if you guys had ay pointers I could use.

Many thanks in advance.

[Raging Insanity]

One thing I know about flushing any Audi engine is they typically require the use of compressed air to completely flush them. You can cheat on this with a high power pressure washer like the Karcher variety. This will ensure any sludgy blockages get cleared - these do occur, even though the system is sealed - don't ask how as I don't know, they just do! Radflush is pretty good at helping break up the really solid bits.

My own overheating prob is looking a bit better. I found a mechanic who will fix it for a max £250 fee. Recently my girlfriend was taking a borrowed M-Reg Renault Clio down the A3 and a spark plug popped out, the thread sheared off and the corresponding cyclinder got compressed and seized. She took it to a garage who said it was written-off (quoted £1200 to fix a car worth no more than £400). Then this mechanic said he could sort it... took taway... came back four hours later with it working with new spark plugs and no threading problem in the spark plug chamber - only wanted £50. Not some fly-by-night either, he owns his own car repair company!

I still cannot think of how he fixed it!! Cannot wait for him to take me through the stuff on my A6 and get the overheating problem sorted.

Where are you CoupeWannabeS2, maybe he can have a look? - he is in Dorking.

Raging Insanity