Hi All,

I have a rather strange problem on my Audi A6 4.2 V8.

It takes a long time to get to temperature. especially in cooler/wet weather.

IT does however get to 90C on the guage. Confirmed by matching diagnostics.
No error codes are retrieved.

I have had the Thermostat, Waterpump, 2 temperature sensors, expansion tank and a small radiator hose to the expansion tank replaced. This was carried out by an audi specialist so am trusting they got rid of air locks.

I have noticed when the temperature gauge drop its when i put the hot air on in the car or when i leave my foot of the gas at speed for a long time.

When the temperature drops the heat from the blowers in the car also drops.

The radiator has been flushed 2 times by 2 different specialists.

I was told the thermostat was faulty and also there was an aparent water leak from the water pump confirmed as i saw the rust on that section of the water pump.

The car does not over heat and does not go above 90C on the guage.

I cant think its anything else mechanical and has to be electrical.

Anyone have any thoughts?

 

I say if it does not overheat, leave it alone. No harm done.

 

Wow. That's definitely an odd one. never heard of a car running too cold before.

 

I'd suspect that the thermostat is stuck open.

 

I used to have a BMW with that same problem. On the highway the temp would drop to the bottom just because of the cool air blowing throw the radiator for a constant time. The heat always worked on mine though. I dont think that your car has a "problem" but it would get annoying if the heat stopped working.

 

NH_USA,

I thought the same as you i took the car to an audi specialist and they replaced the thermostat yesterday. They had said it was the rubber on the thermostat that had gone. Also the water pump was leaking but only small leak. They said that was common as there is a bleed valve on it that they aparently plug. I saw the old parts and there was definately a prob with the pump. Not sure about the thermostat though. I have had the temperature sensor replaced as well.

There is something suspect and i have been going through it all night. I even got up at 4:30 am this morning to have a tinker see if i could see anything else. Took the car for a spin this morning for about 30 mins. The characteristics have changed. Car gets up to temp and stays there when heater is off. It does from time to time but not very often get cooler when i take foot of the gas but i think thats cause i have taken a lot of a load of the engine and its the same effect as putting the heater on but then rises when i gas up again.

I have also had a squeeze of the top and bottom radiator hoses. The top hose has fluid in and feels firm when squeezing. The bottom hose felt a little less firm but when holding each pipe i could feel the pressure when squeezing 1 pipe through the other pipe so i dont think there is an air pocket there.

If it were not for the drop in the heater air temperature inside the car id have put it down to an electrics. But going through the scenarios ive come up with are either:-

Thermostat faulty. I know from what i was told the old 1 was faulty. The chances the new one is faulty are improbable but still possible. I will call up the specialist when they are open to see if they have any other thoughts. Shame as i spent roughly £700 on getting it done.

Water pump. I know from visual inspection that the old pump was leaking a small amount but nothing to write home about. It had metal pump not the plastic one. Apart from the leaking housing there was no corrosion to the part that matters.

Blockage or an air pocket in the system.
If it were a blockage i thought it would have been cleared as its been flushed 2 times can tell 1st attempt i used blue coolant. 2nd attempt red. Fluid was not contaminated. I would also think a blockage would have caused a rise in temperature characteristic. Even if it were a blockage in the radiator. If it were an air pocket in the system i think that would cause a disruption to the flow through the system. Particularly when the cabin heaters are on. Its suspect as i see there are 2 bleed valves on the heat exchange pipes for the cabin heater. I would love to be able to find them but am unsure what im looking for. I know the 1st attemp these were not open and the car heater was not on full. 2nd time heater was on HI but 1st time i used this garage and may not be sure they opened the bleed valves. If anyone knows where i might find these bleed valves or if anyone has any pictures of what they look like i would greatly appreciate it. I had a look this morning and took the top cover of the engine for room and a better view but could not really see alot. Also is there a specific order in which to open them.

If it was a rad temp switch fan would either be off all the time and get too hot or it would be on all the time thats not the case. It comes on at the right time and the charactersitics dont match what im seeing. If it were the electric fan then the problem would only appear when the temperature switch reaches its temperature. If it was this the characteristic would be there all the time and not just when the cabin heater is on.

Cheers for all your responses so far. They have all been good and i have had the help of my wife a theatre nurse here in the uk and have been talking terms of hearts arteries etc. I am looking at this in a whole new light. If i get any answers through the day i will keep the post updated. ITs one of those things. Its winter time and all i need is reduced heating in the car. -2 outside while i was tinkering and driving. Brrrrr :-)

Many thanks from a desperate and cold individual.

 

Thanks for updating about this. I have something close to this. It takes a long time to get to 90, about 30-40 mins with 0-5 celcius outside. It may stay around 80 for a long time.

The plan is to replace the thermostat tomorrow.

The question is: if you open a diagnostic channel on your A/C control head unit - it has coolant temp channel. I believe it has a separate sensor showing temperature of coolant flowing into the heater radiator (input). On my car IT SHOWS 90 5-7 mins after I start. Does anyone know what is that and why there is such a difference in coolant temperatures there and on the cluster gauge?

 

Automotive cooling systems are designed to remove the excess heat developed by the motor. They normally have a higher capacity to remove heat than the motor can develop. System designers added a thermostat that automatically restricts water flow so the motor can operate at an optimum temperature level. Thermostats are available with different temperature ranges – normally 160 – 180- 190 F etc

When you increase the power (press down on the gas) the motor works harder and puts more energy into the water increasing its temperature. The thermostat responds by opening further to lower the temperature. When you take your foot off the gas the energy decreases and the thermostat closes a bit to maintain the temperature.

Likewise, if you turn on the heater it acts like an additional radiator and the Thermostat closes to maintain the temp. ETC. ETC. I think you get the idea!!

If the thermostat is stuck open or by-passed the water flow is not controlled and temperature of the water then varies with the amount of energy input and/or the amount of cooling available. (ie heater, air flow through the radiator, etc.)

In your case the thing to look at is potential water flow control problems. A bad thermostat is the most likely cause, followed by air in the system. If you were overheating, I might suggest looking at a hose that collapses or a plugged radiator but your problem seems to be lack of water flow restriction because it goes from normal to cold.

 

I have, along with my 2 A6's, an A4 2.8.I hadthe same issues. The dealer first told me the heater core was clogged. I double-checked their findings and found that it was not clogged. I took it to another dealer where they connected a sealedflushing machine to it and flushed the antifreeze then installed new green A/F - all while sealed. I have had absolutely no problems with heat or temp gauge since. Even at idle it now maintains interior heat. Air in system. They sure make it difficult to get air out.

 

I’d suggest devising something preventing the air flow to the radiator – like an air flap – piece of cardboard as a screen in front of the radiator.
Observe the coolant system behavior with restricted air flow.
Size the screen from small to bigger; place it away from the radiator to prevent unequally heated areas of the radiator as much as possible. If the screen helps I’d accept the solution. (Unless you have resources to fight for the ideal car… and mental motivation).

This is the way I drive (winter) my ‘86 4kq ever since radiator replacement. Thermostat is new, flushed, flow is ok, but the system can’t hold constant temperature. Summer time temperature gauge gets up to 110C but what is the real temperature is not clear.. . 21 years old car..