A6 4.2 V8 Engine Temp

[cg084]

Hi all,

I have an A6 4.2 V8.

It takes ages to get to temp 1/2 way when starting from cold.

If i put the heater on temp drops to about 1/4 on the gauge.
If i turn the heater off it comes back up.
If i stop in traffic it comes back up with heater on.
If i have the heater off it get to temp after about 10 mins and stays at temp til i coast the car for a distance.

I did have a very small drip leak from the expansion tank and small rad hose that connects to it. At the time the gauge went up to 1st notch after 1/2 way. Noticed immediately and put heater on full.

I replaced the hose and expansion tank.

Since then have had the temperature falls. I have replaced the Thermostat, water pump and 2 temperature sensors. There has been no change. The car is not using water.

I do notice a temperature drop in the heater air when this happens.

I think its an Air lock in the system but the specialist does not agree as they think it would cause overheating.

Does anyone have any thoughts. Also if anyone has any pictures of the bleed valves that are on the 2 pipes going to the heater blower matrix.

Cheers.

[CptMike]

Hi cg084,

I drive a 2.8L 30v... I'm not familiar with the 4.2L. But I think you might just be right about air in the system.

Temp sensors seem to almost always be mounted at the highest point in the coolant flow, excluding the upper hose and the upper part of the radiator. When a thermostat opens, coolant is pumped through the system until it reaches a designated temperature, and then thermostat closes again. Any air still being held in the cooling jackets and passages within the engine block with rise to the top, probably about where your cooling sensor is located. That would account for the low gauge readings - there is no coolant actually touching the sensor, only warm air. When the temperature in the block rises enough to transmit heat to the thermostat, the coolant starts flowing again, air bubbles and all, and the cycle will repeat. The gauge will go up for a while... The heat being on or off just varies the level of coolant that is actually flowing through the system. Air in the system will cause the problems you have described.

I ran into this same problem on an '85 Mercedes 300D Turbo Diesel. I replaced all the hoses once - there were 23 of them of various sizes. What a pain! When all was done I bled off the air through the bleed valve as per the shop manual. But my temp gauge and heat were very erratic. I felt like there was air in the system that could not escape and leave the actual coolant path when the thermostat was closed, even though it was a pressurized system. With engine off and forgoing the bleed valve, I disconnected the top radiator hose at the top of the radiator and, holding the open end as high as was possible, poured it full of coolant mixture. Bubbles did rise from the block. I then held my hand over the end of the hose and positioned it right in front of the radiator hose neck at the top of the radiator. Quickly removing my hand and shoving the hose on the neck, I tried to keep coolant loss at a minimum. Tightened everything back up and found that I had evidently eliminated the air pocket in the system. No problems after that.

With everything else that you have already done, I would give the "fill it up from the radiator hose" method a try. It would cost nothing but a little time an a small amount of coolant spilled when placing the hose back on the neck.

Hope you get it solved...

[cg084]

Hi all,

More of an update.

Doubt its an air lock. Just heard the electric fan cut in.
Engine temp was at 70 had only been driven for about 5 mins. As i pulled up on the car park at work i heard it come on shocked as car was not at temp. Heater was on though. Turn of the ECON and the electric fan stops.

Hmmmm

Any thoughts temperature sensor or heater controls???

[CptMike]

Hi again, cg084...

Was that temperature at 70 a reading from your gauge on the dash though? I would assume that it was since you were traveling. That is why I was talking about an air pocket at the temp sensor - if there is an air pocket at the temp sensor the gauge will read lower than the actual coolant temperature, as the liquid would transmit the heat better than would air. Again, I can not address the 4.2L, but every car that I have owned with an electric fan had a temp sensor for the gauge, and a sensor/relay for the cooling fan (including the aforementioned Mercedes, a SAAB 900 Turbo, and several others.) If the sensor/relay for the cooling fan is actually touching coolant, while the sensor for the gauge is not, this would give you the activated fan while the gauge is reading low. An air pocket (read also as a low level of coolant in the block) would also cause the sensor/relay for the fan to activate sooner as there is less coolant to which the heat from the block can be transferred.

Please know that I'm not trying to play "the guru of cooling systems" here... I'm a long way from that. It is just that the symptoms so closely match my previous experiences that, where I a betting man, I'd put money on it. My concern is not about being right, but in helping you get your problem solved. Hope you don't take my post the wrong way...

Anyway - If I am wrong, I want to know about it, too. That way I will have learned something!

Hope it works out...

[cg084]

CptMike,

Firstly i am glad there is someone out there with ideas. If i had to put money on it id put it on an air lock too. THe symptoms fit that perfectly.

I only mentioned the electric fan as i thought i was on to something. It turns out i was not. Wheat i have noticed is that my problem has worsened since i had my car in to the garage to get the thermostat, water pump and Timing belt changed.

Before i changed these i had the leak in the system. Understanding that its a pressurised system if water gets out air gets in. Before changing them if i had not got the heater on in the car it would get to temp almost normally. i had noth thought of the air lock at this time as people and garage mechanics suggested the thermostat. But thinking about it the symptoms would not suggest it.

I have booked my car back in to the garage that last did work on the car its there problem and they have to fix it. Its the 1st time ive used this garage and only picked it on recommendation and it was an audi specialist.

My usual garage would have done a better job i personnally know them all well only problem was that i now live about 40 miles away from them and is awkward to get there.

Car is back in on tuesday guess it will still bethere in to wednesday. I shall post back when i figure more out.

Thank you greatly for the suggestions.

[cg084]

All,
Have checked the car over Thermostat works ok starts to open at 87oC and fully open at 102oC. Bleed and refilled system. No air locks. They have checked the car over with an infared thermometer. Radiator gets evenly warm as do all pipes. There is a very small difference in one of the pipes leading to or from the heater matrix. Diagnostics have revealed nothing. The temperature sensors report no faults all electric fans operate as they are meant to.
In the end the garage has passed it back to me to try an engine flush with an audi flushing agent. One thats not stocked on the shelf. The reeled of a technical bulletin titles engine not getting to temperature and particles in the coolant. There was no particles in the coolant with my car.
So Not Thermostat.
Not Water pump.
Not Temperature sensors.
Not Air lock.
Any ideas???