LakeTahoeQuattro

So here is the car in question:


I am about to throw in the towel and take her to the dealer. Ugh! Hopefully some of you here can help me as I have already asked this question on a8partsforum UK and audizine. I have also searched the interweb researching this.

So I got the car for pennies in August because it needed a trans swap. See build thread below if you like. Replaced the trans and she drives fine except for the fact that the coolant lives at a steady 100 to 110degC. We know that it is supposed to live at 93degC.

So when I rebuilt the top end of the engine to include gaskets and timing I replaced all the timing components to include water pump and t-stat. I used a Mahle Behr t-stat.

Last week replaced the t-stat with an OEM and tested the Mahle Behr in boiling pot of water. It opened fine at the correct T. My upper radiator hose is hot, my lower is cold. I also replaced CTS (these temp measurements are from VCDS).

Since the car does not over heat over 105 degrees typically I can rule out water pump.

I am leaning towards a clogged radiator? When I replaced the t-stat last week, I pushed some water thru the radiator and it did not flow out as quickly as it went in. This is a symptom of a clogged radiator yes?

LakeTahoeQuattro

So that's the first half of the coolant problem. Second is that even though there is no coolant flow up front, I would still expect coolant flow thru my heater cores. Yes another d2 heater issue I checked the fuses for the aux pump and they were fine, I can also hear gurgling when I turn on the car which sounds like it is coming from the pump. There are no codes in my HVAC. How do I do an output test/ diagnostic in vcds to test this aux pump?

I bled the coolant lines several times and am only able to get 3/10 heat from the vents. Also more heat while driving, almost none when stopped. Both heater core lines going through the front firewall are warm and bled as much as they can be.

Prospeeder

You are going by the gauge, or vag com, or a point and shoot thermometer? The coolant temp sensor is notorious for failing and reading high. If it really is too hot, radiator would be my guess. Sometimes alot of stuff can pack in between the a/c condenser and the radiator blocking airflow.

LakeTahoeQuattro

Yea external fins of radiator are clean. I think it is clogged. I put in a new CTS, still reads high

nyrobbie77

Feel the hoses going to the after-run pump. They are located on the driver side going through the bulk head near the master cylinder. Are they both hot? If one is hot and one is warm or cold you have no flow through your heater cores. my guess on your no heat is that your after run pump is either clogged closed or stuck closed. when they fail, they can fail in the closed position not allowing flow through the heater cores. Or in my case, someone put the wrong coolant in and gelled up inside the after-run pump.

A new one will set you back about $1k from the dealer. I found someone parting their A8 out and got it for about $100. Or if you're handy, you can remove it and bypass the pump all together. Replacement isn't too terrible. You have to pull the master cylinder and brake booster for access but other than that it's fairly straight forward.

Here is a link on my replacement:

https://www.audiworld.com/forums/a8-...-heat-2818957/

LakeTahoeQuattro

Yes I have read thru that thread several times. I will run the diagnosis this afternoon and report back. Shouldn't the solenoids on the pump be pulsing as well?

Prospeeder

Just get one of those point and shoot thermometer guns, and see what the temp differences are on the lower hose versus the upper hose. The upper hose should be all the hot coolant exiting the engine into the radiator, and the lower should be cooler. If theres little to no difference, its the radiator.

You know if the timing belt is not tensioned correctly on the 4.2, the engine will run hot and even overheat if loose enough on that side because the water pump will slip. Have had 2-3 A6 and A8 4.2s in for this a couple soon after a belt job at another shop. Pull the belt covers have a look at everything. Did you use the cam lock bar and have the cam pullies spinning free, and you put that little eccentric idler pulley at a certain point (the manual says) and then pull the pin on the tensioner. Apply just a little tension using an 8mm allen. Then tighten down the cam pullies with the cam lock bar on.

uberwgn

Another simple check is to use VCDS to view the coolant temp sense output after the car sits overnight. It should be close to or at ambient temp.


I'm sure we've all seen problem water pumps where the impeller is worn away or simply spinning on the shaft. Don't know if that is ever seen on an Audi, but can be elusive!


Do your radiator hose checks as prospeeder suggests!

LakeTahoeQuattro

I will look at vcds in the morning when the car cools down and see what the temp is. That is a good idea.

I have done a few 4.2 belts and never had an issue. Yes I did use the lock bar and followed the procedure. I also tonight removed the timing cover to analyze the tension on the water pump and it is taught as it should be.

So I ran some tests tonight. Second, the top radiator pipe is hot hot, and lower is ice cold. Top is 100degC, lower is 30. Same temperature as ambient. I got the car up to operating temperature and then shut off the car. I touched the radiator thru the viscous fan and the top 5" were as hot as the inlet pipe, below that they were ice cold, the lower 80% of the radiator was cold

Fingers point to radiator?

Now on to heater. I turned on the heater when the car was up to temp and can only get "warm" air to come out of the vents if I have the fan at 5 clicks. Any higher it just blows regular air. I dont think the cores are getting enough heat. Also The aux water pump solenoids are not clicking. I can not feel them pulsing when I touch them. Both of the heater hoses going into the aux pump are hot, after the aux pump the upper hoses on each heater core are cooler than the lower hoses, but the upper hoses are still warm.

When I turn off the car and activate the "rest" function I can hear the pump pumping. Who knows if it is actually pumping or not, but I can hear the motor engage...

silverd2

Totally flush your coolant system.

If there has EVER been even a few ounces of the wrong coolant mixed with G12 without being chemically flushed (forma a thick, corrosive brown sludge), it can clog radiator and cores .... and eventually eat away plastic or rubber fixtures.

Is there any dark residue in the bottom of the reservoir or any brown sludge floating there?