Hi AudiWorlders,

My 99.5 A4 [1.8T Quattro Manual Silver] just rolled over 100K recently and some issues are starting to creep up after 15 years of mostly reliable service. This is a summary of the past week of me troubleshooting an overheat condition. I know it is long, but I hope the detail and context can help me get better feedback on where to go with the troubleshooting.

Day One:
I recently overheated about 5 nights ago. Two days prior I saw the coolant temperature go past the middle point for the first time in 15 years. I often wondered if it even worked because it was always so rock solid in the center. Well it works and two nights ago it was two notches to the right and then suddenly kept going until I got the temp warning. I could not slow down immediately as I was in a narrow single lane dark road with a car right behind me. I was able to slow down to a stop a few seconds later. I hope I did not cause any damage in the few seconds I drove the engine too hot. Once I stopped the temperature quickly went down but the coolant had clearly boiled over and some smoke was rising - likely from the coolant hitting the hot engine and cat. Luckily I was almost home and was able to slowly get home without the temp rising again.

The past few evenings I have been troubleshooting what might be going on. I topped off the coolant the same night of the incident and have been testing things since without moving the car.

Day Two:
The AC Clutch does not seem to be engaging and the electric radiator fan does not come on - even when the car gets to temperature while idling and even when the AC is set to the Low temperature.

Here is what I checked so far:
- Checked Fuse on the left of the dash but none of them seem related to the Fan.
- Removed the driver side knee bolster to get to the Relay Carriers and tested the fuses on the 13-way Relay Carrier (50Amp, 40Amp, 5Amp and the two that did not look like normal fuses), all checked out ok with a multimeter continuity test.
- I could not really get to the first and second positions of the 8-Way Relay Carrier which was behind the 13-way carrier.
The second held the 370 Relay (Second Speed Cooolant Fan Control Relay)
The third position held the 373 Relay (Coolant Fan Control Relay)
- Once I removed two bolts from the 13-way Relay Carrier I was able to better reach the 8-way Relay Carrier. I still could not get probes into position 2 because there is a large wiring bundle right in front of that position. So I removed the relay socket from the relay carrier and was able to pull it out a bit so I could get probes to it and pull out relays.
- I tested the 267 Relay which was on the 13-way carrier and it switched, clicked and the multimeter detected continuity across 30 and 87 when 12v power is applied to 86 and 85 on the relay.
- I tested the 370 Relay which was on the 8-way carrier and it switched, clicked and the multimeter detected continuity across 30 and 87 when 12v power is applied to 86 and 85 on the relay.
- I tested the 373 Relay which was on the 8-way carrier and it switched, clicked and the multimeter detected continuity across 30 and 87 when 12v power is applied to 86 and 85 on the relay.
- With the ignition key off, when I tested continuity to ground, I was able to get continuity on both relay sockets in the middle bottom slots. Did not get continuity to ground in any other slots on either relay socket.
- With the ignition key on, I would no longer get continuity to ground, but did get 12v on the two middle bottom slots in both of these relay sockets.

So it seems that power is getting to the relays – which might confirm that the fuses are also ok.

Still get no electric fan action with the ignition on and the AC on Low temp – which I think should trigger the fan to come on.

Currently looking through the Bentley to see where the wires for the fan are so I can test for power.

Day Three:
This morning I decided to put the VAGCOM/VCDS on the car to see what codes might be there.

Here is what I received from the HVAC module:

-------------------------------------------------------------------
Address 08: Auto HVAC Labels: 8D0-820-043-1D.lbl
Control Module Part Number: 8D0 820 043 N
Component and/or Version: A4 KLIMAAUTOMAT D53
Software Coding: 00140
Work Shop Code: WSC 06335
VCID: 2B5E15B6D97EA2461EF-2566
1 Fault Found:

01206 - Signal for Duration of Ignition Off Time
27-00 - Implausible Signal
-------------------------------------------------------------------

The good thing is that when running through the output tests on the VCDS, I was able to turn on the Radiator Fan. Unfortunately when the Output test tried to turn on the AC Clutch, the clutch tried to kick in but did not and the VagCom/VCDS gave an error of 'Function Not Available'

At this point, it seems that the AC Clutch might be a factor here. I will now look up what conditions would cause the AC Clutch not to come on.

Day Four:
After a 24 mile test drive today to see how things went, I can confirm that the aux fan does indeed come on when the temp rises but the car is still running warm.

I took a 24 mile test drive (12 miles in each direction) to see if the car is still overheating, see if the aux fan comes on and see if the bottom radiator hose gets hot (trying to check the thermostat). On my way there the temp slowly rose to the midway mark and got there at about mile 4. Then it remained in the middle until about mile 7. Then it started rising past the middle. When it got to one mark past the middle I turned on the heat at full hot and fans at top speed. This seemed to steady the temp and even dropped it to the halfway point by mile 11. Then it rose again to one mark past the middle and stayed there. When I got there (mile 12) I let the car idle for a little bit and checked the top radiator hose (which was somewhat hot) and tried to locate the bottom radiator hose but could not immediately locate it in the dark. The radiator was hot to the touch. I turned the car off and waited a few minutes but the temp only went down to halfway.

On the way back I tried not turning on the heat but by mile 1 the temp was already rising so I turned on the heat at full blast again and the temp stayed steady at one line past the middle. By mile 11 it started to drop and when I parked in my driveway it was in the middle.

I popped the hood and the electric/aux fan was running. Maybe this is what dropped the temp at the end there. It was running at what seemed full speed. The main viscous fan was spinning but I could not tell if it was spinning as fast as it should be. I tried revving the engine but did not notice any change in the viscous fan speed.

I then went under the car and felt around the hoses there. The hose which seemed like the bottom radiator hose was warm but not hot.

Not sure I learned anything other than the car is still running hot and I assume if I did not turn on the heat it would have overheated.

Going to check more things tomorrow when the sun is out again. At this point I am thinking it is an issue with the circulation of the coolant. Maybe the thermostat, maybe the waterpump - maybe something else.

Day Five:
Ok, so today I put the car up on ramps, got under there and cleaned things up so I can tell if there are any current leaks. I then turned the car on and let it warm up. The top radiator hose warmed up but the bottom one remained cold. Waited a bit longer and checked again and the bottom hose right off the thermostat was still cold. So I then tried the AC just to see if the electric fan came on and what do you know, not only did the electric fan come on but the AC came on! Wt?!

Then I checked the bottom radiator hose and it was warming up. Maybe I did not wait long enough as I doubt the AC caused the thermostat to open up. I guess the AC compressor could have put some more load on the engine and caused the coolant temp to go past the temp the thermostat needs to open.

I left the car running a little longer and it did not move the coolant temp past the middle. But I did start seeing some leaks under the car a drop oil leak and coolant leaking. The coolant was coming down below the transmission so I assume it is something above the transmission.

So I am still puzzled. Could be that the coolant was low due to the leak and that is what caused the overheat the first time. Then I put in more coolant thinking the boil over caused the low coolant condition but maybe it was low before the boil over.

Maybe the leak developed a few days before the boil over and the cooling system was not keeping pressure which is why it is overheating. Next I will go back out there and try to track down the coolant leak. Maybe I’ll go to parts store and get something to pressure test the cooling system.

After running the car for a about 30 minutes again, I could not track down exactly where the coolant leak is. There is definitely a leak and it is about a drop every 45 seconds or so. It is dripping from the transmission but I think it is originally coming from the coolant temperature sensor house area. I see that area wet and when I dry it off, it gets wet again but even with the car on it is not a lot. Maybe it is only seeping but is enough to create a drop by the time it reaches the bottom of the transmission. It is so tight back there that I can't fit a mirror and my hand at the same time to be able to wipe it off and see if it gets wet again. In any case, something is leaking in this area. I guess it is possible there is also another leak somewhere above the transmission but I am not sure what else could be leaking up there. I turned on the heat full blast to see if the heater core made the leak worse when it was in use but the leak seemed to stay the same. Turning on the AC (which now seems to come on consistently) does not seem to change the rate of leaking, it just causes condensation/water to leak down towards the passenger side of the transmission.

Here are some pictures of what I was able to see: https://www.dropbox.com/sh/phjyups59...1jmOKG8Ea?dl=0

Not sure how else to test for leaks in other parts of the system. Seems like I might have to tackle replacing the coolant temp sensor housing - which seems like an involved job..

What else would be good to do while in there? I would change the coolant sensor itself - anything else worth changing at that point? I am tempted to do the water pump and all the belts at the same time while I'm in there. The valve cover gasket was recently done so I'll leave that alone.

Any suggestions would be welcome,

Larry
99.5 A4 1.8T Quattro Manual Silver
91 VW GTI 16V

 

This weekend I spent quite some time replacing the coolant flange. I looked up as many youtube videos and guides as I could. Printed out this one which turned out to be a good guide for me: :: a4mods.com :: - The Premiere Audi A4 Modification Guide and Pictures Library

I removed the ignition wires, fuel rail, coolant expansion tank and stopped short of removing the air pipe which sites right on top of the flange. I could not get the hose off of this pipe. From what I read later, it seems the rubber elbow at the end of this pipe tends to break so its probably good I did not remove it. Removing the bumper let me lean over the engine and reach behind.

The dealer ended up selling me the wrong coolant temp sensor, my 99.5 uses the part without the A at the end which has the larger connectors. After swapping that part out and draining the coolant - I was able to change the flange, coolant temp sensor, the plug on the flange and both plastic clips - all with new OEM replacements from the dealer. I do not currently have a torque wrench that goes as low as the 12 lbs that the flange bolts require so I just tightened it snug without too much force. I cleaned up around the flange area but did not sand it down - hopefully it is clean enough and does not leak.

After putting everything back together, I air bled the cooling system after refilling and let the car warm up until the temp was in the middle. It was great to see that everything still worked as I had never taken this much stuff off the car. I went for a test drive and unfortunately it started running hot. Did not overheat but reached the second mark past center. I drove back home and did not see any coolant leaking - which hopefully is a good sign.

At this point signs point to the water pump and it looks like I am looking at a water pump and timing belt replacement. I was told the timing and water pump was done at 60K but I am not positive it was done - I will find out now it seems.

Found a short video on how to test if the water pump is pumping, going to research if this test will work on the 1.8T AEB engine:
Looks like I will be tackling a water pump and timing belt replacement. Learning lots fast, wish me luck..