MP4.2+6.0

There is no sure thing answer here.

Fundamentally if operating capability of system can not deal with heat generated, it will get too hot. From your description the fact it is not overheating while driving but is while not (if that is a basic summary) can be any of balky fans, clogged/screwed up core or faulty thermostat. The air blowing thru radiator at speed is enough to handle temp load basically, while at slow speeds or stopped it isn't. I've never heard of Audi's adjusting operation based on a driving speed signal BTW; possible on later ones, but haven't heard of it. The MAP ones like W12 and S8 (and probably 4.2FSI) do react to a high load signal and then dump thermo more/full open to drop coolant temp in a second or two.

I had this same issue once on my 1985 5000. I changed thermo. No help. Changed sensor. Nope. Even changed water pump I think in case impeller was screwed up. Nope. Internally failed radiator core as it turned out. Always used correct coolant, no rust, no leaks, etc. Also first car I owned that had plastic side tanks with core (modern type) instead of the old school brazed brass approach.

Or W12, would get the cooling malfunction light show. 115K miles. Got codes too since it has the MAP thermostat function so has a better (but still not exact) idea what car is doing there. W12 and S8 have a heater element so there is MAP control, but still has bimetallic part. Would also run cold blooded. Finally pulled thermo. Thermo physically fine but a piece of rubber seal that fits around the bimetallic part came off and jammed it part open. Might have been able to go full open, but couldn't go full closed since my issues were when running cold in mountain winter driving (like 0-40F).

Or 2000 A6 C5 4.2. Overheated at times. Sort of like your description. Piece of rubber shroud for hood latch fell into one (of three) fans and jammed it. Burned out. Turned out the one affected was the basic engine cooling fan. Separate from the A/C fan and also (on that one) a clutch fan. The other two fans could kind of "cover up the problem" at least while A/C was on. I noticed it because temp gauge would move more than normal and fan(s) would kick to jet engine mode even in moderate weather sometimes.

Back to yours. Next idea since you seem to want to stay with fans. There are of course two, not one. I am not sure of theory of operation of D3 4.2, but suspect one may be basic engine temp fan and the other one the A/C related fan. They may both run together, but my prior Audi experience is each one has a separate function. First determine that. Thus, turn on and off the A/C Econ button and see what that does to each fan. Next heat soak the motor by running up to operating temp and then forcing temp somewhat higher. Do that either by what it is already doing, or with the big piece of plastic over grill idea (used to be cardboard on radiator face, but can't reach there any more). Now you will know which fan does what, and hopefully which one seems faulty--if one is. From your description I expect it is the basic motor related one, not the A/C one but can't be sure. If one is faulty, it can be either controller/electronics or fan.

And if you exhaust fan ideas, then back to radiator or thermo. 1:100 corner cases on sender if has weird internal failure but works a bit.

DA8

Thanks MP. Yea I do suspect fan most simply because it is not coming on. Neither one is on without HVAC on, they both show 10.2% reading from ECU in vag test block but no movement on that value. I don't think there would be any problem if they were both actually moving at 10.2%, the system would probably cool sufficiently. They don't move until the signal jumps up to 14.9%. That isolated alone by itself would suggest the fans are faulty or that the 10.2% signal is not being translated to corresponding voltage at the fans.

With HVAC on the percentage jumps up in stages over a few seconds. #2 goes up a higher percentage, like 60-70%, so I suspect that one is the more AC dedicated one.

What confuses me most is when HVAC is off and temp is going up beyond warning light stage the ECU is not commanding a higher fan speed percentage, yet stays at 10.2. I just assumed the car was smart enough to read the temp signal value too high and push up the fan speed. Then it would not get into the warning heat zone. Any idea why it would not do this?

Also what do you mean by: "1:100 corner cases on sender if has weird internal failure but works a bit."?

BTW, not that symptoms call for it, but last night I ordered up a $30 temp sensor just because it is recommended as a simple rule out in other threads. But I am getting temp readings so I don't see much merit for that being the problem.

MP4.2+6.0

Don't have enough info/understanding of this without delving into something like the SSP for the port 4.2 when it first came out (which is a D2 era publication). Bentley might also have some info in diagnostics.

Something like the thing is internally barely working and cracked internally or something causing sensor element inside to pull away from the metal tip. Then compounding speculation it could be reading more of an ambient temperature from the surrounding shell of it than the true coolant temp. in this scenario driving at some speed it would be less heat soaked from underhood heat than when stopped or moving slowly, plus doubly insidious if fans then aren't going with much oomph to clear air underhood. Again, corner case, but sensors near death can do weird things sometimes. Given the sensor does fail, not a bad investment anyway. Just go in knowing it is way deep down and don't drop plastic retaining clip down under. Easy to lose. Tie some dental floss on it or something in case it falls.

hunterp27

If this turns out to be fan related, I have two used fans. Left and Right. Both are working 100% properly. when chasing my AC issue a few months ago, the Audi dealer told me that I had a bad fan and that was putting my Compressor into protection mode. That turned out to be completely false as I bought these two used fans out of an 09 A8 and installed them. Nothing changed. So, I remove the two fans and there sitting in my garage. If you want them you can have them both for 100 bucks.

Jack88

How good is the insulation to the windings for the fan? If the insulation on the windings are breaking down, I could see it requiring more power to get it going and less speed even when it does. Anytime something electrical is a possible culprit, it's not a bad thing to check the condition of the wires and plugs, either. I might be way off base here, but it is an odd problem.

DA8

So I did a bunch of investigating this weekend and still have no good lead. Here is what I did and found, replaced coolant, replaced $30 coolant temp sensor, did a bunch of testing with vagcom.

No change, still the mini MMI dash 3 beep thermometer pops up and goes back out when cooler.

So the dash warning beeps and lights up at about 95-96C and goes back off at about 92ish. Idling temperature hovers around 91-96C, it doesn't get above that.

FAN CHARACTERISTICS:
-With HVAC off fans are at 10.2% and at 10.2% they are not moving. When it gets above 93ish fans will go to 14.9% and at that percentage they are moving moderately.
-With HVAC on HI fans move from 20-50%
-With HVAC on LO fans move 60-70% and start to make slight jet engine noise

It does not seem to be getting above 100C at all. Measuring blocks in VCDS are under engine block 004 for coolant temp and block 135 for fan speed. Specification when hovering over the coolant temp box says 80=warm and 110=hot.

I cannot find any related fault codes, just the warning light on dash.

So I am looking for ideas here. I am curious about a mysterious fan control module??? What is this and where is it? My main suspicion is that the fans are not running at the base level of 10.2%, other than that they seem to run well.

I also tested with a thermal scanner around the coolant system and seem to be getting about 88C max going into the radiator and 50C coming out. The temps on the VCDS measuring blocks were a few degrees higher than my readings with the thermal scanner direct on the system.

Can anyone help answer these questions who has vagcom?
1) When at 10.2% do your fans run or are they still?
2) what is the normal operating coolant temperature? I am operating at 90-96C according to VCDS. Is that really high or normal.

ltooz_a6_a8_q7

If it fixes the problem then find the correct sensor(s). Find the RELAY that turns on the fan. This problem sounds like my old Alfa Romeo on hot day. Overheat at stop and go, turn on heater, temp goes down because the fan of the heater cool the coolant.
Flush the system, BLEED all the air out then it fixed it, Replaced all sensor and relays.
Cheers
Louis

DA8

I'm wondering if it could just be a bad coolant level sensor or connection for it. I hadn't thought of that like I said I replaced the reservoir about a month ago and the sensor is integrated. Could be connection is goofy too. I drove about 10 miles to a friends tonight and it only came on at max temp right when I got there it was at 95 at top of the hill by his house. Left a few hours later and it came on immediately before it got hot.

Light definitely seems to go on and off when at max operating temperature of 92-96 which is why I was thinking it was an overheat situation. But I'm not sure it is overheating. Operating temp I've been monitoring quite a bit seems to be around 90-96C. I'm not sure if that is hot, but it seems less than the 110C hot I've heard of. Fans do turn as stated below (just not at 10.2%). Curious to know what temps others are getting for their stable warm temp.

storka

It always possible it was the last thing you repaired or changed is the culprit if the error start soon after

DA8

The connection for the coolant level sensor at the bottom of the reservoir was loose when I checked it. Wires were fine, but the little tab that clicks it into place was not locked. Clicked it in and no more warning now, runs perfect as usual.

What threw me off about this one was that the warning only went off when the temperature climbed above 92 degrees and would go back off when the temp dropped back down to 90 or so. It really seemed temperature related. I theorize that the warmer temperatures expanded the tank and the connector disengaged, then would reengage intermittently when the temp went back down.

Still it was an interesting exercise and I learned a lot studying the operating characteristics of the coolant system. I am pretty sure the temp regulates with fans around 90-96C normally. That is what I have observed. Also it is curious why the fans do not turn when VCDS readings are at 10.2%?

Thanks everyone for the help and ideas. Even if they are a shot in the dark it helps to get another look at things and get more ideas.

I am still curious from others the answers to these two questions,
1) When at 10.2% do your fans run or are they still?
2) what is the normal operating coolant temperature? I am operating at 90-96C according to VCDS. Is that high or normal?