hey all, new to the forum, sorta new to audi. Own several Fords some reaching 800whp..

Heres my issue. I was towing a small trailer and experienced overheating to the point of limp mode. My haul weight was no more than 2500lbs. Ive so far replaced the Thermostat. when I pulled it the ears was broken. Ive also replaced the secondary pump next to it. It was make a VERY LOUD noise. Well Ive burped the car several times and im still over heating..my scanner is reading 250+ adn dummy needle hits past 3/4. Ive notice the expansion tank making alot of burping and bubbling noises.. also coolant coming form the passenger wheel well or around area. You c an see the undercarriage soaked in coolant. But when idle i see NO leaks...anyone have any ideas. Ive been web surfing this for a few days now.

 

If you are getting that much air bubbling into the overflow bottle (on top of passenger wheel arch) that it is spilling out, I hate to say it, but first blush sounds like a blown head gasket. That's not a common failure, but it can happen. It might be worth doing a compression test or having it inspected at a good independent shop.

How long have you had the Q7?

 

Top probability is head gasket.
Other slimmer possibilities:
Defective expansion tank cap.
System pressurisation through fuel/egr/oil cooler.
Contamination or blockage in cooling system.
Faulty thermostat.

If it is the head gasket, don’t repair it without finding out what caused it! Overheating will cause a leaking gasket, which in turn leads to overheating.

 

6 yrs i believe

car has 110k , fresh oil . No smoke or fluid coming from pipes. Engine sounds and feels healthy

 

I had a coolant issue (two, actually) on my Q7, and I was able to discover it early on through used oil analysis (I use Blackstone Labs in Indiana). The chemical analysis can detect trace levels of coolant, which would narrow down the possibilities. I had both an oil cooler gasket failure and, later, an EGR cooler gasket failure.

The UOA costs $40 and they generally turn the samples around within two to three weeks.

 

Update : Ive change the reservoir cap and moved the reservoir bleed tube to a bottle. Here's the results... At fuel load in neutral (floored in park ) The bottle is empty ( rules out gasket? ) yet the ground and piping is wet on the passenger side. From what i can see on my back the intercooler tube on the pass. side near the wheel well is wet...when ever it happens it appears to be under load ( id rev engine and ground is more wet ). I really need a asecond hand to take the car past idle while i observe. Till then does any of this ring a bell?

 

Sure, a lesser known issue is that the plastic coolant reservoir can develop a hairline crack and it will only leak once the system fully pressurizes (engine up to temp, etc.), and I've had this happen on couple other older cars, including a Ford Focus SVT, and several members on here with Q7 have had to replace the reservoir bottle also, etc.

This problem can be frustrating to track down issue as most won't consider that the actual reservoir has a crack in it...normally it's leaking from underneath and/or along the mold lines, where impossible to view it directly. Seems to me the fact of where you are seeing the coolant spray/residue beneath the bottle and on ground easily traces it back to the coolant expansion tank, IMO. There are also coolant hoses that run alongside the engine on passenger side, IIRC, so be diligent about where the spray is ending up, etc.

There are also AUX electric coolant pumps located beneath the engine, which can sprout a coolant leak via a cracked housing or at the hose connection point and create a recurring overheating scenario, and while this is rare, it happens and is often overlooked as a source of the air ingress and coolant loss.

So far as the temp feedback loop is concerned, which can account for odd temp problems as well, those coolant temp sensors are also known to go out (2 sensors) on the TDIs and report bad temp readings. This is important, as these sensors control the cooling fans (sensor 1 by radiator outlfow), and the ECU/Coolant gauge feed; sensor 2, behind engine near turbo on metal coolant pipe runs horizontally behind engine, etc.).

Suggest you use live data in VCDS or other scan tool to look at those coolant temp sensors with cold engine and see if things match up to ambient temps/other fluid temps on car, etc. Check them hot too for outlier readings.

There's also a super simple DIY test for a head gasket leak (Block Test kit), and I got one off Amazon when I suspected a head gasket leak on our '15 TDI, given the Audi 'stealers' ineptitude at resolving the coolant leak issue.

Results were negative, which gave peace-of-mind; it turned out to be just the typical coolant leaking into the engine valley, and the Audi dealer tech didn't replace the actual failed/leaking part (replaced oil cooler seals only) which is a known issue and this leak WILL HAPPEN to all TDI engines. Sometime it can be only the oil cooler's seals leaking, but more often it's actually the EGR Cooler coolant bypass valve leaking externally, or EGR Cooler's matrix has fractured internally and is leaking/consuming coolant via the engine combustion path, etc.

The Gen 1 TDI vs Gen 2 TDI motors have slightly diff coolant leak points in the engine valley, but it's easy to verify if you got coolant pooling in the engine valley...scope it or probe it to see what's down there, if anything.

 

Just for clarity, passenger side is the right side? It’s a left hand drive car?

None of this rules out, or confirms, a failed head gasket.

There’s a couple of potential leak sources around the expansion tank but the tank itself is a strong contender.

It’s important to ascertain the chain of events here. When the car overheated, was the coolant level low? Was the warning on the display definitely an overheat warning and not a low coolant warning? If the coolant gets low, the resulting warning can look a bit drastic.

If you’re losing coolant, it will trigger a big low-coolant level warning but your engine temperature will remain normal. If you continue to drive then ultimately the engine will overheat and this can lead to warping and gasket leaks.

If the first warning was engine temperature, followed by low coolant, that points to a more serious issue. Likely the head gasket but possibly not.

 

I rented a coolant pressure pump for the zone and the system hold pressure. The car has thrown p0198 twice. The coolant is coming from the bottles relief line. I figured that out...ive been running it and heres what's happening.
The car takes awhile to get to 200 degrees. There are no leaks of anything. The valley is dry. The car will maintain a 200-210 at idle for 10 minutes. overflow tank is luke warm and the upper hose is spicy hot and hard..the line coming off the thermostat is hard...but no leaks. You try to drive it temp hit 240 and you can here belt get yet, dummy gauge goes just about 3/4. Pop the hood and you can see what im pretty is oil sray near the tensioner / upper hose area. But there are no active leaks. I have done 3 thermostats. 2 was the same and 1 was a different manu. / cap / water pump / aux pump at t-stat(making noise). Ive also cleaned out the auxiliary thermostat on the t-stat housing

 

If you saying coolant is shooting out the overflow tube, which goes down inside the fender well area behind the wheel well liner, then that indicates you either got a failed/defective coolant tank pressure cap (I bought a new one for my TDI for $7), or there is 'something' causing an overpressure situation to occur, or like I mentioned earlier, you may have a hairline crack at a seam on the coolant reservoir tank, which would shoot coolant out at high pressure, so it could go basically anywhere.

Fresh coolant droplets look just like oil down inside the engine area, but they don't feel like oil or remain on the surface in a hot engine bay like oil would. Ask me how I know. A static pressure test of the coolant tank/primary coolant circuit is not likely going to show you any result if you have a hairline crack in the reservoir that only leaks when the pressure inside system is greatly increased. If you want to rule out a head gasket leak, there is a simple test for that using a reagent test kit (search on Block Test Kit) that will sniff out any exhaust gases in the coolant tank. If you've got any oily residue around the top lip of the reservoir, then that's also a dead-giveaway of a head gasket issue. Head gasket failure/leak symptoms include several of the symptoms you are observing, so know the symptoms and eliminate the head gasket as being the underlying issue is my suggestion.

Here's a quick guide to common symptoms of Head Gasket leak/failure: https://mechanswers.com/what-happens...-gasket-leaks/

Other's who've also had odd symptoms with overheating have also found coolant leaks at the electric aux coolant pump beneath the car, which allowed air into cooling system, which caused overheating symptoms. It's not a common failure point, but obviously it happens, as evidenced by Q7 owner who posted it, and he looked at everything twice on that car before going underneath the engine and finding that hose connection was leaking on the Aux coolant pump.

Check the stuff you haven't looked at yet, as with air bubbling into your coolant tank consistently, it's either a plain old leak you haven't yet found, or you got a head gasket issue, or both. I tested on or our TDIs for head gasket leak in parking lot of an Audi dealership after second trip to that Stealership and they didn't fix the coolant leak on it, but claimed they did. I wanted to know 100% that it wasn't related to a head gasket leak, so I tested it myself...it's a simple, quick test; the reagent changes color means you've got a leaky head gasket.

Myself, I'd go under the car and check out coolant hoses at the aux coolant pumps to rule those out. Looking for any signs of coolant loss on those items/hoses, etc. If those are not the leak point, then I'd do the head gasket test. I still think you may have a hairline crack on your reservoir tank, but that's just from the description you gave. Follow the coolant trail/signs and think about how the droplets could have 'sprayed' under pressure to reach the areas you see droplets or dried coolant signs on, etc.