I will share you my experience, that lasted almost 1year to diagnose and resulted in the end with a coolant loss as well, with most of the symptoms described in the post. I will edit thos one into a bigger detailed post later as I just diagnosed my problem.. EGR gas cooler fissured/cracked.
Going to change it to see if it solves.
Later edit (got even worse):
Audi Q5 2.0tdi 2009 (late edition - early 2010).
Mileage: 380.000km (236k miles)
All checks done on time with no major problems, road accidents or leaks until 2022.
How it started: in the autumn of 2022, when outside temps started to drop, I noticed that the car wouldn't put anymore heat inside so and also, the water temp wouldnt rise to the optimal 90°C. Unless I would drive it reved on sport mode, the temps wouldn't climb and the heating wouldn't start. Even if drove aggressively, after going back in cruising mode at around 80mph (2000rpm), the water would slowly go down to 50-70°C and the heating would dissapear.
I assumed it was the thermostat (one of the 3 one) so I went to the mechanic and had him change the small one (circular round, around 10€). His opinion- the thermo was cooked and had to be replaced, still... no improvement in the heating on the road.
Personal issues started to appear later that winter so I didn't had time to invest in the car. No leakage, no smoke, no power loss, the car was driving like a champ so I had the problem ignored for the next 6-9 months.
During the next summer, when outside temps would be back up, the water temp would climb and stay at the normal temp so I let it be for that time.
Autumn of 2023 came so I had the car at another mechanic (the one that usually does all the checkings for it, since 10y) and told him the issues, so he changed the biggest thermostat (around 80€), i believe somewhere close to the front radiator.
Still no other symptoms except for the lack of heating and water not reaching the optimal temp.
He took the engine diagrams out and found there's a 3rd thermostat on some hose, close to the engine block.
Ok, had that replaced also and the distribution (water pumps, belts, tightners, timing and accessories belts, etc, filters, fluids).
Back on the road, everything was heaven with no problems.
Lots of heating, water temp would easily reach and maintain the 90°C in all types of cold weather and slow driving/idling).
I was thankful at that point that everything was sorted out.
1,5 months (maybe more) later, I had my first medium-long trip (around 370miles). At that point, no symptoms.
Went of on the highway and started eating the track at 100-110mph. The car didn't last for 30minutes of 2300rpms and issued a "check coolant levels - turn of engine" error.
Hell started...
Got to the nearest parking, checked the coolant level, couldn't identify how much was left in the expansion vessel so I opened it up and the coolant would rapidly flush back in, filling it like 1-2 inches under the minimum level, with lots and lots of bubbles and air in it.
Called the mechanic on the spot, he said that maybe the boys that had the water pumps changed, didn't flushed the installation properly. I filled it with coolant till top top and went back on the road, again. For the next 330miles, the car didn't had any symptoms. Water was ok, heating was ok, no smoke, no nothing, no coolant level error. I didn't checked the coolant level again until the next error appeared.
Got back in town after 1 week, had some snow, had some -10°C temps, had some revving in the city (very crowded, jams, need to be quick if don't wanna spend a lifetime at the streetlights) and the error would appear once more.
Same thing, coolant vessel opened but this time the level came back.even lower. Had it reffiled, had it flushed 10minutes opened on idling, seemed ok and I was starting to suspect a hose (when they changed the thermostat / water pumps).
Next 2-3 days i constantly monitored the expansion vessel for every short trip I had. The leakage was becoming slowly obvious because it kept required reffiling, but not all the times. When drived easy, steadily, the coolant wouldn't escpaed as fast as when drived aggressively.
Still, a new sign appeared- I found wetness and liquid behind the vessel and on some hoses (at that point, got me some relief as I was pretty sure that one hose was fisured but was opening the crack only under pressure)
Ok, phoned the mechanic to let him know the problem persisted and started to document/research the issue (this is how I found the thread).
*until this point, never did the expansion vessel boiled or throw up coolant opened when revving it (i wanted to eliminate the head cylinder/Gasquet)
Got the car to the mechanic and after some checkups, he phoned me: EGR (DPF) gas cooler cracked / fisured and was losing coolant.
Ok... lets have it changed.
2 hours later, my mechanic phones me again.
The head cylinder was gone (had a crack somewhere) and the gasquet was also terminated (lossing coolant on al 4 cylinders) and, if left, undiagnosed it was only 20miles away from a total engine death.
He knew there was another problem, except for the gas cooler, because after he changed the cracked one, the car started smoking white and the coolant started to flow with pressure/boil out from the vessel (the cooling circuit became pressurized from the gasquet leaking).
It took me a lot to find out that my engine was to blame (slowly dying) because the coolant / pressure was being lost trough the egr gas cooler. So nobody suspected the head cylinder, not even I. The system had 2 faults and only by solving the first (gas cooler), did the second became obvious (head cylinder + gasquet).
Had the egr cooler changed.
Had the gasquet and the head cylinder changed.
Took around 5 hours for the entire operation to be done.
Had the car for a test-drive... 70 miles of mountain serpentines, only sport mode, noticed it could reach 5000rpm again (before it stopped at 4000-4500)...
no more shaking, no more vibrations.
Has a smooth acceleration without ever feeling the gear upgrade/downgrade.
