3.0TDI (ASB) Air In Coolant Issue

RyanNZ

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Hi Guys,
I'm hoping someone here can help me, I have a 2007 (B7) Audi A4 3.0TDI, and it has air in the cooling system. The worst symptom of this is that the heater works intermittently/poorly as most of the air seems to collect in the heater core and pipes, but I fear other issues could be imminent (eg overheating of the engine). There's a bit of history here, I'll start from the top:
  • About 6 months ago, my mechanic noticed some milky substance in the coolant during a routine service. At this stage I had also noticed that occasionally the temperature indicated on the dash would drop from the normal 90 degrees to 65 or so sometimes, usually at high speeds.
  • My mechanic said that the substance could not be engine oil, as this would be black (sound logic) so he went about investigating if the car had a transmission cooler - answer was no it doesn't, so it couldn't be transmission fluid either. Perplexed, he decided the best course of action was to flush it all out, replace the coolant and see if it came back. After several thousand kilometers, it hasn't come back.
  • After flushing the cooling system and replacing the coolant, I noticed the heater no longer worked. He had a few more battles with bleeding the air from the system (the pipe coming out of the heater core was observed to be cold and empty) to no avail.
  • I asked for the heater core to be replaced, as I was sure it must be blocked up, probably with the weird sludge that we'd flushed out. Replacing the heater core did not resolve the issue.
  • The issue of the temperature dropping whilst driving at high speeds got more frequent, so we replaced the thermostat - this seemed to resolve that issue. I still get the odd little brief drop in the temperature, presumably as air bubbles pass the sensor, but it no longer correlates with increase in speed so I'm pretty confident the thermostat was faulty.
  • Still battling with not being able to get the air out of the system, and the heater not working, we decided to replace the water pump. The car is at 250,000km and I have had it since 93,000km, and have never done the water pump, so it seemed like a good idea to do it anyway. This did not fix the problem either.
  • My mechanic spent several more hours trying to bleed air out of the system, with the result that the heater would work well for a few minutes after bleeding, and then air would build up and it would go back to luke warm again. I tried bleeding it manually myself a few times with the bleed hole on the top of the heater core hose where it goes through the firewall - I observed the same thing, I would get the heater working really well (and no more air coming out the bleed hole), push the hose back on, let it idle for a few minutes and it would go cool again.
  • Having got to the point where my mechanic didn't know what to do next, I took the car this week to the local Audi dealership, who seemed confident that they just needed to bleed the system with their vacuum system (which my mechanic had already done) and it would be resolved. To their surprise, after spending 2.5 hours on it (when they said it should be less than an hour) they found that there was still air getting in from somewhere. They said that it had passed a pressure test, and the only suggestion they had was that it might be air coming from the cylinders (head gasket failure), but they had no suggestions for a way forward other than leaving it with them for an undetermined number of hours (at dealership rates - NZ$169 per hour, ouch) with no guarantee that they would find the fault.
So I'm left not knowing where to go next with this - I'm asking around various Audi specialists in Auckland, but so far everyone's a bit perplexed. I obviously can't sell the car like this, and it's not great not having a properly working heater in winter, plus I don't like the fact that air is building up everywhere in my cooling system. I am willing to spend a reasonable amount of money to get it sorted, but only if I know I will get it sorted. The car has done several thousand kilometers since the issue started, and it doesn't use any coolant (I never have to top it up). There is no white smoke coming out of it at any time, so coolant is not entering the cylinders. There is no coolant in the engine oil, and no engine oil in the coolant. A lot of times recently when I switch off the engine, the radiator fans continue to run for 30 seconds or so afterwards. This has never happened in the past (I've owned the car for 6.5 years).

Has anyone got any ideas on this? Perhaps seen something similar in the past? I really feel like the key might be figuring out where all the white residue in the coolant 6 months ago came from - if it wasn't engine oil and it wasn't transmission fluid, perhaps it was a result of some internal component of the system corroding/dissolving?
 

simon bowler

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hell having the same problem on my audi 3.0 tdi asb engine , did you resolve the problem
regards Simon
 
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