Coolant leak / heater issue

[Steve2.7T]

Apologies for the long post - hopefully someone might have some ideas!

Car - A6 2.7 bi-turbo, 1999

Symptoms - as the weather got colder I noticed that the heater was only blowing out lukewarm air even when I wound the temperature control up to "HI". Also, the engine didn't seem to warm up that quick. After a 5 mile drive to work it might just about be up to 90 degrees.

I took the car to a local VW/Audi specialist that I'd used before.

They diagnosed and fitted a thermostat but then phoned to say that it was still not blowing hot air and diagnosed the water pump. Having fitted that and after hours of trying to bleed the air out of the system they announced that all was now fine. Oooodles of hot air on demand and the engine warms up to 90 degrees in a mile or so now

However, the first cold frosty morning since the repair (after about 100 miles of driving the previous day) I went out to it in the morning, started it and set everything to demist (i.e. blower on full, temperature on "HI"). After 5 mins I went out to the car again. Temperature guage now on 90 degrees but no hot air, but as soon as I revved her up a little, after 3 or 4 seconds, loads of hot air.......

When I went to drive home that night, the low coolant warning comes on. No sign of any coolant in the expansion tank so I had to have that topped up. Air in the system suspected.

The same thing has happened the last 2 cold mornings - no hot air until the engine was revved up a bit. Yesterday I took it back to the garage to have the coolant topped up again. Because there is no apparent coolant leak, they have also tested the coolant for exhaust gas content i.e. head gasket failure. The results were inconclusive as the test solution only changed colour minutely after 5-10 minutes of testing.

This morning I checked the coolant level which was fine but still no hot air until I revved it up a bit......I'm now very confused.

The next stage is a coolant system pressure test next week.

Does anyone have any other suggestions as to the cause??????

[nskeates]

Not sure if this will help or not but i had an A4 that had a similar problem but not the coolant lose, on tick over you got just warm air but when revved it got better, had the thermostat and waterpump changed but in the end it ended up being the heater matrix. once a new heater matrix was fitted problem solved. You should be able to check if the flow is getting thro the heater matrix as there are two pipes that go thro the bulkhead, one feed and the other return. If the heater matrix is blocked or leaking the feed pipe will be hot and the return will be either cold or just warm.

have you checked the passenger footwell to see if that is damp ? if so most probably heater matrix leaking.

[Steve2.7T]

Not sure if this will help or not but i had an A4 that had a similar problem but not the coolant lose, on tick over you got just warm air but when revved it got better, had the thermostat and waterpump changed but in the end it ended up being the heater matrix. once a new heater matrix was fitted problem solved. You should be able to check if the flow is getting thro the heater matrix as there are two pipes that go thro the bulkhead, one feed and the other return. If the heater matrix is blocked or leaking the feed pipe will be hot and the return will be either cold or just warm.

have you checked the passenger footwell to see if that is damp ? if so most probably heater matrix leaking.

Thanks for that. Will let it warm up without revving it in the morning and check those hoses. I'm fairly certain there is nothing wet inside the vehicle. Will check that too.