Expansion cap release - Heater not hot

[Alistair E]

Having read through other posts regarding problems with the heater blowing warm not hot, I decided to see if I could find the source of my problems.

Coolant level was low so I topped up with the intention of bleeding the system. Ran the engine for a few minutes, then checked the temperature of the pipes for the matrix at the bulkhead. Both were getting warm, whilst some of the other hoses were still cold. This confirmed something I'd read on another post, that the water runs through the matrix before the thermostat opens. Heater temp was just off cold.

Ran until stat opened, pipes all hot and heater blowing hot. Checked the bleeds on the matrix pipe and the top of radiator. Both produced water, no air.

Left car to run until fan kicked in. Before it did, the expansion cap released some of the coolant, twice. Another minute and the fan kicked in.

I believe the lack of heating is due to the coolant system being low. I know the system was overfilled but should the cap release water like that. I also think that when it does it allows too much coolant out leaving the system low. Can't see leaks from anywhere else and the level does get to point where it remains constant.

Any suggestions / help gratefully received.

Alistair

[John.]

Your best case scenario is the cap being defective, the cheapest too!! If the system pressure is excessive then that would be bad news.
Buy a cap and take it from there.

[kulster]

Yeah I'd buy a genuine cap and expansion bottle

[Alistair E]

I assume that when you say excessive system pressure you're suggesting that the problem could be head gasket based or something similar? I would have thought that if it were that, the water system would be pressurised as soon as the engine was started, not just when it warmed up.

I'll try the new cap route initially.

Thanks

[Ryss]

Not the easiest cars to get all the air out the system. Any air in the system will expand rapidly and force water out the cap.

I run, as you described with the cap off, heater on and keep topping up water to max. Until operating temp is reached. Plenty squeezing of main pipes, in an effort to dislodge any trapped air.


A blown head gasket will often show up as a miss on cold start up for 2 secs. Then air may get into the water system. Unlike when flushing a system, this air just keeps coming. The higher the revs the more air. Fairly unmistakable.

Best way to test is to go for a test of CO2 in the water system.

[Alistair E]

Pretty sure it's not a cylinder leak. As you say, the pressurising would just continue.

Does anyone know of a cap being sold? Maybe expansion bottle as well?

Thanks

[Cottis]

I had the header tank fracture on my 3B last year. I too thought it was leaking from the cap, as I was experiencing water loss but only gradually. Turns out it was actually cracked around the part that the return hose joins to and was leaking as the temp rose.

DO NOT buy an aftermarket tank. I think I recall hearing people on here saying they had expanded under pressure and ended up deformed. The actual OEM unit from Audi is not that much (think I paid about £40 from memory without a cap) The cap is more than likely still available and the price is negligible in terms of peace of mind and knowing you are getting quality.

Good luck.

[macspring]

Pretty sure it's not a cylinder leak. As you say, the pressurising would just continue.

Does anyone know of a cap being sold? Maybe expansion bottle as well?

Thanks

Given the nature of the problem and the comparatively minor expense, go dealer oem for the Bottle and the cap, then you know they are right, if you do still have a problem.

Consider little or no antifreeze until you sort the problem as each time you drain it you waste coolant.

With that in mind change your thermostat and rubber ring seal for a dealer oem at the same time, and spend some time flushing all parts of the coolant system

You only want to be in there once, if you can help it.

Let us know what it is in the long run when you find it.

Good luck.