Under pressure

[wrestlerdad]

Hey All,
Every time I remove the coolant cap on my '98 740il there is some residual pressure that has accumulated. Not enough to blow off the lid but about like opening a 2 liter soda. It doesn't matter if the car has sat in the driveway for a day or a week this is a constant occurrence. Is this normal, and if not what does this excess pressure indicate? Fluctuations in operating temp are leading me to replace the water pump and thermostat, anything else I should figure into the equation?

wrestlerdad

[Chamberlin]

If you're sure it's positive pressure escaping, and not a vacuum filling rushing in when you loosen the cap, you may have a head gasket leak.

Sent from my MHA-L29 using Tapatalk

[xboss]

Tiny leaks can lead to air pockets that make pressure.

[racer2086]

There is always a hiss like that when I open mine on the E38 and E39. I'm pretty sure mine is a vacuum though as the engine cools. Also may depend on ambient temp.

[wrestlerdad]

Thanks guys,
It's definitely positive pressure, and a leaky head gasket would contribute to the temperature fluctuations I'm getting. Not seeing anything in the oil or exhaust yet, so this info comes at a great time!!

wrestlerdad

[Timm]

I would look at this from the other direction:

Fluctuation in temperatures is pressurizing the system.

Fix the temperature fluctuation first as detailed here:

https://www.meeknet.co.uk/e31/BMW_M6...ing_System.htm