Coolant pressure...

[Guest]

Hi, following on from the thread about filling the expansion tank (glad mine is showing normal signs!), I have a concern about the amount of pressure thats building up in the cooling system.....

I haven't driven my 944 much yet, due to gearbox issues, but it seems to build up a lot of pressure, and the system will stay pressurised for up to 24 hours...for example, I drove it 5 miles last night to test the new gearbox, and, when I removed the cap today, there was a whoosh as the air escaped....I've never experienced this with any other car.....

This seems to me to be a faulty rad cap, but any advice would be appreciated, and is there a way to test the rad cap?

 

[sawood12]

The pressure in the system is directly related to the engine temp and since the cooling system is sealed the pressure should return to atmospheric when it cools so you shouldn't get that whooshing when taking the top off when the engine is cold. It could very well be the expansion cap as you suggest as it could be that the pressure relieving cap (as Riverside explains the operation of on the other thread) might be weak and allowing air to escape when hot therefore when the engine cools you are getting a partial vacuum and the wooshing noise you are hearing is air being sucked in rather than blown out of the system.

 

[John Sims]

I've been told that it could also be indicative of head gasket problems. The pressure in the cylinder forcing air into the water way.

I guess if it were a very small hole that would only allow passage at high pressure that might be the case; but I like Scott's explanation better.

Either way it must be worth getting a new cap. Also, clean any crud off the sealing ring on the reservoir to ensure a good seal.

 

[geoff Ives]

And check that there is no oil in the water..[:-]

 

[Peter Empson]

I can't be of any help I'm afraid, but I have the same on my engine at the moment, the system is always pressurised when I remove the cap (2 days is the longest I ever leave it without using it). It had a new cap a while back for another reason (which I suppose could be faulty) and had my garage check for signs of the head gasket before the last service. I'm loosing a fair bit of coolant too, but there's no sign of oil mixing. It doesn't seem to be getting any worse but clearly I want to get to the bottom of it, I think I need to book more time at my garage to see if they can spot it.
Best regards,
Peter

 

[Diver944]

I thought this was fairly standard and have always noticed it on my cars.

However in the interest of science I've just opened both my expansion caps and:

The S2 which ran last night was pressure free
The Turbo which ran on Monday had a small whoosh

 

[edh]

If you've recently filled the cooling system, it may be just the last bits of air that wasn't bled out (944's are not easy to bleed completely)
That's the best case....

If you're losing coolant from the expansion tank, it will be blowing out of the drainage tube. Try fixing some sort of container to catch anything that comes out of the tube
- I re-routed the tube so that it exited just behind the n/s headlight & I could fit a bottle in there. It's a fairly easy thing to test before you go taking anything else apart.

The other easy thing to do is to test if you are getting combustion gases in the water, An friendly MOT station will pick up hydrocarbons using their tester - takes about 30 seconds.

FWIW, mine blew its headgasket without ever mixing any oil & water, but it was pressurising the cooling system. It was also losing coolant from the a radiator connector. There's a plastic connector that screws into the radiator just below the top hose on the n/s. This connects to a hose from the expansion tank & is prone to cracking when overtightened.

 

[Guest]

Thanks for all the replies, I hope its not the headgasket- thats always my biggest fear about cars.....

I've been outside, running the engine...when the engine is cold and the cap removed, there are no bubbles, but when the car warms up, it starts to spew ( but not blow, if you know what i mean) water out, I am thinking this is normal behaviour, but hopefully someone can advise....

Also, It doesn't overheat- the gauge gets to about 3/4, then the fan kicks in and it goes down to about 1/4....

Guess I'll get it MOT'd and run it for a while, and see how it goes...

 

[edh]

hmmm....

mine did this with a failing h/g

when you get it MOT'd - get the coolant checked for hydrocarbons

if it is headgasket - better to get it done sooner rather than later. It's not the end of the world though - I think a clutch is more expensive than a headgasket on these cars - and if you're due a belts service, you can roll it into the same job.

 

[John Sims]

ORIGINAL: dick dasterdley

...... the gauge gets to about 3/4, then the fan kicks in and it goes down to about 1/4....

[][] If Beaky gets near ½ on the gauge, at any time, I get jumpy. I don't want to frighten you but yours doesn't sound right to me.

 

[Guest]

It only gets that hot if its idling, on the road, it hovers just under 1/2

 

[John Sims]

That still seems hot. ¼ or just above is more the norm.

IIRC Paul Smith did a poll on what peoples temperature gauges read - this was years ago at the start of the forum.