21 replies to this topic

[gazza82]

I'm not sure that pressure cap type is designed to allow it to be drawn back ... unless the seal is poor or too low. Pressure caps are available in different ratings of lbs/sq.in. so what is stamped on the top of yours?? Think the common ones are 7 and 13.

[Lplus]

I'm not sure that pressure cap type is designed to allow it to be drawn back ... unless the seal is poor or too low. Pressure caps are available in different ratings of lbs/sq.in. so what is stamped on the top of yours?? Think the common ones are 7 and 13.

They will drain back, because the lower pressure cap part has a spring loaded valve in it to avoid  a significant vacuum forming.  The important thing is a good seal at the higher level which may happen with steel as shown but is iffy.  Also the overflow pipe must be fully sealed into the rad cap holder.

[Spider]

This is my setup. No outer rubber seal but it looks like there's quite a good metal to metal contact there.

 

The Cap has 2 clips that latch on to the Neck and pull it down as it's rotated, a bayonet arrangement that has the main relief spring pushing against it. When the Cap is off, it all might appear nice and flat, but when fitted, the neck is pulled out of shape and any sealing that may appear to be there when apart, is lost..
 

[mbolt998]

I'm not sure that pressure cap type is designed to allow it to be drawn back ... unless the seal is poor or too low. Pressure caps are available in different ratings of lbs/sq.in. so what is stamped on the top of yours?? Think the common ones are 7 and 13.

It says "15lbs" on it, which I assume means 15 psi and that's the pressure at which it's supposed to blow out at. It's definitely not designed to draw back and as Spider says probably doesn't seal very well. Maybe I could just make a rubber gasket. But if it ain't broke etc.

[KTS]

I have a pretty tuned 1380cc engine and during an engine overhaul last winter I decided to upgrade the cooling system with:
* The water pump is the large plastic impeller version from minispares
* The radiator is the performance v shaped 2 core version
* 82c thermostat
* 15 bar radiator cap.

I can't see any leaks in the system yet there is always coolant under the car that looks to come from the overflow tube.

When I check the coolant levels it is always below the coolant fins which leads me to believe the engine somehow is burping more coolant up than it should.

When I refill it the level is once again lower than the fins after a drive.
The temperature seems normal on the dashboard temperature gauge, so I don't think it is overheating.

And just to make sure, I have done a compression test with 16 bar on all cylinders, I see no white smoke and the engine oil looks completely normal, so I don't suspect any internal coolant leaks.

What can I do to keep the coolant at the right level and what causes this burping when the temperature gauge says I am not overheating the system?

 

 

as per Whistler's post;  have you checked you have the correct length cap fitted for your radiator ? 

 

a short cap fitted to a long neck radiator would allow pressure to be released (..and coolant spat out..) at normal operating temperatures, which sounds very much like what's happening

 

the link in Spider's post on the 24th provides all the details you need to measure and check

Edited by KTS, 26 October 2023 - 04:10 PM.

[imack]

I've been running the same type of expansion tank as mbolt998 for probably 30 years with no issues.
I use a one pint washer bottle and bracket fitted between the inner wing and rad and a unipart 15lb cap with a metal outer seal. Rad is always full when I remove the cap when cold.

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[68+86auto]

I would start by pressure testing the radiator cap, they do fail. I would then pressure test the cooling system. Once both those tests are ok I then keep the pressure tester on the radiator and run the engine to see if combustion gasses are pressurising the cooling system due to a blown head gasket.

 

It should be fine without an expansion tank but I do intend to fit one to my next mini.

Edited by 68+86auto, 29 October 2023 - 11:30 PM.