Pressure cap not releasing pressure!!

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dannybays78

New Member
Posts
42
Location
Rural Argyll
System keeps pressurising after 20 miles. When the engine is cold you can squeeze the hoses and hear venting from the expansion tank cap, but when it's hot, hoses can be solid so why is the excess pressure not being released?
Just bought a brand new cap and still doing it. Breather is clear. Baffled.
 
Diesel yes, sorry. Convinced it's not major. The thing can run great for a few days, then whack, solid hoses.
Regardless of what's causing the pressure, surely the sole purpose of the cap is to release it?
 
Well if you're convinced that the pressure is too high then it's possibly a cracked head. It's been covered on here before by blokes who've had this but usually the top of the rad gives. Don't ask me why the cap doesn't do it's job even with the pressure from a cracked head but it has been reported before.
 
I know I've read dozens of threads but noone ever seems to have identical symptoms, or indeed ever really follow through with a definitive solution!
Ah well, garage it is.
 
dogsbody said:
Well if you're convinced that the pressure is too high then it's possibly a cracked head. It's been covered on here before by blokes who've had this but usually the top of the rad gives. Don't ask me why the cap doesn't do it's job even with the pressure from a cracked head but it has been reported before.
Click to expand...
I would guess it depends on how quickly the pressure is being raised as the expansion cap release port is quite small and as such the volume flow to release the pressure is not very big, so if what is causing the over pressure is generating faster than the poor little cap can expell it at, would lead to constant over pressure...thats the way I see it anyway!
 
Saint.V8 said:
I would guess it depends on how quickly the pressure is being raised as the expansion cap release port is quite small and as such the volume flow to release the pressure is not very big, so if what is causing the over pressure is generating faster than the poor little cap can expell it at, would lead to constant over pressure...thats the way I see it anyway!
Click to expand...

I agree with Saint and assuming this is what is happening on your engine you should be able to detect signs that the cap is releasing pressure albeit not enough.
 
The pressure cap is there for two reasons. Firtly to pressurise the coolant to raise it's boiling point. 2 degrees F for each pound of pressure. Secondly to release pressure that would compromise the cooling system. Burst a hose Etc. Under normal running conditions the coolant should never, ever, be hot enough to raise the pressure to such an extent that the cap needs to vent. I suggest you go to a garage and get a static test done. The cap vent pressure can be tested and the pressure the coolant is reaching under running conditions can also be checked. If with the engine at normal temp, the static test gauge shows a pressure above 15 psi it is odds on you have a gas leak into the coolant.

PS. Just read through the posts, if the engine temp is in the blue and you still have over pressure you definatly have a gas ingress problem.
 
Last edited:
wammers said:
Think that is obvious. With a thermostat in it running at normal temp, it would probably blow the cap through the bonnet. He has a problem.
Click to expand...

Indeed, i fear gas must be the only option left! Got a purge and fill kit today, will try it to see if it makes any difference. The annoying thing is i could plod along the quite roads here all day, but as soon as i go up steep hill its a killer.
Will need to just live with it for another week or two before i can get to / afford the garage.
 
dannybays78 said:
Indeed, i fear gas must be the only option left! Got a purge and fill kit today, will try it to see if it makes any difference. The annoying thing is i could plod along the quite roads here all day, but as soon as i go up steep hill its a killer.
Will need to just live with it for another week or two before i can get to / afford the garage.
Click to expand...

I suppose your viscous fan is working correctly, the water pump has not shed its blades and the rad is not sludged up?

Overheating under load like towing or hill climbing is unfortunately a symptom of a cracked head, as is the missing thermostat:eek:
 
I had somewhat similar symptoms, or maybe still have....:)

On another tread called overheating still....

One thing i found was rad overflow outlet was deffo blocked. I took the small hose off and reved car with cap off (cold) and got nothing but dribbles. In fact you should see water flowing into header tank so dont need to take hose off

After a quick flush, now jets out and into header tank and gets stronger jet as car warms up and coolant expands, and for 2 days now fingers crossed anyway, seems less pressure in hoses.
 
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