Pressure caps.

[Richard the old one]

I noticed that the water pump on one of my 1500cc Dolomite was leaking coolant recently and I was surprised how much coolant I had to put into the radiator. I also removed the pressure cap from the expansion tank and topped the plastic tank up and it has not leaked again. I was all set to replace the water pump but when I was at the local Triumph owner's group meeting it was suggested that the pressure cap might have stuck and over pressurised the system. I am now assuming that might be the case as it has not leaked since but I have not used the car much. The 1500 dolomite uses a 13 psi pressure / radiator cap, part GRC114, which does not seem stocked by a lot of the common triumph suppliers. So I would like to ask is it used on many other cars?

I decide to go ahead and purchase a couple from one of the well known suppliers. When it arrived yesterday I did find that I could not get it to fit onto the plastic tank until I bent the two lugs out slightly and then I found it very difficult to turn and I had to file a slight lead in on the lugs. I have not yet run the engine up to full temperature to check it out but Jeroen has now worried me with his comment in the other post which was in respect to Sprint caps but I will copy it here .

"As for the caps, plastic tanks do stretch so these need actually oversized caps but these don't exist. You can use a cap with a higher rating, stronger spring then. Most current caps are way off what's on the label. NOS caps, unipart or similar are the ones to have but with a stretched tank you maybe need an 15 or 18. Most bicycle pumps have some sort of pressure gauge on them. Put the cap on the empty tank, pump and see when it opens. 8 out of 10 times your new cap with the correct ratings printed on is opening too soon causing all kind of troubles and wrong diagnoses. The time of buying a new correct working part for a classic what does what it say on the box is long gone. Fit, test, check and know."

The pressure testing will be something I will be doing when I next visit my son in law as I do not have a bicycle pump with a pressure gauge.

Could some also explain to me how the pressure cap works as it looks a bit more complicated than I first thought.

[simonr]

Could some also explain to me how the pressure cap works as it looks a bit more complicated than I first thought.

Hi Richard

I needed the same answer recently and found this explanation useful

https://www.freeasestudyguides.com/engi ... r-cap.html

Simon

[Richard the old one]

Many thanks for sending that link Simon.

I had not thought about the need to allow air back in when the engine cools down. The link explains how it works very well.