Coolant residual pressure.

[DMT]

Afternoon folks

Can you please help me with a ballpark figure for the cooling system pressure after the engine has stopped. How long should it hold pressure for? I opened the cap after about an hour and I did get a hiss, is this good/bad/normal?

I am trying to find if I have a leak, even though I cannot see one.

Cheers

Dave

[barkerwilliams]

Dave,

It depends how hot it was when you turned the engine off, the outside temperature and how strongly the wind is blowing. Just too many variables to be a useable criteria.

I have a friend who left an Hillman Imp engine for a few minutes and took the radiator cap off, and was then taken to hospital with burns over his back, playing with fire, (Ok, boiling water).

Far better is to rig a foot pump to one of the smaller hoses with a T piece and raise the pressure of a cold engine and let it stand for some time, it should hold pressure. Perfectly safe and as you work around the engine looking for leaks there are no hot surface to trouble you, or evaporate a drip or two.

Better still, radiator pump test kits are available on ebay for £50'ish.

Alan

[DMT]

Thanks Alan, I was hoping for an easy way out.
Dave

[trunt]

If you have a Mk2 the overflow pipe is an Ideal spot to add a small length of hose (fuel?) with a schrader valve from a bike or something to pressurise it up with aforesaid foot pump. in theory it should hold that pressure for quite a while, plus you may be able to spot leaks without scalding yourself

Of course that eliminates the overflow bottle and cap, so check those as well.

[DMT]

Hi Terry. I have removed the overflow pipe when I modded the rad to add a fitting for the header tank.

I think the general consensus (sensible option) is to pressurise it cold.

I will try this at the weekend. Do you think 30psi is too high?

Dave

[Lingen]

Dave. In my opinion yes! The pressure cap is set for 20 PSI and that is a good deal more than many cars.. I think 30psi would be putting the radiator at risk.
Mike.

[trunt]

I will try this at the weekend. Do you think 30psi is too high?

Dave

You can try but the pressure cap will give way before you get there! No just pump it up to 20 ( as long as you have a 20psi cap) leave the header tank and pressure cap in the test if at all possible, they can leak too..

I think I went up to 30 when I tested my radiator alone, I then used soapy water to check it but I wouldn’t go that high on the whole system.

[DMT]

Cheers Gents

[jbuckl]

Get one of these, or similar https://www.ebay.co.uk/itm/392977869...3ABFBMlIiXmfFf

[DMT]

Hey Rockefeller, I just ordered 4 off Schrader valves for 2.99. (free postage) I will cut the end off one and jubilee a pipe to it.

Hey presto pressure tester.

[flying farmer]

If your system holds pressure with no obvious leaks for a long period of time (hours not minutes) all is good.

That is unless you release the pressure cap when the engine is stone cold and it hisses at you. That is definately not good.

Neil

[DMT]

Sorry for the dumbness Neil, what would the hissing be from.

[Ian Durrant]

Sorry for the dumbness Neil, what would the hissing be from.

A blown head gasket, unfortunately.

[flying farmer]

A blown head gasket, unfortunately.

Exactly, a cooling system can hold pressure even with a blown head gasket.

Cylinder pressure is so high on the combustion stroke it can pump small amounts of gas into the cooling system but water cannot get back through the tiny fissure the gas gets through.

Many years ago I had a progressive head gasket failure in the Stag engine in my TR. The system was pressurising for an entire year and many thousand miles.

It never overheated because a header tank was fitted and it allowed the gas to escape without blowing coolant. Taking the cap off the header tank on a stone cold engine was always accompanied by a pssst from the cap as the pressure escaped.
I spent the Summer building a new tuned engine with more power. When Winter arrived I swapped engines.

The engine I removed had both gaskets showing signs of failure between the rear bore and the waterway. Because it never overheated nothing was warped. I totally stripped the engine to check for wear. It had done about 70,000 miles since I first built it. Nothing was needed but a gasket set and new timing chains.

That engine has been in my Estate for the last 10 years and hasn't needed touching since.

Neil

Actually I have remember the water pump sprang a leak, I binned it and fitted an electric pump

[DMT]

Cheers Ian/Neil.

My engine seems to be good. The only problem is low iish pressure on #7. Rings passing. Before I bought the car, the motor had done very few miles over 4 years. I am hoping they are only stuck!

Dave