Pressure Build Up In Coolant

[mof]

Just had head gasket replaced when coolant was over pressurised and overheating.

Head was skimmed first.

New pump also fitted.

It was okay for a few miles but now get build up of pressure in coolant again leading to overheating and running on and a lot of gurgling when engine stops.

Compression tester still shows all cylinders in the green zone, about 150psi. The book says 160psi.

I suspect I've got a leak in the head gasket again, but can anyone else think of a simpler solution I may have missed?

What about the bypass hose. I suspect mine is blocked. Is that anything important?

[MartinB]

I had something similar with my BMW328. It turned out to be porous exhaust ports just under the seat inserts. I was loosing coolant which was passing into the exhaust port at idle/low power due to normal coolant pressure, and at higher revs/more power the back pressure in the exhaust was reversing the flow of the leak and really pressurising the cooling system forcing it out of the radiator cap. Had a lot of gurgling noises when switched off.

Are you loosing coolant?

Martin

[mof]

Don't seem to be losing coolant.

Today's trip to work has convinced me we are talking about a bubble in the system.

1. Warms up okay via thermostat.
2. After about 5 miles the temp starts to climb.
3. Then the guage goes to zero.
4. Back to 3/4 of max temp.
5. Back and forth every 30 seconds or so.
6. Get to work - switch off - gurgle gurgle - runs on and then stops.

Horrible to watch!

[IslipMinor]

Mof,

What engine and spec do you have? Is the head standard? What valve guides?

[PSL184]

Sounds like either a faulty stat or a big air lock.....?

[Leo]

Could be worth checking the radiator is not blocked also flushing out the entire cooling system.
Does the heater work?



Leo 69 Traveller

[mof]

I'm pretty well sure it's an air lock now. I've just run it to normal temp with the cap off and suddenly the heater actually heats the inside of the car and all the pipes are hot!

Loads of movement in the header tank water, so the new pump is obviously working fine.

The bubble must have escaped, so now the test will be to see how long it takes to come back.

Can you get such a build up of compressed air from a head gasket without any noticable loss of pressure or coolant?

[mof]

In answer to Islips question, the block is a 1098 cc circa 1970 and the head is from the same cc but circa 1960's.

Just a bog standard engine set up.

[PSL184]

Yes you can - The heater is the most obvious place for air to get trapped... My method of removing air locks is as you have done - to run with cap off - that usually shifts them. Run as normal now for a week then try cap off again and see how you get on...

[mof]

I only need to use the car until the end of March at which point I can address the problem with an engine change.

I never do more than 12 miles to and from work. Plain sailing 50mph all the way.

It seems to me that I could leave the cap on the first notch, i.e. held in place but system not pressured up.

That would get me to work well before I lost very much water through evaporation and the excess air could escape.

Does the pressure cap do anything other than stop the system drying up?

Comments welcome.

[bmcecosse]

Never ever had an 'air-lock' in many many A series engines. Sounds like gas is getting into the coolant - could be cracked head or cracked/porous block. Was there anything wrong with the old head gasket when you took it off ? The temp gauge going to zero indicates no water at the sender - caused by gas. Is the engine expelling water ? Put a plastic bottle on the overflow pipe to catch the water -so you can see how much is coming out.
Keep the cap ON !

[eastona]

never had an airlock either.

Sounds like it has good compression, so presumably the head gasket is sound and torqued down properly.

airlocks can be shifted by keeping the cap off, the heater on and squeezing hoses to shift the bubbles around, BE CAREFUL OF THE FAN BLADES! Never had to do it on an a series. Don't think a blocked bypass would cause an airlock.

Can you see bubbles or gas coming out of the radiator when the car is running? You can get stuff to check for exhaust gas in the coolant.

Andrew

[mof]

Lots of food for thought there.

While running with cap off to clear air lock the coolant does change colour slightly, taking on a slightly lighter and perhaps more 'fizzy' look.

There has been very little loss of coolant, perhaps less than an inch in the header.

I never saw the old gasket, as it was done for me.

The job had to be done as the existing gasket was letting by between 2 and 3 and the head had to be skimmed.

I've never had this airlock issue either in 25 yrs of ownership. Same car!

I suspect the entire block may be warped and I have a replacement in mind.

The issue is just the most effective way to limp through the next couple of months

Thanks again gents.

[mof]

Oh....and just a thought.

The head is torqued as per the book.

Assuming I may as well nip it down a wee bit more, has anyone got any figures they could suggest?

[Kevin]

The head is torqued as per the book.

Well it should be fine then.
Does the heater now work normally, I have had an airlock in the heater in the past and have just burped it by quickly pulling of a heater hose and reconnecting straight away with the engine running, but it can be a little messy.

[bmcecosse]

Head can be re-torqued to 44 ft.lbf with the standard studs. Then reset the valve gaps of course! Heater pipes may be kinked and the valve may not be fully open ? I would top up - fit cap - put a catch bottle on the overflow - and drive it! If the gauge starts to do way down again then (indicating loss of coolant) then there is indeed a problem which will need further investigation. But I would have thought 12 miles in Fife with open roads would be 'do-able'!

[ColinP]

I've just changed the head gasket on mine - there was a small piece of ? (rather like gasket paper) partially blocking the heater drilling on the block.
It looked like something from when the last head was put on - not by me...
I wonder if your gasket is correctly aligned - but there is only one way to find out.

BMC - 12 miles in Fife - ok, I've done that (not in the Minor), but "open roads" ?- usually I'm either trapped behind a local doing 20 mph in the middle of the road - or scared witless by someone overtaking at 70+ when I know there's a blind hill/corner coming up.


Colin

[bmcecosse]

Ahh - that would be ME doing the overtaking! You must have been driving too slowly.

[rayofleamington]

Never ever had an 'air-lock' in many many A series engines. Sounds like gas is getting into the coolant - could be cracked head or cracked/porous block.

Never had an air lock that lasted more than a drive round the block - persistent gurgling is a very bad sign.
The only time I've had symptoms like that was leaky head gasket... If that is ok then a leaky block or head would be my guess.

Aneqdote form a non Minor:
Fixed a Punto 2 years ago - had 180psi on all cylinders... It overheated at idle and medium revs but temperature gauge went down if revved high. Had all the symptoms of an air lock but turned out to be porosity in the head. A small gas leak into the coolant gave air in the head which caused the overheating. High revs pushed the water round quick enough to compensate.

[bmcecosse]

Just a thought - who changed the head gasket for you - not M Motors I hope !