Coolant system over pressure - diagnosis help needed please

[andyfb78]

Hi All,

I'm stumped and so is the garage...

The bongo had a new AMC head after an over-heating event and has done around 2k miles with no problems since...
Then, on a long drive the other day, did 200 miles ok, then we stopped for coffee and on re-start it blew out water from the header tank over flow.

Called the AA, and -ve combustion gas check, but rapid build up of coolant pressure. towed home... cried... then calmed down..

Local BF-garage said same, no gases in the water (so hopefully head and gasket ok), and rapid build up of pressure, which is then retained even when the engine has fully cooled ie overnight. He is stumped. Said he's never seen a cracked block so thinks that is highly unlikely.
He said that when he tried to bleed it the bottom hose never got hot, which suggests a blockage, but where, and why does the pressure get retained after cooling.
Radiator is brand new from BF shop.
Oil is clean, water is clean (ie no emulsion)
Suggests a blockage or cracked block, but engine starts and runs fine.
Thermostat works in a pan of hot water, just tried it..... of course it does; it is new from the BF shop. and the housing looks clean, so no blockage there.

just don't understand why the presure is retained so completely.... over a few days of sitting cold, it still hisses like a coke bottle when you take the cap off.

too nervous to run it now in case of head issues.

Any ideas, or previous experience with this?

Thanks

Andy

[g8dhe]

Radiator, Thermostat all OK ,no leaks, that leaves the Water pump has that been checked ?

[andyfb78]

Hi yes, water pump new from BF about 18 months ago, def pumping water.
No leaks anywhere.
System staying pressurised..

Garage says def not the head or head gasket..

something must be blocked, but haven't found what yet....
hmmmm

[teenmal]

Hi andyfb78,

A strong possibility that the heater matrix is gunked up.Might be worthwhile bypassing to eliminate it from the problem.

Cheers..

[Northern Bongolow]

a hot bongo that is normal will /should drop its pressure to 1 pound NEGATIVE when it is cold. that is to say that all the pressure that builds up (NORMALLY 7 OR 8 POUNDS) is dropped apart from 1 pound in about 4 hours.
are you sure the release of pressure is not negative going into the system and not possitive coming out.
this pressure drop is controlled by the small brass disc on the end of the expansion cap, under it sits a little light spring. pull the disc to test if its stuck.
the positive pressure in the system is vented by a much larger spring in the same cap.
so the cap allows pressure up control and also pressure down control.

your symptoms suggest the expansion tank cap is faulty on the light pressure drop side so the system is constantly topping up the pressure to a point where the system is at max and vents.
fit a gen mazda 1.1 expansion tank cap (not a copy) then see what happens.
to test this you can fit a 0.9 cap for a brief test, but dont leave it on.

bleed the system again to be sure.

[chipvan]

this all rings a bell!

[Northern Bongolow]

no yours is different jez, this one is retaining pressure even when cold.

[andyfb78]

Hi all,

Thanks.

Yes def retaining +ve pressure, it sprays water out when the cap is released.

AA seemed to think the pressure build up was "too fast" rather than "too high", but it was a side-of-the-road assessment. but he was measuring it with his own cap, ie not the bongo one fitted.

It would be ace if it was just a expansion cap issue.....

I'll have a look...

Andy

[Driver+Passengers]

Did you ever resolve this?

[andyfb78]

yes, the brandnew thermostat didn't work. even though it seemed to in a pan of water when tested.
put the old one back in.
problem solved....

Grrr

[Northern Bongolow]

what make/brand were the stats involved please.

[andyfb78]

absolutely no idea.
i can have a look and see it it has a stamp on it.

assuming i didn't throw it away in disgust...