P38A intermittent coolant pressure issue

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kermit_rr

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This is really doing my head in now!!
I can drive for miles and miles with no coolant leaks and no overheating or over pressuring.
Then i can get a few miles down the road, still no overheating but the pipes are trying to burst at the seams!!
I can only put this down to a intermittent blockage somewhere as a foreign object moves about.
I've previously hooked out a lump of what looked and felt like silicone sealer from the radiator bleed back to the expansion tank, but i can't rule out more lurking.
The engine has had stop leak in it, so i can't rule out that causing havoc even though I've flushed as best i can
I'm also trying to convince myself that the block isn't cracked on the basis it would consistently be over pressurising and not just sometimes
Tempted to ask the local garage to put his MOT exhaust sniffer near it to see if that picks anything up
 
My previous one played "overheat and dump all my water" games on me shortly after I bought it.
Dealer could find no faults, but said that they changed the heater matrix, the radiator and took the heads off to check for cracks HG failures etc. NOTHING!
Then it ran for a few weeks OK and then started playing up again. If you did not have RAM-AIR cooling it, it would boil-over in traffic after a decent run.
I never checked the Viscous-Fan though (I would today). Never did a "sniff-test" for hydrocarbons in the water (I would today).
Strange thing is I chucked one small canister of Bars-Leaks in it,in total desperation and magically the problem stopped and never, ever re-appeared.
 
thats the thing.. it doesn't dump water.. just gets over pressured
last night the top hose was obviously rounded and there was a slight hiss from the expansion cap. There was some evidence of coolant from the overflow but the level was low, nowhere near the top. I loosened the cap slightly to let more pressure off and there was no further leakage of coolant, though it did rise up close to the top.
on a side note, I've never figured out why the cap doesn't dump the excess pressure more readily to stop the pipes bulging
 
I've just been out in it tonight, lots of low great work to give it a work out and get nice and warm, no issue what so ever! Reached 94c while using for a while, no hotter - i believe the thermostat fully opens at 92c
 
Have you checked the water pump?
I had a car many years ago that would pressurise and it turned out to be the impeller was loose on the shaft.
If it pressurised, a blip up to high revs got it spinning again and all was well for a while.
Once the pump was replaced it was fine.
 
I can't honestly recall if i checked whether the impeller was loose, just that it was a metal one and looked fairly new
The engine stays between 91-95, even at prolonged idle so I'm assuming the pump is ok
 
thats the thing.. it doesn't dump water.. just gets over pressured
last night the top hose was obviously rounded and there was a slight hiss from the expansion cap. There was some evidence of coolant from the overflow but the level was low, nowhere near the top. I loosened the cap slightly to let more pressure off and there was no further leakage of coolant, though it did rise up close to the top.
on a side note, I've never figured out why the cap doesn't dump the excess pressure more readily to stop the pipes bulging

If you get it up to running temperature, release all pressure then replace the cap does it repressurise?

Cap has a vent in it I think but don't know what pressure it goes at.

I bought an Omega that kept pressurising and discovered the previous owner had used windscreen wash from a garage rather than coolant to top it up!
 
Never tried that! Will do and report back.
The cap is a 140 which i believe vents to about 20psi. How an engine over pressurises when the cap vents I'm not quite sure!!
I'm sure it's only antifreeze and water, flushed and filled myself
 
I don't know anything about Killing Pascal, as far as I am aware the P38 runs at 14 psi, 20 psi would take some holding.
just checked RAVE and it is indeed 1.4 bar (no killing) :p which is about 20psi:eek: you're right though.. it is rather high!! takes the boiling point of water to around 123c I believe
 
Bonkers thought probably...
Is it is a dodgy pump on a smaller than usual pulley? That might give over pressure symptoms when the pump begins working properly, and "normal" behaviour when the pump is slipping/failing perhaps.
It is really puzzling the intermittent nature of it for sure.
 
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