P38A intermittent coolant pressure issue

kermit_rr · Jul 18, 2019

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

DanClarke · Jul 18, 2019

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.

kermit_rr · Jul 18, 2019

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

biketeacherdave · Jul 18, 2019

Which engine is this ?

kermit_rr · Jul 18, 2019

4.0 bosch

kermit_rr · Jul 18, 2019

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

biketeacherdave · Jul 18, 2019

Have you at any stage pulled the spark plugs out to see if any are steam cleaned ?

kermit_rr · Jul 19, 2019

Yes even had the heads off, all looks perfectly normal

Wazzajnr · Jul 19, 2019

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.

kermit_rr · Jul 19, 2019

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

RangeRoller dt · Jul 20, 2019

Any pin holes in your rad or pipes?

kermit_rr · Jul 20, 2019

I don't believe so. I have had all sorts of leaks here and there as I've gone on, seems to be ok at the mo

Grrrrrr · Jul 20, 2019

kermit_rr said:
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!

kermit_rr · Jul 20, 2019

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

Datatek · Jul 20, 2019

kermit_rr said:
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

140 is I think 14 psi.

kermit_rr · Jul 20, 2019

Datatek said:
140 is I think 14 psi.

i always assumed it was 140kPa, incidentally my bmw has a cap marked 140

Datatek · Jul 21, 2019

kermit_rr said:
i always assumed it was 140kPa, incidentally my bmw has a cap marked 140

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.

kermit_rr · Jul 21, 2019

Datatek said:
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)

which is about 20psi

you're right though.. it is rather high!! takes the boiling point of water to around 123c I believe

DanClarke · Jul 21, 2019

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.

Saint.V8 · Jul 21, 2019

Pressure in the system is a function of temperature not pump output....the pump provides flow, the temperature provides pressure.

Low flow, temp rises....pressure increases....

P38A intermittent coolant pressure issue

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