Thread: Please not a head gasket? TD5 D2a

How are you pressurising the coolant system using air?

Why does the heater stop flowing when the pressure is high?

Or have I answered my own question...

Regards

Originally Posted by peter51

Pressure monitoring is very useful. It detects head gasket problems pretty quickly.It is very fast to detect an issue.
This how I believe a typical overheat due to head gasket failure occurs
1. Pressure builds
2. Flow in the circuit slows
3. Indicated temperatures are still OK while ever the hoses and cap can hold the pressure.
4. Coolant pressure settles at higher value. Temprature is normal at cruise because primary circuit is functioning sufficiently and coolant flow is high. ( Very different result idling at the lights with very low coolant flow)
5. Suddenly cap releases or hose splits - zero pressure means instantaneously boiling at those places in the head and oil cooler where the temp is greater than 100 celcius.
6. All water vapour naturally contained in the solution comes out of solution.
7. Loss of pressure and vapour coming out of solution is exacerbated by the suction at the pump entry - the pump cavitates and only foam enters the pump - all flow stops.
8.The head is cactus shortly thereafter.
Water temperature is a lagging indicator and rises well after the pump stops flowing - the damage is done.

Pretty well sums it up! Re my early post there is really no escape from head removal once this scenario has taken place.
Nick

@peter51 i can only raise my hat for a such well documented answer and can't argue with it, the only thing i want to add is that any fluid will start to emit microbubbles with 10-15 degrees below the "official" boiling point and that's important to know also if the pressure is rising so will the temperature cos theyr's proportion must stay constant so i can hardly see a scenario when the pressure is rising but the temp reading stays OK

Not if the rising pressure is caused initially by compression though. I believe this is where Peter's claim of early detection using pressure measurement is correct.

micro bubbles will still form under externally created pressure but they form at a higher temperature, theres a correlation between the temp that the micro bubbles form and the boiling point. as the pressure goes up they both rise.

Originally Posted by peter51

As above - if the top radiator hose gets very hard during warm up then there is a head gasket breach. This overpressure also stops flow through the heater matrix so there will be no ho air from the heater.
Normal coolant pressure in a TD5 is 0.5 -0.7 bar. This pressure allows you to slightly squeeze the top hose. So if the hose is rock hard it means head gasket failure.

So change the overflow bottle cap. If it still presurising then the head comes off


Normally no head work is required. I have done 2 gaskets - my car and a another. In both cases the heads were flat and needed no work.

In both these cases there was no white smoke any time including at start up.

heater flow doesnt stop because of the pressure, it stops because of an airlock. The heater is effectively a high point trap.

This is why I have people turn the heater on when bleeding the TD5, if the vents dont start making hot air it hasnt bled up correctly. (as opposed to having to turn on an old school heater to open the valves for the coolant to flow through the heater)

Thanks for the replies all, and the subsequent discussion.

I am pulling the head off this week.

And there is now a Prado in the drive.

A) join the club, this was me yesterday
ewww, prado

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