help with coolant leak/ overheating 79' 308

You would not think so in a closed system.....I guess with failing seals and shaft wear it's possible...

A mystery....

I just routinely crack the rad screw, when I think about it, and see if any air exists there..

 

I'm just being persnickety and technical: Technically, cavitation is a microscopic low pressure point (such as on a rapidly rotating impeller blade) which causes a microscopic amount of fluid to turn to gas. Microseconds later the pressure increases again such that the bubble collapses back to liquid causing a microscopic fluid hammer on the metal. True cavitation cannot generate permanent gas. https://en.wikipedia.org/wiki/Cavitation

Its a different phenomenon than cavitation that is causing air to permanently enter the system.

 

About cavitation, we are in a closed circuit pressurized on 1 bar and the liquid is not water but mainly alcohol !
The pressure is here to have an higher ebullition point AND to repulse the cavitation phenomena.

But I agree that doesn't "produce" gases !

One other point is the thermostat : some have a little hole, others no (mine) : this may have a role ?

 

Does this happen when you stop or after its cooled down. Ie Is air in the rad as soon you shut down the engine. Check the expansion tank connections and are clear.

Pressure test is fine for checking for leaks but does not prove when under negative pressure

 

I bleed the air in the front radiator when everything is cold, so after cooling down some hours after the run.
When the engine is hot just after shutdown : I don't know but I won't open the expansion tank ...

What do you mean by "negative pressure" ? The system is pressurized when running with 0,9 bars, as the atmosphere is 1 bar, the system is 1+0,9 = 1,9 of absolute pressure when running : if there is a leak it should be from the circuit to the outside, not the contrary.

If you suggest that air may come in the cooling down phase, why the level in the expansion tank goes back to its previous level as the air in the front radiator is bleeded (no difference of level) ? To me that is a conservative volume transfer :

- air in the cooling tank <-> air in the front radiator
- bleed
- normal level in the expansion tank, no air in the front radiator

 

First I would try to isolate when the problem occurs, obviously before you start, make sure the system is full and fully bled, no air in rad and expansion tank at the correct level. Go for a drive.

Carefully check the rad when hot and see if air is present by opening the bleed valve, if it is then the problem occurs when the car is running. If no air then check when the car is cool.

If air is present then you know

1. its sucking in air via a leaking joint as it cools down or.
2. The pump is drawing air from the ET when running.

Joints expand and seal up when hot & sometimes and leak when cold or vice versa, a static pressure test only tells you have no leaks when cold. Although the system is above ATM pressure (0.9bar) when hot it does not stop air being pulled in the system, remember the water pump has a positive and negative discharge when operating and can pull in air via a weak joint or seal. This can happen when cooling down as well as the expanded water is forced back into the system.

The expansion tank (ET) connections are taken from the thermostat housing and so will both be under negative [pump] pressure when running to stop circulation in the ET.

Static or running, air could be sucked out of the top part of the ET and then end up in the rad, the shift in volume will allow water to rise up into the ET, you wont get any “over expansion” as the trapped air in the rad acts as the diaphragm.

Make sure BOTH tappings on the thermostat housing are clear, one that is slightly blocked can cause a shift in pressure and help the pump to pull air out of the ET.

What happens if you just leave it and let it find its own equilibrium, ie just leave it alone.? The rad has top connections and will never realistically airlock under operating conditions.

 

Hello Tony,


Thanks for your help.

The engine has had an complete check up, all the seals are new with sealant (including the thermostat housing seal, water pump seal and so on ...), circuit full bleeded several times, all fluids level are correct.

Next run I will check if air in the radiator as soon as the engine is off, this will help to know the origin of the air (I still think this is only the air from the ET that is send to the radiator).

The check valve was there to prevent air from the ET sucked by the water pump by the top ET hose, as seen this do not solve the problem ...

One other test could be to stop bleeding : if the phenomena goes on this is because air is sucked somewhere (water pump axle), if the phenomena stops this is only a transfer of the air from the ET to the front radiator.

 

By the way here is an update: original ez-bleeder still working like a charm. Temps always as should be and no need to manually bleed or drip coolant. This after entire engine , hose restoration. This leads me to believe that perhaps cavitation or air being sucked in through heater valves or elsewhere is a culprit in some cars.


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