Help please - Pressure in coolant when cold

[CrispyEdd]

HI,

AMF tdi75 no mods all stock, no AC but with webasto.

Coolant level was checked, its at low line. Drove to work all good. Went out just now engine cold, coolant level dropped about 2cm form low line. I undid the cap and ''POP'' with some splurge out the vent of the expansion header tank.
Level back up to near min line again.

One thing I did do is change the coolant cap for 'similar' one I have in the garage, this was because I could hear it letting some air out, or sucking in, one of other when hot.

Engine sounds sweet, runs smooth and heats up normal time and doesnt overheat. Is the header cap the type that lets pressure in and out?

Other than popping old one back on tonight, any ideas good people?

 

[A2Steve]

There's only one way there's pressure getting into the cooling system, from the combustion chamber into the water wells via a leak in the headgasket.

 

[CrispyEdd]

Ive checked the gasket using a tester and its saying its okay? Did a fair few tests on it. Maybe its a slight head gasket split, small for now! So to fix would need a head skim and new gaskets?

Ill see what heppend when I put original cap back on...

 

[PlasticMac]

HI,

AMF tdi75 no mods all stock, no AC but with webasto.

Coolant level was checked, its at low line. Drove to work all good. Went out just now engine cold, coolant level dropped about 2cm form low line. I undid the cap and ''POP'' with some splurge out the vent of the expansion header tank.
Level back up to near min line again.

One thing I did do is change the coolant cap for 'similar' one I have in the garage, this was because I could hear it letting some air out, or sucking in, one of other when hot.

Engine sounds sweet, runs smooth and heats up normal time and doesnt overheat. Is the header cap the type that lets pressure in and out?

Other than popping old one back on tonight, any ideas good people?

Ed,
You sure it was pressure out, and not air in, (vacuum in tank), as there would likely be a splurge as the air drawn into a coolant system under negative pressure balanced itself out?
Mac.

 

[CrispyEdd]

I didn't see which way it was going but yes after doing some interesting research, you may be correct, as the level came back up that would be relief of downward pressure? Could be just the cap not equalising correctly. Have to wait until tomorrow now.

 

[PlasticMac]

I didn't see which way it was going but yes after doing some interesting research, you may be correct, as the level came back up that would be relief of downward pressure? Could be just the cap not equalising correctly. Have to wait until tomorrow now.

Vacuum on the other end of the circuit, would pull liquid up, so level at the other end would go down, until vacuum is released.
Mac.

 

[CrispyEdd]

What would vacuum mean?

 

[PlasticMac]

What would vacuum mean?

If you top up a sealed system, when it's warm, a vac will created as it continues to cool, as the air shrinks. The vacuum will pull the liquid up, so at the opposite end of the system, the level will drop. When you remove the cap, the levels will equal out. The coolant levels will change, but not the total volume.
Not sure if different caps have different release pressures.
Either way, if the level recovers, there no problem, imo.
Mac.

 

[CrispyEdd]

If you top up a sealed system, when it's warm, a vac will created as it continues to cool, as the air shrinks. The vacuum will pull the liquid up, so at the opposite end of the system, the level will drop. When you remove the cap, the levels will equal out. The coolant levels will change, but not the total volume.
Not sure if different caps have different release pressures.
Either way, if the level recovers, there no problem, imo.
Mac.

Thanks Mac, makes sense!! Will see what happens tomorrow then with old cap. Will go out now and see if it pops again.

 

[CrispyEdd]

Coolant was bit warm still just now, cap seemed to let air in as I undid it slowly, the coolant level rose back. I'll leave it overnight and put original cap back cold.

So apart from warm coolant when putting cap on should any vacuum appear anyway?

 

[CrispyEdd]

Refitted the original cap this monring stone cold, drove 35 miles to work for 45 minutes. Left it sat there, just been out, stone cold engine. I gently removed the cap and again it puffed air (in or out who knows for sure) and the coolant level rose about 1.5cm up the reservoir.

Is this normal? Cna anyone with a tdi see if their coolant cap and level does same pleasae?

Also the level is below the sensor yet it doesnt ping on the dash! Tad annoying that it doesnt register the level being below the min. Tried reconnecting and drying the plug.

 

[PlasticMac]

Refitted the original cap this monring stone cold, drove 35 miles to work for 45 minutes. Left it sat there, just been out, stone cold engine. I gently removed the cap and again it puffed air (in or out who knows for sure) and the coolant level rose about 1.5cm up the reservoir.

Is this normal? Cna anyone with a tdi see if their coolant cap and level does same pleasae?

Also the level is below the sensor yet it doesnt ping on the dash! Tad annoying that it doesnt register the level being below the min. Tried reconnecting and drying the plug.

Edd,
I have given it a lot of thought. It seems you are not actually loosing coolant, do you agree? So whether it's pressure or vacuum, I'm at loss as to where the coolant is going to, and coming back from.
Mac.

 

[Evripidis]

Hi,

This means that the coolant was warm and under pressure. When you removed the cap, the pressure dropped so the coolant heated up and expanded in volume, hence the rise.

I haven't got a tdi but the observations suggest so. I could be wrong though.

The sensor will do that sometimes until it resettles inside the tank. Mine does it and the connector is fine, but it can equally be corrosion on the contacts (either side) or loose connection/wire.

Evros

 

[CrispyEdd]

Thanks for the input, would be nice to know if anyone elses does the same though. I'll monitor the level over coming few days make sure its not dissappearing slowly....

 

[Evripidis]

Based on my observations, I would say check the level when the radiator hoses are cold. Or when completely cold.

 

[Guest 4123]

Maybe you have some air in the system? When the system heats up and the air is compressed by the pressure, the liquid level drops. When you remove the cap (and release the pressure) the level rises.

RAB

 

[Jongriff]

My TDI 75 hisses when removing the cap. I havent checked for a while and I was meaning to get one of those exhaust gas tester things to see if I had a head gasket issue. But the car has been absolutely fine and I forgot about it . End of last year I checked it last and when removing the cap I get a hiss and some coolant splurge from the vent in the cap like you do.

I am away at the moment but I will check it this weekend after a drive and when cold to see if its the same now. I dont think I have lost any coolant and the temp stays at bang on 90.

 

[Jongriff]

My TDI 75 hisses when removing the cap. I havent checked for a while and I was meaning to get one of those exhaust gas tester things to see if I had a head gasket issue. But the car has been absolutely fine and I forgot about it . End of last year I checked it last and when removing the cap I get a hiss and some coolant splurge from the vent in the cap like you do.

I am away at the moment but I will check it this weekend after a drive and when cold to see if its the same now. I dont think I have lost any coolant and the temp stays at bang on 90.

Should have said that i don't know if its air going in or air coming out causing the hiss.

 

[PlasticMac]

Maybe you have some air in the system? When the system heats up and the air is compressed by the pressure, the liquid level drops. When you remove the cap (and release the pressure) the level rises.

RAB

I wondered about this, but the air would expand quite a lot as it warms up. No idea how the volume expansion due to heat compares with compression due to pressure, but my guess is expansion of the air due to 80 or so degrees increase in temperatures would be the greater.
I do think expansion of the coolant itself, as it heats up, will be small it's about 4% between ambient and 100C.
for water, antifreeze would reduce this.
Mac.

 

[Guest 4123]

I wondered about this, but the air would expand quite a lot as it warms up.

How much difference it makes would depend on the amount of air. If you go for a short run, that will normally get rid of most of it but the heater controls would have to be fully open!

RAB