Pressurising coolant

[anto]

Hello folks, got a problem with my engine pressurising its coolant.
Its a peugeot 306 1.9td xud9te and very similar to the 1.6td you guys use in your veedubs.

Here are the facts;

Car has been suffering from pressurised coolant and slight loss of water.
No temperature problems at all
Opening rad cap will causing water to gush out after a run

So;

Thermostat checked and is opening as it should
Radiator replaced with a new one.
Water pump works as it should
head pressure tested and is ok
head skimmed
payen headgasket added with new head studs tightened down as spec.

What else could cause my coolant to be pressurised?
Please suggest anything even remotely related as this is frying my head trying to work out the problem.

Sorry for the essay.
thanks in advance

Anton

[Rabbit on Roids]

did you have the head pressure tested cold or warm? ive seen cracks that would not present themselves if the metal was not warm.

is there any possibility that the block is cracked?

[8v-of-fury]

Opening rad cap will causing water to gush out after a run

Your opening a rad cap when it is hot?!? I did this once to an F-350 (stupidest hard life lesson I have ever learned) and had second degree burns on my face... BE CAREFUL, please!

Opening any rad cap while hot will cause the coolant to literally boil out of the reservoir, I mean after all it is at or around 180F. Leave the car over night, and open it first thing in the morning. Get in and start the car, and see if while idling it starts to bubble over.  

[Rabbit on Roids]

a careful mechanic can vent a hot cooling system with no burns. especially one that has a nice big gas pocket in the radiator.

[8v-of-fury]

a stupid mechanic can vent a hot cooling system with no burns. especially one that has a nice big gas pocket in the radiator.

fixed it for you Kevo The one i was attempting to vent was a nicely over pressurized mix of water, oil and dawn dish soap. The hottest thing I have ever felt... Don't be stupid.

[Vincent Waldon]

Was thinking about this the other day...as I was fussing with my MK3 that looks like it's tossed the head gasket:   how come whenever I open the rad cap the system picks that exact time to boil over?  Then I realized.... it's elementary physics, Watson.

Cooling systems are pressured to raise the coolant's boiling point... allows the engine to run more efficiently without the system forming pockets of steam, boiling over, etc.

However, if the engine is nice and hot and you open the rad cap the pressure in the system suddenly drops. The coolant is now *above* its boiling point, and some portion of it flashes directly to steam, geysering super-heated liquid coolant and steam out of the system.    ouch  

That's why they always say "never do this to a hot engine" or "if you must, wear gloves, drape a big rag over the rad cap, and prepare to still get burned."    

[8v-of-fury]

had a rag, gloves and my jacket draped over it.. the cap shot up dented the hood, and covered the entire front of the truck in a nice gooey mess.. it was not a fun ride to emerg lemme tell you.. NEVER AGAIN!

[anto]

Soooo....
Could i just be imagining this build up of pressure then?
I know coolant systems require some pressure to raise the boiling point but my car is able to make the top radiator hose be rock hard!
Tested our other car (same engine) and it never builds pressure like mine does.
Still losing some coolant as well.

Rabbit - it was pressure tested warm and came out AOK. I am obviously going on the engineers word on that as i wasnt present for the test.

With regards a cracked block, it seems unlikely as its usually the head/ headgasket that blows 1st on these engines.
But it was my next point of thought.

[Rabbit on Roids]

with soo many things checked off your list. the list gets shorter and shorter. it may be one of those illusive leaks that you will never find and just live with. the head gaskets in my 89 4runner have been leaking a tiny bit since right after i changed them. it uses about 3 cups of water a week. and its always consistent. so i just drive it..

[arb]

A few weeks ago my 1.6TD in my Dodge caravan began venting coolant out the rad cap after a few hours of hard running... If the engine was running, it was venting a lot of coolant. Remove the cap to relieve pressure, and replace it, start the engine (operating temp) and it would immeadiately start venting again a litter in a minute's time.

I had resurfaced the head last fall and it has ran great until now. Its got to be combustion gasses - HG or Head. Either way, I've ordered a new head and will start fresh in the next week or so when it gets here. I figure the resurfacing (twice) was simply borrowed time, so I'm not complaining.

[Mark(The Miser)UK]

BP of water above atmospheric.

"110 C - 6psig, 120 C - 14 psig, 130 C - 24.5 psig, 140 C - 37.6 psig, 150 C - 54.3 psig".

So you're looking at about 125C 
Release cap SLOWLY, and ensure gas is escaping as you do so...

[arb]

Hello folks, got a problem with my engine pressurising its coolant.
Its a peugeot 306 1.9td xud9te and very similar to the 1.6td you guys use in your veedubs.

Here are the facts;

Car has been suffering from pressurised coolant and slight loss of water.
No temperature problems at all
Opening rad cap will causing water to gush out after a run

So;

Thermostat checked and is opening as it should
Radiator replaced with a new one.
Water pump works as it should
head pressure tested and is ok
head skimmed
payen headgasket added with new head studs tightened down as spec.

What else could cause my coolant to be pressurised?
Please suggest anything even remotely related as this is frying my head trying to work out the problem.

Sorry for the essay.
thanks in advance

Anton

When you say it is "pressurized" - what PSI are you talking about ? As said, all modern auto cooling systems are pressurized - usually 14 - 17 psi, depending on the system. Have you measured it? Many auto parts stores will measure this for you - both the pressure your cap vents at as well as your cooling system.  If you are over heating, you will over pressurize you system and the cap will vent as designed.

My HG / head problem is not over heating... the engine at temp w/o pressure in the system will immediately dump coolant out the cap the moment the engine is started, and stops immediately after it is shutdown.  Over heating has some lag at both starting and stopping.

[Dirtrag2]

How about the heater core? may be leaking a bit there and still not show up on pressure test.

[anto]

Why would that cause pressure to build up in my coolant which is my main problem?

[8v-of-fury]

It wouldn't cause the pressure, but it may be the cause of the loss.