Please help with my coolant pressure problem!

[Vitwagen]

I'm really in need of help, can anyone shed light on this? Standard AAZ engine.

Basically, my coolant is pressurising. I thought head gasket. Done that (head skimmed, 3 motch metal gasket), same problem. leave it for an hour and a half and the system is still pressurised (coolant is lukewarm). Undo the expansion cap and it blows right out. the small feed to the expansion tank is feeding properly. Doesn't overheat.

When you rev the engine (with the cap off) you can see all the coolant disappear in half a second. it flow right back as soon as you're off the throttle. if I drain and refill the system, you can let it idle perfectly for ages.

This is my daily car and really need it working properly.

I'm desperate now, please help.

Steve

[blackdogvan]

Put a pressure gauge in the system near the pressure tank. IIRC 10lbs is optimal, if you go for a long drive & its 15-18 i think its still combustion gas so head... Did you ahve the ehad pressure tested when you did the skim & gasket?

[Vitwagen]

No I didn't have it pressure tested, wish I had now.

What I can't figure out is why the pressure doesn't bleed back? is it possible that the gasket is just leaking a little?

[maxfax]

How long does it take for the system to build pressure from startup, if you have all the pressure released?? 

Have you tried another coolant cap? 

With the system holding pressure for so long after shut down I'd think that everything is sealed okay..  Unless it is a small crack or something that takes a lot of pressure to make leak..  A lot of pressure say like compression versus the pressure building int he cooling system...

[coke]

I replaced a head gasket in my 90 Jetta because coolant was pressurizing, it wasn't immediate, but it didn't take long. Needless to say, after the head gasket it was still blowing coolant all over the place like an infant throwing up his formula.  It was highly annoying. Well, I said to hell with it and I replaced the radiator. Never had a problem after that.

Apparently, the radiator was blocked, and the pressurizing was coming from the coolant getting hot and having no place to go. It wasn't circulating through the radiator so it wasn't cooling at all and all it was doing was expanding and blowing out the tank.

Blocked radiator hoses can usually be confirmed by feeling the radiator while its running and somewhat up to temperature.  I believe mine was blocked in the lower core, where the lower radiator hose goes. It will usually be cool in some spots, and hot in another. You should be able to confirm by doing this.  If you have spare radiator from another car, even if it doesn't fit yours, you can hook it up and let it sit in there and see if the coolant problem still persists.

I noticed you had your had skimmed and replaced the gasket and all that. Did you check the block for warpage? If it overheated at all and the block warped (uncommon, at least not as common as heads) then all that work you did would be for nothing. This is really a worst case scenario.

[Vitwagen]

Thanks for the replys guys.

The rad is almost new (7K miles). I don't know if the block was warped, never thought to check. I'm letting it cool overnight, but the hoses are rock solid. I mean solid. These are silicone ones too, they have some give in them. they almost look ballooned. I'll see what residual pressure there is tomorrow.

I thinking that the leak I saw was actually due to this pressure problem, but I never knew there was a pressure problem, as it was escaping.

 

I torqued the studs as described, 30nm, 60nm, 90 degrees then 90 degrees, is it worth giving them all another quarter turn just to be sure?

[nathan_b]

dont do it

[GEE-BEE]

Are you getting good flow from the hose from the radiator to the overflow tank ?

I have seen those clog before

my vote is pump or thermostat ?

GB

[Smokey Eddy]

If you are getting combustion pressure into your coolant (as i did with my 1.6TD) that pressure is many many many times greater than what the cooling system could ever hold so it would make sense for it to only go one way.

Never re-torque those bolts further, they will snap.

I would take off everything cooling system related. Hoses and what not. And if you have access to a compressor try blowing air through all the passages.
Either

A) you have cracks between the valves putting combustion pressure in the coolant (as i did)
B) something is blocking the flow of coolant somewhere

but honestly that water pump can't produce pressures like that. It's coming from else where and im betting its coming from the diesel exploding.
  Its not news you want to hear but when its your DD you don't want to beat around the bush...

I am a terribly listener, even to the voice in my head when im reading, did you say there were or werent cracks between the valves when you had the head off? I see you had it skimmed...

[nathan_b]

Not trying to contradict all your good info, but I don't think anyone meant that the wp was building the pressure, but more that the lack of the wp pumping was causing coolant to become too stationary and take on too much heat and boil over. Again, not my opinion, just attempting to clarify.

[Vitwagen]

Thanks for all the replys guys, here goes on the answers:

There is good coolant flow (the AAZ has a slightly different coolant arrangement to the 1.6, there is a return to the expansion tank from the head, not rad, the flow from that is good)

There were cracks between valves. Why the hell didn't I get it pressure tested??? they didn't look too bad, but one crack was bigger than the others...

The pressure is there long before the system is upto temp. Deffo pressure from exhaust gasses.

Smokey Eddy, This:

Its not news you want to hear but when its your DD you don't want to beat around the bush...

 
...is awesome. I hadn't been thinking about this side of it, and needed to hear it. So, although this engine isn't 'dead', (although the head may be), I know of a low mile AAZ for reasonable money, and I'm gonna bung that in. I can then sit this engine in the garage and give her a rebuild. If nothing else, it gets me back on the road!


Thanks guys, I'll update when I know what's going on. For interest only, the engine is in this: