Hi, first off I'm fearing this is a headgasket issue but thought some other opinions may help.

1994 MK3 polo coupe, 1043cc AAU engine. 1.3 throttle body and injector, golf elbow and cone air filter.

I've been losing coolant, meaning I have to top up about once a week, but can't find a leak. Also, the expansion tank and cooling system remains under very high pressure even after the car is left until cold hours later. Removing the cap means a rapid expansion and loosing more coolant as it explodes out. To me this indicates combustion gases are getting into the cooling system.

However, there is no 'mayo' on the oil cap or dipstick, the oil level is normal, the car performs well and does not overheat, no steam or smoke from the exhasut and my mpg is normal.

The only thing I have noticed is that sometimes when cold, the car will idle lumpy for a few seconds before it smooths out and is fine after that, could this be caused by water settling in one cylinder due to the leak and taking a few seconds to be cleared?

I'll be trying to find a garage who will sniff test or compression test it for me tomorrow.

Any advice / ideas? I will be tackling a headgasket change myself if it has failed.

Cheers, JJ

Edited 15 June, 201214 yr by poloJJ

It wouldn't be a stuck closed thermostat would it?

The pipes to the radiator get hot, so I wouldn't have thought so mate

Cheers

Compression test is the way to go.

Thought it was, I'm not best pleased as I had a compression tester and it seems to have been thrown out with some rubbish....

Think I'm going to be trying to find a garage to diagnose tomorrow morning. Cheers

I've got one if you're anywhere near coventry

as above, I'm in basingstoke

I think the collant cap has a valve in it that opens during high pressue? Might be that thats fubard but it doesnt explain the high pressure.

If a water way on the headgasket has cracked between a cylinder I would assume it would cause the pressure to be very high.

Edit: I meant collant loss! doh

Edited 15 June, 201214 yr by Matt G40

Ahh thanks anyway lads but I'm in Newcastle.

Task for tomorrow sorted, I'll report back and hopefully head towards fixing it.

Cheers, JJ

You can get a block tester that goes on the expansion tank that changes colour if exhaust gas passes through it.

You could also ask a garage to put a gas tester over the expansion tank to see of there's any exhaust gasses coming out of there.

With out seeing it, it's hard to say but I would say head gasket, just because oil isn't mixing with water it doesn't mean the head gasket hasn't gone / on its way out.

I recently had the same thing and in my case it was the head that had gone as opposed to the gasket. Decided to fill number 1 with water, but the car otherwise ran as it should untill it eventually died.

Had a Saab doing this, HG had cone between the water jacket and the cylinder, had the slightest of steam out the back but when we pulled the expansion tank cover you could see the bubbles rising in it.

Remove the coolant cap and start the engine, you should see the bubbles coming up fairly quickly.

Cars in the garage for a sniff / compression test. Mechanic agreed that it sounds like water gallery / bore HG break.

Cheers guys, will report back soon as car should be done for 12.

Right, good news!

Compression test results, 14 bar in each cylinder! exactly the same in each and very good compression. No HG failure :)

Also, the occasional rough idle was traced to one faulty spark plug, found when removed for the compression test, two large cracks across the ceramic insulation bit around the electrode, I fitted a new set of plugs and the idle and engine is much smoother.

Mechanic was 100% convinced the pressure problem was a large airlock causing the pressure build up. I managed to bleed some air out of the system and refilled so lets see what happens over the next few days. I am going to replace the header tank cap as this should really not be letting the pressure get this high without venting it off.

Had a look under the engine in an inspection pit, very dry and no obvious water leaks. However the radiator around the bottom hose looked slightly damp, so maybe this is where I'm loosing water from when the engine heats up and any potential leaks appear through expansion.

I'll report back hopefully with it sorted! Cheers, JJ

did he do a leakdown test or just the compression one?

Just compression.

That might not be the complete story. You might well get good pressure when the valves are closed, as its in the valve chamber that the water is mixing. As above, run the engine with the cap off and watch for bubbles.

That might not be the complete story. You might well get good pressure when the valves are closed, as its in the valve chamber that the water is mixing. As above, run the engine with the cap off and watch for bubbles.

Thats why a leak down test would have been better in my view.

Indeed. Though I have just lived through the exact same thing with my engine. It's a very weird experience, the engines completely buggared, yet it still pulls like a train. And it will continue to do so right up untill it never works again. Towards the end i was using a litre of water per mile!

Hmmmm :(

Oh noes!! Flush the system out and try again.

=/ not sure what to do now. Bled the system and bubbles remain.

:(

Looking alot like you'll be cracking the head open

Ergh. Just grafted all week to earn some money to take away on holiday, which is now going to have to go on fixing the car. Gutted

Hg repair isn't much, price parts on ecp, use 25% discount code, get head skimmed. you'll be reet

This is worst case scenario