Why do headgaskets fail?

[The_Burg]

Some engines it's inherent due to design. K series springs to mind. Though having owned many and still have 2 i've never had an issue. And if built proper;y have no issues and produce big power per litre.

Had a Passat 20v non turbo than blew it's top and the youth has an A3 with the same issue. 115hp from 1.8 so not exactly stressed.
This is a known tough engine that can go 300hp plus in turbo form.

So why?

[gowmonster]

because they are designed to if the engine gets to hot no? better the gasket than the block or head?

[bigjobbo]

Poor quality gaskets, faulty engine cooling, poor previous repairs. Generally.

[Eighteeteewhy]

I think poor/cheap coolant has a lot to do with it.

ETA...or even the "wrong" coolant, cheap or not.

[aw51 121565]

Because the head and block can "move" relative to each other in certain circumstances (eg with an alloy head and iron block, with their differing expansion rates as the engine warms and cools)? Admittedly less of an issue with modern all-alloy engines, but they can still have steel liners which can cause this "movement".

The K-series (and I'm a BLMC/BL/ARG/Rover fan ) was originally a good engine with minimal HGF 'issues', before its design was cheapened (the dowels used to locate the head, block and head gasket were changed from metal in the original engines to plastic in an attempt to save money, for example). The thermostat setup (on the cold side of the raditor, somehow) wasn't ideal and caused the engine extra thermal stress as it warmed up then was quickly filled with a rush of cold water from the radiator (rather than the gentle bleed of cold water in engines with conventionally located thermostats as the thermostat opens) - but the original K-series engine could cope with this OK without potentially hastening HGF (the plastic-dowelled engines couldn't).

(The earliest KV6 engnes had 'issues' with cracking cylinder liners as well as failing seals at their bottom end where they seated against the bottom of the crankcase.)

Away from the K-series, the issue can be lack of antifreeze allowing (in general terms) the head to corrode around and under the head gasket therefore not supporting the Head Gasket enough for it to seal properly and HGF follows...

Or overheating, as alluded to upthread...

Or... Or... I think most of the usual suspects are as mentioned above .

[morgrp]

A few common things in my experience, overheating (often through water pump failure), poor engine design, poorly rebuilt engines and age/mileage - they simply deteriorate with age and use -

[vrooom]

metal expand when it get hot but gasket doesn't...

[GAjon]

Well, they stop paying attention at school, fall into the wrong crowd, petty crime, bit of a laugh, goes to far then it all gets serious and a downward spiral to drug abuse, crime and ultimately prison.
It's the governments fault.

[DKS]

I'm willing to say 95% of failed HG, poor design aside, is due to old anti-freeze. How many people know it should be renewed every 2 years? Even the long life pink stuff that's supposed to last 10 years just doesn't.
I've changed about 12-14 HG in my time, mostly Vauxhall, some Nissan, one Citroen. The Xantia had failed across 2 cylinders for some reason, ALL of the Vauxhall ones has corroded between pot 4 and the large water channel next to it, after 10 years and around 100k. No over heating, none of this head warping RUBBISH. Don't even get me started on skimming (which should only be done to remove damage). One of mine needed doing due to a large weep of oil from the back, again, no idea why but it's not re-appeared in 75k.
In my experience, if there's a trickling noise behind the dash, it's due to an airlock caused by pressure from the breached gasket. Eventually a misfire from probably one cylinder will follow for a few seconds after first start. If the coolant it kept topped up, many more months can be bought, but the gasket is on the way out and on a long journey it will drink all it's coolant and over heat.

Also there's no protection from the gasket corroding from the outside in.
In short, run an engine long enough and like any seal or gasket, it will fail.

[The_Burg]

But why on a VAG 20v 1.8? Terribly feeble engine?
Gutless POS. If it was a boosted turbo i'd expect troubles.

[kambites]

Eighteeteewhy said:

I think poor/cheap coolant has a lot to do with it.

ETA...or even the "wrong" coolant, cheap or not.

Poor quality water? Surely the additives you add to your coolant aren't so much to make it cool the engine better so to stop it from freezing and/or corroding things.