Head gasket failure its causes and diagnosis

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Head gasket failure its causes and diagnosis was created by Cobber

Posted 1 month 5 days ago #206909
Due to the fact that there are a great many enquiries about Head gasket failure. This issue does seem be be of immense concern to many owners and prospective buyers of the MGF and TF . I thought that I would place this here, it is actually a reply I wrote to a query raised by a member in a thread.
I figure by placing it here it would be easier for all to find in the future.

N.B From here on the term "head gasket failure" shall be referred to by the initials HGF.

HGF is a bit of a boogyman for these cars, to much has been made of it and as such everyone is overly paranoid about it.
Yes it is a real problem, but no more so than on many other cars.
So lets look at the various reasons why head gaskets can fail on our cars.

The early MGFs came out with plastic head locating dowels, a bloody stupid idea which didn't do much of a job of keeping the head located. And if fitted it's most likely that they have been replaced with steel ones by now.
The cylinder head can fret (move about a tiny bit) thereby fatiguing the gasket.
There are other causes of cylinder heat fretting that have effected our engines, early head gaskets were of a different design and inferior to later designs.
and the fact that the head studs go straight though the engine block to also secure the crankshaft main bearings as well, can cause problems.
Such long studs can stretch a bit more that shorter studs, and it is critical that they are replaced and tensioned properly when the head is removed. There are up rated (better quality) head studs available.
Some later engines had a more rigid stud girdle, often referred to as an oil rail on our engines as it also distributes the oil to the bottom end. This also uses the go through head studs and increases the rigidity of the engine.

These engines have separate cylinder liners sometimes the cylinder liners aren't sitting properly, which makes the surface that the head gasket seals uneven.
This problem is most likely to occur when an engine has had its cylinder head removed for repair, the cause is sloppy workmanship, by the mechanic that did the work.

Over heating due to a variety of reasons such as loss of coolant and various failures like: waterpump, fan, drive belts, hoses, radiator etc. Another problem that is common to all cars, but our cars in particular is the coolant filler cap, it has a pressure relief valve integrated into it, this isn't always the best quality, and it can leak away your coolant. In fact I recommend carrying a spare in the boot or glove box at all times.
Proper preventative maintenance is key here. That and and the fitment of a "Low Coolant Warning Alarm" is essential for ALL cars, IMHO, but it is perhaps, more so with our cars.

Sometimes a cylinder head will warp with over heating. So unless the head surface is flat and clean when it is assembled, HGF will most certainly follow.
The head should be checked with a straight edge, and re machined if is isn't flat. It should be noted that there is a practical limit to how much a head can be re machined.

Corrosion and pitting of the surface of the cylinder head is another cause of HGF, this can possibly be cured, by welding the damage and re machining the head as I did to save the impossible to find head of a Mercedes, but generally it is more expedient to replace a head in such poor condition.

Heat stress over time, can shorten the life of heat gaskets. many heating and cooling cycles can cause stress to a lot of engine components and the limited airflow of a mid engine layout inherently contributes to this. This can be minimised by keeping the cooling system in top condition.

There are other more exotic and unlikely reasons for head gasket failure, which I won't bother to mention now, otherwise this post will read like bloody War and

So what are to most important lessons here?

* A low coolant alarm is essential for our cars and if not already fitted one should be fitted ASAP!

* Proper maintenance is of the utmost importance and should be done by a competent person.......no elcheapo shortcuts and quick fixes!

* Where possible uprated components should probably be used whenever major engine surgery is done.

Now we can go to properly diagnosing a buggered heat gasket.
A little snot on the oil cap can be due to condensation, but if noticed a proper diagnosis must be done.
A proper diagnosis should be done if an unexplained loss of coolant is noticed a or if there is any other reason to suspect HGF, such as over heating. and/or globs of oil in the coolant.

* Check the oil, pull the dipstick and look at the quality of the oil is it nice and clean, darkly blackish, dirty black sludge or greyish white snot the latter 2 should be attended to immediately, the black sludge indicates poor maintenance and needs changing ASAP but doesn't necessarily have any implications of HGF, however if the dipstick shows greyish white snot then a buggered head gasket is highly likely. There are other potential causes for water getting in to the oil but HGF is the most likely.

* Remove and inspect the spark plugs, water droplets on the plugs is a dead give away, as is great gushes of water coming out of the plug hole whilst the engine is cranked over by the starter! An unusually clean spark plug is also cause for suspicion, any coolant getting into the combustion chamber can effectively steam clean the spark plug. whist you have the sparkplugs out you can go to either of the next two diagnostic checks.

* A compression test should be done this will indicate poor compression which can be caused by HGF, cracked cylinder liners, cracked cylinder head, poor valve sealing, or bugged rings and or bores. the Gauge is fitted to the sparkplug hole and the engine wound over on the starter to see how much compression is available.
you are looking for consistent readings across all the cylinders within @ 10% of one another, the exact pressure reading doesn't matter much. other tan to say there must be at least 80 psi.
Any discrepancies in the readings can indicate HGF, if only one cylinder is low then all of the options above may apply. If two adjacent cylinders have the same or very close low readings then the most likely cause is siamesed cylinders, this happens when the head gasket fails between the two cylinders, but can also indicate a cracked cylinder head. Either way the head has to come off!

* There is a similar test to the compression test, this is a cylinder leakdown test, like a compression test a pressure gauge is fitted to the sparkplug hole, but this test involves you you aligning the cylinder under test to top dead centre with both intake and exhaust valves closed. Then compressed air is applied the cylinder' a pressure reading is then noted. Once the pressure has been supplied the supply is turned off and the gauge observed any quick loss of pressure can indicate problems, if you ca hear air leaking from the exhaust then a damaged exhaust valve is the most likely problem, the sound of air leaking from the intake, will tell you the inlet valve is the most likely culprit, Is air leaking it to the cooling system? then bubbling may be seen under the coolant filler cap. and if air can be heard leaking into the crankcase then that's also bad. the latter two could be HGF or other problems needing more investigation.
The advantage of the compression test is it's relatively quick and easy in comparison with the cylinder leakdown test, which has to be done one cylinder at a time.
Different equipment is required for these tests.

* A coolant system pressure test involves fitting a test unit to the coolant filler cap, this unit has a gauge and a pump, you pump the pressure up to around 15psi and note how long it takes for the pressure to drop, a quick loss of pressure can indicate HGF, but it can also point to other problems, such a leaky radiator, welch plugs, cracked block, buggered water pump bearing, bad hoses or connections to name a few. While the system is under pressure you can do a thorougher inspection around all the various components that carry coolant around the car for leaks. The same equipment can be used to test the pressure relief valve in the coolant filler cap if the kit comes with the appropriate adapters.

* A coolant sniff test, is another way to check for HGF, a cracked head or cracked liners. The process involves a simple tool that is fitted in place of the coolant filler cap that consists of a clear chamber that contains a blue fluid that turns yellow if there are combustion gasses present in the coolant, indicating a problem.
Like the coolant pressure test this test can't test for all types of HGF only those that concern the cooling system they may but won't always indicate if the HGF is only either siamesed cylinders, combustion gasses or coolant are leaking in to the crankcase and other less common types of HGF like oil, combustion gasses or coolant leaking to the out side of the engine, though a coolant pressure test might show the latter if it's bad, but the more common weep may not show.

Another potential cause of overheating and therefore HGF is a knackered thermostat, these can either get stuck open allowing over cooling of the engine, or worse stuck shut thereby overheating it!
Removal of the thermostat is required to do a visual check, a thermostat that is stuck open should be obvious and should be replaced, you should never run an engine with it removed as the coolant may not circulate around the engine in the ways it was designed to causing localised hot spots.
A thermostat stuck or sticking closed will obviously cause over heating, this can be tested by placing the thermostat in a suitable container such as a small saucepan and heating it on a stove with a with thermometer to monitor the temperature that is required to open the thermostat.

Now there are probably a few points that I've missed, but I've had enough of composing this great volume for now and if you've read this far I assume that you've had enough of reading it!
I have no doubt there will be others that will come along and add to this, and they're bloody well welcome to do so.

I shall probably have more to add to this subject as time goes by and I will add further posts here as they occur to me.
Watch this space!

"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"

Last Edit:1 month 3 days ago by Cobber
Last edit: 1 month 3 days ago by Cobber. Reason: Some additional points added and edited for better readablity
The following user(s) said Thank You: David Aiketgate, sworkscooper, MGB281, neilpinleeds, Sculler, Notanumber

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Replied by Airportable on topic Head gasket failure it's causes and diagnosis

Posted 1 month 5 days ago #206911
My word Cobber, let me be the first to congratulate you on your epic posts. I can’t see there being much you have omitted & I will certainly watch this space for future inspiration.

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Replied by generous_dad on topic Head gasket failure it's causes and diagnosis

Posted 1 month 4 days ago #206927
This needs a "sticky" or something similar.

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Replied by David Aiketgate on topic Head gasket failure it's causes and diagnosis

Posted 1 month 3 days ago #206932
I have introduced this into our 'How to' Guides. Cheers Cobber


Last Edit:1 month 3 days ago by David Aiketgate
Last edit: 1 month 3 days ago by David Aiketgate.

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Replied by Airportable on topic Head gasket failure it's causes and diagnosis

Posted 1 month 3 days ago #206933
Is there not a sound argument for a special Cobber’s guide to Life, The Universe, Motorised things & everything else.
I would create a computer short cut for that without a moment’s hesitation.

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