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[chrisgixer]

Pps, if i did a coolant pressure test and the HG had gone, i'd definitely see bubbles in the expansion tank, right?

No, other way round if engine off. You'd be forcing the coolant out past the hg, so into the bore, or in to fresh air if hg failed externally.

[Webby the Bear]

So really then to be absolutely certain of a HG diagnosis (as we cant see in to the bore) and block testers are unreliable is a leak down test?

[joff]

Pps, if i did a coolant pressure test and the HG had gone, i'd definitely see bubbles in the expansion tank, right?

No, other way round if engine off. You'd be forcing the coolant out past the hg, so into the bore, or in to fresh air if hg failed externally.

On the TD5 engine the cylinder head moves as they used plastic locator pins on build which over time snap and head moves just a small amount. This allows exhaust gas to get into coolant system which is then pressureised by the gas, and that's how you pick it up with the kit.

No plastic pins on a Mig

I would use a compression tester if you have that, but that could show a low reading for a few things

1. Valves
2. Piston rings
3. Head gasket to air

to name but 3

[Marks DTM Calib]

I would say that you need to consider what this test is telling you , it does nothing more than detect to see of there is carbon dioxide in the coolant system.

Similarly, leak down tests tell you that there is some escape of pressure from the bores.

Compression test tells you peak compression for a cylinder.

As there is no such thing as a 'head gasket failed' test you have to use observation of symptoms to make the diagnosis and its this that modern mechanics are bloody awful at. 

Classic example, on a 3.0 Omega V6 not one of the above tests will give any indication of the classic head gasket failure on that engine type.....

[Webby the Bear]

Pps, if i did a coolant pressure test and the HG had gone, i'd definitely see bubbles in the expansion tank, right?

No, other way round if engine off. You'd be forcing the coolant out past the hg, so into the bore, or in to fresh air if hg failed externally.

On the TD5 engine the cylinder head moves as they used plastic locator pins on build which over time snap and head moves just a small amount. This allows exhaust gas to get into coolant system which is then pressureised by the gas, and that's how you pick it up with the kit.

No plastic pins on a Mig

I would use a compression tester if you have that, but that could show a low reading for a few things

1. Valves
2. Piston rings
3. Head gasket to air

to name but 3

Thats right. Then a leak down would pin point where the failure was 

Really was just trying to find out if there was any other way cos although i have a compression tester, i dont have compressed air for a leak down test.

[Webby the Bear]

I would say that you need to consider what this test is telling you , it does nothing more than detect to see of there is carbon dioxide in the coolant system.

Similarly, leak down tests tell you that there is some escape of pressure from the bores.

Compression test tells you peak compression for a cylinder.

As there is no such thing as a 'head gasket failed' test you have to use observation of symptoms to make the diagnosis and its this that modern mechanics are bloody awful at. 

Classic example, on a 3.0 Omega V6 not one of the above tests will give any indication of the classic head gasket failure on that engine type.....

Very true Mark. but with a leak down test youd see bubbles in the coolant confirming HG failure so surely that is the one and only sure fire test.

struggling to understand why particularly this wouldnt work to find a hg failure on a 3.0 v6 mig? 

[joff]

I would say that you need to consider what this test is telling you , it does nothing more than detect to see of there is carbon dioxide in the coolant system.

Similarly, leak down tests tell you that there is some escape of pressure from the bores.

Compression test tells you peak compression for a cylinder.

As there is no such thing as a 'head gasket failed' test you have to use observation of symptoms to make the diagnosis and its this that modern mechanics are bloody awful at. 

Classic example, on a 3.0 Omega V6 not one of the above tests will give any indication of the classic head gasket failure on that engine type.....

Very true Mark. but with a leak down test youd see bubbles in the coolant confirming HG failure so surely that is the one and only sure fire test.

struggling to understand why particularly this wouldnt work to find a hg failure on a 3.0 v6 mig? 

That would be the same as system being pressurised when running, are you losing coolant?

[Marks DTM Calib]

I would say that you need to consider what this test is telling you , it does nothing more than detect to see of there is carbon dioxide in the coolant system.

Similarly, leak down tests tell you that there is some escape of pressure from the bores.

Compression test tells you peak compression for a cylinder.

As there is no such thing as a 'head gasket failed' test you have to use observation of symptoms to make the diagnosis and its this that modern mechanics are bloody awful at. 

Classic example, on a 3.0 Omega V6 not one of the above tests will give any indication of the classic head gasket failure on that engine type.....

Very true Mark. but with a leak down test youd see bubbles in the coolant confirming HG failure so surely that is the one and only sure fire test.

struggling to understand why particularly this wouldnt work to find a hg failure on a 3.0 v6 mig? 

You might see bubbles but the pressures used on a leak down test are but mere fractions of that seen during the combustion stage so don't consider it a good test.

Your also making a fundamental error in that you assume that a head gasket always fails around the fire rings.......

[Webby the Bear]

Thanks Mark,

But as I'm in danger of getting boggled by basic science....

if these tests wouldnt work whats the procedure for diagnosing a bad HG on the omega? obviously compression test will tell you if youre losing pressure. a leak down will then tell you if theres a leak around fire rings. and a coolant pressure test will show you external leaking cylinder head (thus HG).

Surely theres no more tests you can perform other than the block test and collating other symptoms (over heating, mayo in the oil etc)?

[Webby the Bear]

PS not trying to be awkward.... just trying to understand it

[Marks DTM Calib]

Well given that 3.0 V6 head gasket tends to result in coolant loss at the rear of the 2-4-6 bank cylinder head, this is only like to show up with a coolant system pressure test.

Similarly, you could do a pressure test and find loss coolant into a cylinder, diag a head gasket and find the liner is dropped or cracked.

so many possibles, variation on engine type and design etc......no conclusive answer and hence why a good mechanic looks at all symptom's before making a judgement.

Not what you want to hear I know... 

[joff]

Well given that 3.0 V6 head gasket tends to result in coolant loss at the rear of the 2-4-6 bank cylinder head, this is only like to show up with a coolant system pressure test.

Similarly, you could do a pressure test and find loss coolant into a cylinder, diag a head gasket and find the liner is dropped or cracked.

so many possibles, variation on engine type and design etc......no conclusive answer and hence why a good mechanic looks at all symptom's before making a judgement.

Not what you want to hear I know... 

Take the head off and have a look

[Webby the Bear]

Well given that 3.0 V6 head gasket tends to result in coolant loss at the rear of the 2-4-6 bank cylinder head, this is only like to show up with a coolant system pressure test.

Similarly, you could do a pressure test and find loss coolant into a cylinder, diag a head gasket and find the liner is dropped or cracked.

so many possibles, variation on engine type and design etc......no conclusive answer and hence why a good mechanic looks at all symptom's before making a judgement.

Not what you want to hear I know... 

No, that makes sense. But essentially the more tests you do the more evidence you have.

I';ve not heard of a cracked liner on one of these cars. Have you? i have heard of them on rovers.... 

[Marks DTM Calib]

Dry liners so much less likely to occur....hence the 'different design' statement

[Webby the Bear]

As for the aforementioned Rover.... i'm sure what happened was the symptom was water in the oil. HG was replaced (being a Rover) and it kept happening. Stripped the sump off and watched water pee out in to the oil. Terrible design.