Loss of compression

[jathastings]

My car started missfiring and after checking the ignition I did a compression test. The result was no compression on cylinders 3 and 4. I assume that this means a head gasket failure between 3 and 4.
Obviously I need to take the head off to investigate but wondered if others have experienced this problem and if it was as simple as the gasket?
This was a simple change from the car running normally to suddenly running on 4 cylinders without any other symptons. No apparent water in oil or oil in water.
Yout thoughts will be appreciated.
Cheers
JohnT

[oldtimer]

John,
Sorry to hear of your problem but not unusual. Your assumption is most likely correct but with the head off all will be revealed. My memory ain't what it was but I seem to recall that this occurred mainly on 4.2 engines as the barrel or bore spacing within the block changed in that the amount of material between 3/4 bores was reduced. If examination reveals that the head gasket has failed at this point it is crucial to examine both the block and head to ensure no metal erosion has taken place. Failure to observe this will most likely result in premature failure of a replacement gasket.

Let us know how you get on

[PPV]

Hi, I would have thought that if there was zero compression then it would be worse than a misfire (probably sound like a badly galloping horse with the firing order of 1,5,3,6,2,4). Did you pull off plug leads in turn to confirm that it was cylinders 3 and 4 that were not doing anywork? However, if you are certain there is very much reduced compression and your valves are not sticking then unfortunately taking the head off looks like the option to follow. Best of luck. It is fairly straightforward but heavy work. Just follow the manual.
Regards,

[Puddinhead]

What was the actual compression readings for 3+4 ? Did you take only one plug out at a time during the compression tests ?

Thanks,
Patrick
'66 FHC

[jathastings]

Yes I took all the plugs out and the readings for 3 & 4 were zero.
I've now finally got round to doing the job and in fact the gasket between 3&4 was completley burnt away.
The block looks ok - no cracks and is flat so I assume the head is the problem. I' ll check for flatness and if needed will get it skimmed. What I am surprised about though is that if the head has gone out of flat then its across the head rather than longwise.
Any comments will be welcomed especially info on the best quality gasket to use.

[oldtimer]

John,
Am pleased you have got the head off and identified the problem - well done.
Do you have a magnetic based pedestal onto which you can attach a DTI ( Dial Test Indicator ) Set the DTI to zero and the magnetic base switched off so that you are able to slide the block across the surface of the engine block. With the top surface of the engine block impeccably clean, slide the magnetic base in a manner that allows the DTI probe to contact the engine block in the vicinity of the area between cylinder3 and 4 and see what readings you get! Might I suggest a sheet of A4 paper with six circles, then take a series of readings all over to get the feel of what the hell is going on. A quality straight edge and feeler gauge can be quite useful too. Pity the head studs get in the way.

I'm afraid I can't be helpful in respect to head gaskets. Back in the sixties I could get a head gasket set for 30 shillings or £1.50p in todays money. How times have changed. Gaskets for 3.4 and 3.8 were thin steel corror joint gaskets, like a pressing around each cylinder and water ports. I always used a thin film of Red Hermatite jointing compound on both sides and never had a comeback. I seem to recall that copper asbestos head gaskets appeared for the 4.2 engine. I believe you will need to consult your supplier to see what is fashionable to use these days. Perhaps others with more recent experience will come forward and advise. I would prefer to use a Jaguar approved item if it is available especially in view of the work necessary to replace a substandard gasket.

John, are you taking the opportunity to carry out any other work while the head is off ----- do let us know

I hope my ramblings are of some help --- Oldtimer