3.9 compression test

hiya its time to start work on my 3.9 sisco,its been stood a while with a suspect coolant leak?poss head gasket,theres pressure in the coolant system.ive looked and where the front of the valley gasket sits is oily/wet?maybe valley gasket leaking?question is what does a compression test do,apart from the obvious..i,m thinking if it shows worn cam lobes etc i may aswell change camshaft etc.what do you experts reckon.cheers

A compression test can tell you a number of things. It is not a precise diagnostic procedure but can help in pointing you in the right direction.

A compression test has to be done twice - first dry, then again wet.

From your post it sounds like you want to see if you have a blown head gasket. If the head gasket is blown, a compression test might normally show this, typically, two pots will be down. It is possible that both head gaskets are blown and so you might see low results on both banks.

Once you've done the dry compression test (and written down the results!!) run teh test again wet - pour a small amount of oil into the bore and check compression. If the reading goes up then worn rings, if it stays the same then head gasket or sticking valves.

To be honest if you have already decided you might want/need to pop the heads off you might as well just get on with it. Allow a couple of days to do it, take your time, make sure you have all the parts to hand and that you have the right manual to do the job.

With the heads off you will be able to assess the condition of the engine a bit better. It is also a good opportunity to lap the valves in, refresh oil seals and of course do those head gaskets and stick a new valley gasket in. You should also take a careful look at the variety of hoses etc that you would normally ignore. Don't forget to clean the plenum and ram housing while you're in there.

cheers matey,to be honest i bought a 2.5dse rr with the intention of getting rid of the disco but being a v8 fan i,m struggling to get used to the lack of grunt the rr has to offer(has good a car as it is)so i,m 99% sure it has to go and the disco,s a keeperbut before the rr goes i need to be sure i,ve solved the coolant pressure/loss problem i,ve had for the last year or so,also its a little lumpy so i,m thinking maybe camshaft/timing gear change just a general refresh.i,m a lttle reluctant to mess with the head gaskets since reading about them leaking once they,ve been messed with? so if compression test comes out good i,ll leave well alone.thanx for reply craig

bart2181 said:

hiya its time to start work on my 3.9 sisco,its been stood a while with a suspect coolant leak?poss head gasket,theres pressure in the coolant system.ive looked and where the front of the valley gasket sits is oily/wet?maybe valley gasket leaking?question is what does a compression test do,apart from the obvious..i,m thinking if it shows worn cam lobes etc i may aswell change camshaft etc.what do you experts reckon.cheers

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hope it hasn't been sitting too long, cause could be water in pot/pots would have made right royal mess of things be now.

bart2181 said:

i need to be sure i,ve solved the coolant pressure/loss problem i,ve had for the last year or so,also its a little lumpy so i,m thinking maybe camshaft/timing gear change just a general refresh.

i,m a lttle reluctant to mess with the head gaskets since reading about them leaking once they,ve been messed with? so if compression test comes out good i,ll leave well alone.thanx for reply craig

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coolant pressure/loss is a common sign of head gasket problem.

As for head gaskets leaking after being changed that is very much open to debate, mine has been fine.

and porus linner as well buy a combustion gas tester to see if you have petrol fumes in the coolant

vougese39 said:

and porus linner as well buy a combustion gas tester to see if you have petrol fumes in the coolant

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porus liner,not really what i wanted to hear,i,ve had a combustion gas test done and it came out clear.disco,s on hold atm whilst i fix my bad starting rr,i new i should have kept well away.thanks for replies