A few years ago I was given an 85 XJ6 which would not start (or even crank initially). Using the guides from these forums I managed to iron out quite a few small problems with the car and have since brought it back to life.

It now runs, but only just. Since it runs rough, I tested the compression and found that cylinders 2, 4, 5 and 6 all made 150 psi which seems quite good for its age and the odometer reading (200,000 km). However, cylinders 1 and 3 made absolutely no pressure at all.

What is most likely causing this? Exhaust valves? When I find some more time I will have a go at removing the head and have a look.

 

Pressurize the cylinder with compressed air using your own medley of fittings or a leakdown tester. See if it will maintain 15 to 35 psi (1 to 3 bar). Then listen to the hisssss....if the air is escaping through the dipstick tube or oil fill then you have a scuffed cylinder wall, cracked piston or worn rings. If the air escapes through the exhaust then you have a faulty exhaust valve or very worn guide. If the air is coming out the intake manifold you have a issue with your intake valve. If the air is coming out of the radiator cap you may have a cracked cylinder or bad head gasket. Remove the valve covers so you can observe TDC for the cylinder in question. For cyl 1 this is not necessary as you can use the timing marks on the balancer but for #3 it will be difficult to divine TDC any other way.

 

If it has been sitting for a long time prior to getting it running, you quite possibly have stuck/seized piston rings.

On those two cylinder, rotate the motor so they are down the bottom of their stroke and fill with ATF fluid, or a penetrating fluid to help loosen up any possible rust/carbon deposits that are holding the rings stuck to the pistons, let the fluid soak overnight.

 

Thanks for the tips guys.

I finally did a leakdown test and found that the problem is with the intake valves in both cylinders. Surprisingly, the exhaust valves appear to be fine in this case. Looks like I'll have to remove the head.