Compression test. Spark plugs in or out?

South Gippy Rover

Active Member
Hi All,
When doing compression tests in the past I have always removed all the spark plugs at the start and then tested each cylinder in turn. I mentioned this to a mate recently and he told me that you should only remove the spark plug from the cylinder that is being tested and leave the rest in situ.
Who is correct and does it make a difference?
Thanks in advance.
Paul.
 
you would normally take them all out, but the other method would work. occasionally i only take out the plug out on a suspect cylinder, but in that case im merely trying to establish whether it has a reasonable amount of compression or nothing at all on just the one cylinder
 
I do not want to push in on this topic, but I remember some years ago I had a compression done on my 3500S by GRD of Woking, and they did test with throttle both wide open and closed. They did explain the logic of this, but I forget. Anyway the result was they confidently diagnosed a worn camshaft, not a 'lazy tappet' as the cause of tappet noise, and changed the camshaft. The old one was a revelation:- most cams worn nearly circular, but the hydraulic tappets had kept the car going. I had only owned the car a year, so not my neglect. I suspect that the large number of previous owners had neglected oil changes.....
Previous owner had used a 'mobile engine tune' guy who had only talked of replacing the 'lazy tappet' and never mentioned camshaft wear. Similar from a local Rover agent. I guess we know better now.

Robin.
 
Hi Robin.

Feel free to push in.
We all learn from other people’s experiences.
All input is gratefully received.

Regards,

Paul.
 
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