I have a 1983 240d with zero compression on the #3 cylinder. Car has about 200k miles. All the other cylinder messure between 340-400 compression.
I pulled the motor about 6 months ago and it's been sitting in my garage since then.

I squirted some oil in the #3 cylinder and the compression instantly jumped to over 300 psi so I assumed I had bad rings/piston/cylinder.

Yesterday, I pull the valve cover and set the cam so that there was valve clearance between both the intake and exhaust valves. I assume by doing this both valves would be seated tightly and there would not be any air coming out of the intake or exhaust ports.

After doing the above, I shot forced air into the injector port and all the air came out the intake port. I did the same thing to the other cylinders and the cam/engine rotated as should happen if the valves were seated.

Now I think the problem has always been with the intake valve on #3.

The questions I have are the following:

Why would squirting oil in the #3 cylinder cause the compression to go to over 300 if the intake valve is stuck open?

Is it possible to fix this problem without removing the head?

Will a machine shop need to rebuild the head or is this something I would be able to handle with the proper tools? I do have many tools/gauges but not a commercial press.

What should be my next step in resolving this problem?

Thanks for any advise.

pete