Cylinder #6 of freshly rebuilt motor has ZERO compression - what's possible cause?

Question

This is a freshly rebuilt 400 c.i. Olds motor. AIll other cylinders checked out with 135-150 psi. I just got the motor back from the machine shop and installed it soon thereafter. New cam, pistons, piston rings, etc. I'm guessing the valves weren't placed correctly and a valve job may be in the works. OR the block is blown.

This is a freshly rebuilt 400 c.i. Olds motor. AIll other cylinders checked out with 135-150 psi. I just got the motor back from the machine shop and installed it soon thereafter. New cam, pistons, piston rings, etc. I'm guessing the valves weren't placed correctly and a valve job may be in the works. OR the block is blown.

BTW, Its carbureuted (1969 Olds big block)

Answer 1

You probably have a valve hung open or not closing tightly - a bad valve job.

If it is popping through the intake side, it is an intake valve, but it is probably on the exhaust side and the cylinder charge of gas is being pushed out the exhaust.

Not good, it tells the oxygen sensor the system needs more gas and it turns up the fuel injector system if FI - wasted gas and fouled plugs.

Take the valve cover off and see if the cam is moving the rocker arms and if the valves for that cylinder are opening and closing. If they are then the valve seat was not ground correctly and you need it done again.

Worse case - you have a hole in the piston and the gas / air mixture is being pushed into the crankcase and being circled through the PCV valve back into the system OR the compression ring on that cylinder is stuck in and doing the same thing.

Sorry.

Sonny

Answer 2

or the valve stem height was over looked.--- remove the rocker arm pedestal and put a 5/16 washer under each bolt on that cylinder only then recheck compression.

Answer 3

The most common is misadjusted valves.

ADDED:
135-150 psi seems kind of low too.........