Cylinder compression pressure

[hykam]

My V8 engine back-fires. Try to find out if it is the valve leaking.

Tested the compression pressure on all 8 cylinders, got the reading from 90 - 125 psi.

Does anyone know the normal compression pressure ?

[mike_sekinger]

Hello Hykam.

That pressure range is on the low side. Compression of a healthy cylinder is stated in the manual as being 155 +/- 15psi.

Cheers,

Mike S

[hykam]

Thanks.

Will try to find out where is the leak.

[hilly]

My V8 engine back-fires. Try to find out if it is the valve leaking.

Tested the compression pressure on all 8 cylinders, got the reading from 90 - 125 psi.

Does anyone know the normal compression pressure ?

Did you do the test with the throttle wide open, if not the throttle plates will limit the amount of air suck in so reducing your compression figures ??????

Hilly

[mike_sekinger]

Forgetting the backfiring, but IIRC if you add some oil into the combustion chamber for the test, an increase in compression would suggest worn/damaged rings while no change would indicate valves/valve stem seals, gasket etc..... but I have been wrong before.

Cheers,

Mike S

Edited August 12, 2009 by mike_sekinger

[Vulcan Grey]

also the value will go down with increasing altitude, so it depends on where you live.

But you should also have all cylinders within a fairly close pressure. Varying from 90-125 is a big range.

Re-do the test and make sure to follow what Hilly and Mike said.

[Günter]

third way is:

if you fabricate an adapter to fit into the sparkplug hole -than you can fill the cylinder with compressed air from outside. Use an pressure reader/manometer gauge, and rotate the crank for each cylinder separately and hold the piston for an amount of time in the same position. If you notice an drop in pressure on an specific position you have found an single fault in the liner sidewall.. .

[hykam]

Thanks for all the suggestions.

The compression test my mechanic did was with the air intake pipe dismantled, while the engine was still in the car. So there is no restriction on the air intake. The pressure gauge was mounted to the plug hole and each cylinder was measured individually, by cranking with the starter.

Suspected the leaking is from the valves, rather than the rings. Polishing the valves needs to take the engine out and special tools. The engine was put back in last week after fixing the exhaust manifold. So will tune the valves in the next round of engine out maintenance.

Found one of the primary injector has a very small leaking. Suspect it is the cause for the backfire. Will replace that injector plug lead and check.