If the air and fuel system are functioning correctly, perform a compression check to determine whether the problem is:
• Piston ring sealing
• Valve sealing
• Cylinder head gasket sealing or a crack in the cylinder head
Piston Ring Sealing
If the compression is low but can be increased significantly by squirting lubricating oil into the cylinder, the
cause of low compression is inadequate sealing between the piston rings and the cylinder walls.
Valve Sealing
If the compression is low on one or more non-adjacent cylinders and the pressure cannot be increased by oiling
the rings, poor valve sealing is to be suspected.
Valve leakage is often audible from the intake and/or exhaust manifold.
The parent valve seats can be re-ground to a depth of .254mm [.010 inch]. Re-ground seats are identified with a
mark on the cylinder head. Service valve seats must be installed in previously ground valve seats.
Head Gasket Sealing
If the compression was found low on adjacent cylinders and the pressure cannot be increased by oiling the rings,
the head gasket is probably leaking between the cylinders.
Low compression on a single cylinder can be caused by an external leak or a leak to a coolant passage.
A compression leak to the coolant will normally be detected by loss of coolant as the coolant is blown from the cooling system.
