Interpreting Compression Readings

AllData Editors Note - Ford does not provide a cranking pressure specification, only an acceptable range.

Examine the readings for all of the cylinders. Pressure variation between the highest and lowest cylinders should be within 75 percent of one another.

Variations between cylinders will have a greater effect upon engine performance than overall readings that are even but slightly below specifications.

When taking compression readings, watch the action of the gauge needle. When it raises only a small amount on the first stroke, a little more on succeeding strokes, and results in a very LOW reading; burned, sticky or warped valve(s) are indicated.

A low build-up on the first stroke with a gradual build-up on succeeding strokes to a moderate reading can mean worn, stuck, or scored piston rings.

If two adjacent cylinders are low, a blown head gasket or warped cylinder head to cylinder block surface could be responsible.

Add one teaspoon of heavy (50W minimum) engine oil to a cylinder with a low reading. Insert the compression gauge and recheck the cylinder. Crank the engine for a few extra compression strokes and watch the gauge. If the compression goes up 10 percent or more, worn piston rings are indicated. If the addition of the oil produces no significant change, valve trouble, a broken piston ring or a blown head gasket is probably causing the low reading.

If the compression pressure exceeds specifications, there is a build-up of carbon on the head of the piston and on the cylinder walls. IF CARBON BUILD-UP IS PRESENT, AND IS CAUSING PINGING THAT CANNOT BE STOPPED, THE CARBON SHOULD BE REMOVED BY RETARDING THE TIMING OR BY DISASSEMBLY OF THE ENGINE.

Another sign of excessive carbon is "dieseling" (the engine continues to run after the ignition switch is turned OFF). Dieseling can be caused by glowing bits of carbon. Hard cranking can also indicate excessive compression from carbon build-up.

EXAMPLE READINGS





Example: If the highest cylinder reads 924 kPa (134 psi) and the lowest cylinder reads 696 kPa (101 psi), then the 696 (101) reading is within 75 percent of the 924 (134) reading. The above chart shows maximum compression readings and their corresponding 75 percent minimums.