Compression Check: Testing and Inspection
AllData Editors Note - Ford does not provide a cranking pressure specification, only an acceptable range.
OVERVIEW
Before performing the Engine Compression Test, check the battery (10655). The battery must be fully charged, in good condition and properly connected. If the battery is not fully charged, recharge or replace it. The engine must be at normal operating temperature and the throttle valve must be fully open.
COMPRESSION GAUGE CHECK
1. RUN the engine until normal operating temperature is reached.
2. Turn the engine OFF.
3. Remove the spark plugs (12405).
4. Disconnect the distributor connectors to eliminate fuel flow and spark.
5. Install Rotunda Compression Tester 164-R0250 or equivalent, tightly in the spark plug hole.
NOTE: The engine MUST be at normal operating temperature and the throttle valve must be fully open.
6. Depress and hold the accelerator to Wide Open Throttle (WOT) and crank the engine.
NOTE: Normally, the first compression stroke will run the gauge indicator needle a considerable distance up the scale. Succeeding strokes will raise it more until the highest level is shown (this will require at least four or five compression strokes).
7. Record the highest reading.
8. Repeat Steps 5-7 above for each cylinder.
COMPRESSION READINGS - INTERPRETING
Examine and compare the readings for all of the cylinders. Compression should be 1412 kPa (204 psi) with a minimum of 1030 kPa (149 psi).
Pressure Variation - between the highest and lowest cylinders should be within 75% of one another. See Example Readings chart. The chart shows maximum compression readings and their corresponding 75% minimums. Variations between cylinders will have a greater effect on engine performance than overall readings that are even but slightly below specifications.
Low/Slow Readings - 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 ends up with a very LOW reading; burned, warped, or sticky valves are indicated. A moderate reading with a low buildup on the first stroke and a gradual buildup on succeeding strokes can mean worn, stuck, or scored piston rings (6149). Cylinder Leakage Test
Two Low Readings - If two adjacent cylinders are low, a blown head gasket (6051) or warped cylinder head to cylinder block surface could be responsible. Add 4ml (1 teaspoon) of heavy (30 weight 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% or more, worn piston rings are indicated. If the addition of the oil produces no significant change, valve trouble, a broken piston (6108), or a blown head gasket is probably causing the low reading.
High Readings - If the compression pressure exceeds specifications, there is a build-up of carbon on the head of the piston and on the combustion chamber walls. Another sign of excessive carbon is "dieseling" (the engine continues to run after the ignition is turned OFF). Dieseling can be caused by glowing bits of carbon. Hard cranking can also indicate excessive compression from carbon buildup.
EXAMPLE READINGS
If the highest cylinder reads 924 kPa (134 psi) and the lowest cylinder reads 696 kPa (101 psi), then the 696 kPa (101 psi) reading is within 75% of the 924 kPa (134 psi) reading. See compression chart for maximum and minimum compression readings.