Compression Test
- Make sure the oil in the crankcase is of the correct viscosity and at the correct level and that the battery is correctly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove all the spark plugs.
- Set the throttle plates in the wide-open position.
- Install a compression gauge in the No. 1 cylinder.
- Install an auxiliary starter switch in the starting circuit. With the ignition switch in the OFF position, and using the auxiliary starter switch, crank the engine a minimum of 5 compression strokes and record the highest reading. Note the approximate number of compression strokes necessary to obtain the highest reading.
- Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.
Compression Test - Test Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75% of the highest reading. Refer to the Compression Pressure Limit Chart.
Compression Pressure Limit Chart
NOTE:
The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75% of the highest reading. Refer to the Compression Pressure Limit Chart.
| Maximum Cylinder Pressure - Minimal Cylinder Pressure | Maximum Cylinder Pressure - Minimal Cylinder Pressure | Maximum Cylinder Pressure - Minimal Cylinder Pressure | Maximum Cylinder Pressure - Minimal Cylinder Pressure |
|---|---|---|---|
| 134.0 -100.9 psi (924 -696 kPa) | 164.0 -123.0 psi (1, 131 -848 kPa) | 194.1 -145.0 psi (1, 338 -1, 000 kPa) | 223.9 -168.0 psi (1, 544 -1, 158 kPa) |
| 136.0 -102.0 psi (938 -703 kPa) | 166.1 -124.0 psi (1, 145 -855 kPa) | 195.9 -147.1 psi (1, 351 -1, 014 kPa) | 226.0 -169.0 psi (1, 558 -1, 165 kPa) |
| 138.1 -104.0 psi (952 -717 kPa) | 168.0 -126.0 psi (1, 158 -869 kPa) | 198.0 -147.9 psi (1, 365 -1, 020 kPa) | 228.0 -171.0 psi (1, 572 -1, 179 kPa) |
| 140.0 -105.0 psi (965 -724 kPa) | 170.0 -127.1 psi (1, 172 -876 kPa) | 200.0 -150.0 psi (1, 379 -1, 034 kPa) | 230.0 -172.0 psi (1, 586 -1, 186 kPa) |
| 142.0 -107.0 psi (979 -738 kPa) | 172.0 -128.9 psi (1, 186 -889 kPa) | 201.9 -151.0 psi (1, 392 -1, 041 kPa) | 232.1 -174.0 psi (1, 600 -1, 200 kPa) |
| 143.9 -108.1 psi (992 -745 kPa) | 174.0 -131.0 psi (1, 200 -903 kPa) | 204.1 -153.0 psi (1, 407 -1, 055 kPa) | 233.9 -175.1 psi (1, 613 -1, 207 kPa) |
| 146.1 -109.9 psi (1, 007 -758 kPa) | 176.1 -132.0 psi (1, 214 -910 kPa) | 206.0 -154.0 psi (1, 420 -1, 062 kPa) | 236.0 -176.9 psi (1, 627 -1, 220 kPa) |
| 147.9 -111.0 psi (1, 020 -765 kPa) | 178.0 -133.0 psi (1, 227 -917 kPa) | 208.0 -155.9 psi (1, 434 -1, 075 kPa) | 238.0 -178.0 psi (1, 641 -1, 227 kPa) |
| 150.0 -113.0 psi (1, 034 -779 kPa) | 180.0 -135.0 psi (1, 241 -931 kPa) | 210.0 -157.1 psi (1, 448 -1, 083 kPa) | 240.0 -180.0 psi (1, 655 -1, 241 kPa) |
| 152.0 -114.0 psi (1, 048 -786 kPa) | 182.0 -135.8 psi (1, 255 -936 kPa) | 212.0 -157.9 psi (1, 462 -1, 089 kPa) | 242.1 -181.0 psi (1, 669 -1, 248 kPa) |
| 154.0 -115.0 psi (1, 062 -793 kPa) | 184.1 -138.1 psi (1, 269 -952 kPa) | 214.1 -160.0 psi (1, 476 -1, 103 kPa) | 244.0 -183.0 psi (1, 682 -1, 262 kPa) |
| 156.1 -117.0 psi (1, 076 -807 kPa) | 185.9 -140.0 psi (1, 282 -965 kPa) | 216.0 -162.0 psi (1, 489 -1, 117 kPa) | 246.0 -184.1 psi (1, 696 -1, 269 kPa) |
| 157.9 -118.1 psi (1, 089 -814 kPa) | 188.0 -141.0 psi (1, 296 -972 kPa) | 218.0 -163.0 psi (1, 503 -1, 124 kPa) | 248.0 -174.3 psi (1, 710 -1, 202 kPa) |
| 160.0 -119.9 psi (1, 103 -827 kPa) | 190.0 -142.0 psi (1, 310 -979 kPa) | 220.0 -165.1 psi (1, 517 -1, 138 kPa) | 250.0 -187.0 psi (1, 724 -1, 289 kPa) |
| 161.0 -121.0 psi (1, 110 -834 kPa) | 192.0 -144.0 psi (1, 324 -993 kPa) | 236.6 -166.1 psi (1, 631 -1, 145 kPa) | 251.9 -189.0 psi (1, 737 -1, 303 kPa) |
If one or more cylinders reads low, squirt approximately one tablespoon of engine oil meeting Ford specification on top of the pistons in the low-reading cylinders. Repeat the compression pressure check on these cylinders.
Compression Test - Interpreting Compression Readings
- If compression improves considerably, piston rings are worn or damaged.
- If compression does not improve, valves are sticking or not seating correctly.
- If 2 adjacent cylinders indicate low compression pressures and squirting oil on each piston does not increase compression, the head gasket may be leaking between cylinders. Engine oil or coolant in cylinders could result from this condition. Use the Compression Pressure Limit Chart when checking cylinder compression so the lowest reading is within 75% of the highest reading.