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 such as the Compression Tester 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
| Maximum Pressure | Minimum Pressure | Maximum Pressure | Minimum Pressure | Maximum Pressure | Minimum Pressure | Maximum Pressure | Minimum Pressure |
|---|---|---|---|---|---|---|---|
| 924 kPa (134 psi) | 696 kPa (101 psi) | 1,131 kPa (164 psi) | 848 kPa (123 psi) | 1,338 kPa (194 psi) | 1,000 kPa (146 psi) | 1,544 kPa (224 psi) | 1,158 kPa (168 psi) |
| 938 kPa (136 psi) | 703 kPa (102 psi) | 1,145 kPa (166 psi) | 855 kPa (124 psi) | 1,351 kPa (196 psi) | 1,014 kPa (147 psi) | 1,558 kPa (226 psi) | 1,165 kPa (169 psi) |
| 952 kPa (138 psi) | 717 kPa (104 psi) | 1,158 kPa (168 psi) | 869 kPa (126 psi) | 1,365 kPa (198 psi) | 1,020 kPa (148 psi) | 1,572 kPa (228 psi) | 1,179 kPa (171 psi) |
| 965 kPa (140 psi) | 724 kPa (106 psi) | 1,172 kPa (170 psi) | 876 kPa (127 psi) | 1,379 kPa (200 psi) | 1,034 kPa (150 psi) | 1,586 kPa (230 psi) | 1,186 kPa (172 psi) |
| 979 kPa (142 psi) | 738 kPa (107 psi) | 1,186 kPa (172 psi) | 889 kPa (129 psi) | 1,303 kPa (202 psi) | 1,041 kPa (151 psi) | 1,600 kPa (232 psi) | 1,200 kPa (174 psi) |
| 933 kPa (144 psi) | 745 kPa (109 psi) | 1,200 kPa (174 psi) | 903 kPa (131 psi) | 1,407 kPa (204 psi) | 1,055 kPa (153 psi) | 1,055 kPa (153 psi) | 1,207 kPa (175 psi) |
| 1,007 kPa (146 psi) | 758 kPa (110 psi) | 1,214 kPa (176 psi) | 910 kPa (132 psi) | 1,420 kPa (206 psi) | 1,062 kPa (154 psi) | 1,627 kPa (154 psi) | 1,220 kPa (177 psi) |
| 1,020 kPa (148 psi) | 765 kPa (111 psi) | 1,227 kPa (178 psi) | 917 kPa (133 psi) | 1,434 kPa (208 psi) | 1,075 kPa (156 psi) | 1,641 kPa (238 psi) | 1,227 kPa (178 psi) |
| 1,034 kPa (150 psi) | 779 kPa (113 psi) | 1,241 kPa (180 psi) | 931 kPa (135 psi) | 1,448 kPa (210 psi) | 1,083 kPa (157 psi) | 1,655 kPa (240 psi) | 1,241 kPa (180 psi) |
| 1,048 kPa (152 psi) | 786 kPa (114 psi) | 1,255 kPa (182 psi) | 936 kPa (136 psi) | 1,462 kPa (212 psi) | 1,089 kPa (158 psi) | 1,669 kPa (242 psi) | 1,248 kPa (181 psi) |
| 1,062 kPa (154 psi) | 793 kPa (115 psi) | 1,269 kPa (184 psi) | 952 kPa (138 psi) | 1,476 kPa (214 psi) | 1,103 kPa (160 psi) | 1,682 kPa (244 psi) | 1,262 kPa (183 psi) |
| 1,076 kPa (156 psi) | 807 kPa (117 psi) | 1,282 kPa (186 psi) | 965 kPa (140 psi) | 1,489 kPa (216 psi) | 1,117 kPa (162 psi) | 1,696 kPa (246 psi) | 1,269 kPa (184 psi) |
| 1,089 kPa (158 psi) | 814 kPa (118 psi) | 1,296 kPa (188 psi) | 972 kPa (141 psi) | 1,503 kPa (218 psi) | 1,124 kPa (163 psi) | 1,710 kPa (248 psi) | 1,202 kPa (186 psi) |
| 1,103 kPa (160 psi) | 827 kPa (120 psi) | 1,310 kPa (190 psi) | 979 kPa (142 psi) | 1,517 kPa (220 psi) | 1,138 kPa (165 psi) | 1,724 kPa (250 psi) | 1,289 kPa (187 psi) |
| 1,110 kPa (161 psi) | 834 kPa (121 psi) | 1,324 kPa (192 psi) | 993 kPa (144 psi) | 1,631 kPa (222 psi) | 1,145 kPa (166 psi) | - | - |
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 that the lowest reading is within 75% of the highest reading.