Section 03-00: Engine, Service | 1996 All F-150, F-250, F-350, F-Super Duty and Bronco Vehicles Equipped with 4.9L, 5.0L, 5.8L or 7.5L Gasoline Engines, and 7.3L Diesel Engines Workshop Manual |
Compression Gauge Check
Make sure oil in crankcase is of the correct viscosity and at proper level and battery (10655) is properly charged. Operate vehicle until engine (6007) is at normal operating temperature. Key off, then remove all spark plugs (12405).
Set throttle plates in wide-open position.
Install a compression gauge such as Rotunda Compression Tester 164-R0250 or equivalent in No. 1 cylinder spark plug opening.
Install an auxiliary starter switch in starting circuit. With key off, and using auxiliary starter switch, crank engine at least five compression strokes and record highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
Repeat test on each cylinder, cranking the engine approximately the same number of compression strokes.
Test Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest. Refer to the following chart.
| Maximum kPa | Maximum psi | Minimum kPa | Minimum psi |
|---|---|---|---|
| 924 | 134 | 696 | 101 |
| 938 | 136 | 703 | 102 |
| 952 | 138 | 717 | 104 |
| 965 | 140 | 724 | 105 |
| 979 | 142 | 738 | 107 |
| 993 | 144 | 745 | 108 |
| 1007 | 146 | 758 | 110 |
| 1020 | 148 | 765 | 111 |
| 1034 | 150 | 779 | 113 |
| 1048 | 152 | 786 | 114 |
| 1062 | 154 | 793 | 115 |
| 1076 | 156 | 807 | 117 |
| 1089 | 158 | 814 | 118 |
| 1103 | 160 | 827 | 120 |
| 1117 | 162 | 834 | 121 |
| 1131 | 164 | 848 | 123 |
| 1145 | 166 | 855 | 124 |
| 1158 | 168 | 869 | 126 |
| 1172 | 170 | 876 | 127 |
| 1186 | 172 | 889 | 129 |
| 1200 | 174 | 903 | 131 |
| 1214 | 176 | 910 | 132 |
| 1227 | 178 | 917 | 133 |
| 1241 | 180 | 931 | 135 |
| 1255 | 182 | 938 | 136 |
| 1268 | 184 | 952 | 138 |
| 1282 | 186 | 965 | 140 |
| 1296 | 188 | 972 | 141 |
| 1310 | 190 | 979 | 142 |
| 1324 | 192 | 993 | 144 |
| 1338 | 194 | 1000 | 145 |
| 1351 | 196 | 1014 | 147 |
| 1365 | 198 | 1020 | 148 |
| 1379 | 200 | 1034 | 150 |
| 1393 | 202 | 1041 | 151 |
| 1407 | 204 | 1055 | 153 |
| 1420 | 206 | 1062 | 154 |
| 1434 | 208 | 1076 | 156 |
| 1448 | 210 | 1083 | 157 |
| 1462 | 212 | 1089 | 158 |
| 1476 | 214 | 1103 | 160 |
| 1489 | 216 | 1117 | 162 |
| 1503 | 218 | 1124 | 163 |
| 1517 | 220 | 1138 | 165 |
| 1531 | 222 | 1145 | 166 |
| 1544 | 224 | 1158 | 168 |
| 1558 | 226 | 1165 | 169 |
| 1572 | 228 | 1179 | 171 |
| 1586 | 230 | 1186 | 172 |
| 1600 | 232 | 1200 | 174 |
| 1613 | 234 | 1207 | 175 |
| 1627 | 236 | 1220 | 177 |
| 1641 | 238 | 1227 | 178 |
| 1655 | 240 | 1241 | 180 |
| 1669 | 242 | 1248 | 181 |
| 1682 | 244 | 1262 | 183 |
| 1696 | 246 | 1269 | 184 |
| 1710 | 248 | 1283 | 186 |
| 1724 | 250 | 1289 | 187 |
Compression Readings — Interpreting
It is recommended the Compression Pressure Limit Chart be used when checking cylinder compression so that the lowest reading number is 75 percent of the highest reading.
If one or more cylinders reads low, squirt approximately one 1.5 ml (1 tablespoon) of SAE 50 weight or equivalent engine oil on top of the pistons in the low reading cylinders. Repeat compression pressure check on these cylinders.
If compression improves considerably, piston rings are at fault.
If compression does not improve, valves are sticking or seating poorly.
If two adjacent cylinders indicate low compression pressures and squirting oil on pistons does not increase compression, cause may be a cylinder head gasket leak between cylinders. Engine oil and/or coolant in cylinders could result from this problem.
Example Readings
For example, check the compression pressures in all cylinders. If the highest reading obtained was 1351 kPa (196 psi) and the lowest pressure reading was 1069 kPa (155 psi), the engine is within specification and the compression is considered satisfactory.