Section 03-00: Engine—Service
1994 Mustang Workshop Manual
SERVICE PROCEDURES

Compression Test

 

Compression Gauge Check

  1. Ensure oil in crankcase is of the correct viscosity and at proper level and battery (10653) is properly charged. Operate vehicle until engine is at normal operating temperature. Turn ignition switch (11572) to the OFF position, then remove all spark plugs (12405) .
  1. Set throttle plate in wide-open position.
  1. Install a compression gauge such as Rotunda Compression Tester 059-00009 or equivalent in No. 1 cylinder.
  1. Install an auxiliary starter switch in starting circuit. With ignition switch in the OFF position, 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.
  1. Repeat test on each cylinder, cranking the engine approximately the same number of compression strokes.
 

Test Conclusion

The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest. Refer to the Compression Pressure Limit Chart.

COMPRESSION PRESSURE LIMIT CHART
Maximum PSI Minimum PSI Maximum PSI Minimum PSI Maximum PSI Minimum PSI Maximum PSI Minimum PSI
134 101 164 123 194 145 224 168
136 102 166 124 196 147 226 169
138 104 168 126 198 148 228 171
140 105 170 127 200 150 230 172
142 107 172 129 202 151 232 174
144 108 174 131 204 153 234 175
146 110 176 132 206 154 236 177
148 111 178 133 208 156 238 178
150 113 180 135 210 157 240 180
152 114 182 136 212 158 242 181
154 115 184 138 214 160 244 183
156 117 186 140 216 162 246 184
158 118 188 141 218 163 248 186
160 120 190 142 220 165 250 187
162 121 192 144 222 166    

If one or more cylinders read low, squirt approximately one tablespoon of engine oil XO-20W50-QR or equivalent meeting Ford specification ESE-M2C179-A on top of the pistons (6108) in the low reading cylinders. Repeat compression pressure check on these cylinders.

 

Interpreting Compression Readings

  1. If compression improves considerably, piston rings are at fault.
  1. If compression does not improve, valves are sticking or seating poorly.
  1. If two adjacent cylinders indicate low compression pressures and squirting oil on pistons does not increase compression, cause may be a head gasket leak between cylinders. Engine oil and/or coolant in cylinders could result from this condition.

    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.

 

Example

If, after checking the compression pressures in all cylinders, the highest reading obtained was 196 psi and the lowest pressure reading was 155 psi, the engine is within specification and the compression is considered satisfactory.