Yo B,
Since I see you are here now, I am posting some easy to do things first, then onto the compression test. Will add more in new reply after my physical therapist visits here.
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I assume that you have good battery voltage over 12.6 volts. Have it load tested for free at local oarts store, same for starter that needs to be removed.
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Inspect battery clamps, cables and wire strands for corrosion, damage or looseness. Include ground cables from negative battery post to engine, intake manifold, to firewall, to frame, etc.; and positive cable from battery to relay to starter for corroaion, damage, loose connection, etc.
Is relay securely mounted to a Rust/Grime Less section of fender?
This can also be due to a dead battery cell. Sometimes, a battery will have just one cell go bad or short out. When this happens, the battery will seem fully charged, but fail to start the Bronco, or it may start the Bronco right after charging, but then fail to crank. I had a dead cell that did same; no crank then after turning to start a few times, it would crank.
"...Why are the cables and battery so important? The starter circuit pulls a lot of amperage, up to 500 amps depending on the starter, the engine load, and battery condition. This kind of amperage stresses all of the components in the starter circuit, including the battery, battery terminals, the battery disconnect switch, the cables including the ground path, and any remote solenoids. Problems with these components are hard to find because they appear fine at rest or under a light load, but generate high resistance under heavy amperage draws. The result will be low voltage to the starter during cranking, resulting in heavier amperage draw and increased internal heat in the starter. Over time, this will cause starter failure. Voltage measured at the starter during cranking should always be above 9.5VDC..."
Source: by powermastermotorsports
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Compression Test
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.
COMPRESSION PRESSURE LIMIT 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.
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