Ford Aerostar 1986-1997 Repair Guide

Compression Testing


See Figure 1

A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits here. A check of each cylinder's compression will help you locate the problems.

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Fig. Fig. 1: A screw-in type compression gauge is more accurate and easier to use without an assistant

As mentioned in the Tools and Equipment part of Routine Maintenance , a screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole. Although it takes slightly longer to use, it's worth it to obtain a more accurate reading.

To check engine compression:

  1. Warm up the engine to normal operating temperature.
  3. Remove all spark plugs.
  5. For vehicles with distributor ignition systems, disconnect the high tension lead from the ignition coil.
  7. For vehicles with Direct Ignition Systems (DIS), detach the ignition coil pack wiring harness connector.
  9. Disengage all fuel injector electrical connections.
  11. Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.

Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily ruined.

  1. Have an assistant depress the accelerator pedal fully. Then, while you read the compression gauge, ask the assistant to crank the engine two or three times in short bursts using the ignition switch.
  3. Read the compression gauge at the end of each series of cranks, and record the highest of these readings. Repeat this procedure for each of the engine's cylinders. Maximum compression should be 175-185 psi. A cylinder's compression pressure is usually acceptable if it is not less than 80% the highest cylinder's reading on the engine. The difference between each cylinder should be no more than about 12-14 psi. (82-96 kPa).
  5. If a cylinder is unusually low, pour a tablespoon of clean engine oil into the cylinder through the spark plug hole and repeat the compression test. If the compression comes up after adding the oil, it appears that the cylinder's piston rings or bore are damaged or worn. If the pressure remains low, the valves may not be seating properly (a valve job is needed), or the head gasket may be blown near that cylinder. If compression in any two adjacent cylinders is low, and if the addition of oil doesn't help the compression, there is probably leakage past the head gasket. Oil and coolant water in the combustion chamber can result from this problem. There may be evidence of water droplets on the engine dipstick when a head gasket has blown.