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GM Astro/Safari 1985-1996 Repair Guide

Checking Engine Compression


See Figure 1

Click image to see an enlarged view

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

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.

As mentioned in the Tools and Equipment part of General Information & Maintenance , a screw-in type compression gauge is more accurate that 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.

  1. Make sure the battery is fully charged.
  3. Warm up the engine to normal operating temperature, then shut the engine OFF .
  5. Disable the ignition system by disconnecting the primary ignition wiring from the coil. On early models equipped with an external ignition coil it is also possible to disconnect the secondary wiring (coil-to-distributor lead).