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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 138 (ENGINE CONTROL SYSTEM - BASIC DIAGNOSTIC PROCEDURES)PRELIMINARY INSPECTION & ADJUSTMENTSMECHANICAL INSPECTIONCYLINDER COMPRESSION PRESSURE
1999 GMC Forward Control 5.7 R, Automatic

Cylinder Compression Pressure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Cylinder Compression Pressure
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

Ensure battery is fully charged and starter motor is in good operating condition. Remove engine cover from engine. Remove 4 ignition coils from the cylinder head cover. Remove spark plugs. Remove EFI main relay from fuse/relay block 1. Install a compression gauge into a spark plug hole. Depress the clutch pedal on vehicles equipped with manual transmission. Hold throttle in Wide Open Throttle (WOT). Crank engine through 4 compression strokes for each cylinder being tested. Measure the highest pressure reading. See ENGINE COMPRESSION SPECIFICATIONS  table.

ENGINE COMPRESSION SPECIFICATIONS

Application Specification - psi (kPa)
Normal Compression Pressure (1) 218 (1500)
Minimum Compression Pressure (1) 145 (1000)
(1) No cylinder compression reading should be less than 15 psi (100 kPa) of highest compression reading.

After testing all cylinders, install all parts removed. Compare results with the following for diagnosis:

  • Normal Operation

    The pistons, piston rings and valves are operating normally if the cylinder compression pressure increases quickly and evenly to the compression specification.

  • Faulty Piston Rings

    If cylinder compression is low on the first stroke or cylinder compression increases on the following strokes but does not reach the compression specification, this indicates that the piston rings are faulty.

  • Faulty Valves

    If cylinder compression is low on the first stroke or cylinder compression does not increase on the following strokes, this indicates that the valves are faulty.

RENDER: 1.0x

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.