Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSPONTIAC1991LEMANS LE, 2D HATCHBACK, AUTOMATIC, 3T40/MD9REPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - BASIC TESTINGPRELIMINARY INSPECTION & ADJUSTMENTSMECHANICAL INSPECTIONTESTING COMPRESSION
1991 Pontiac LeMans LE, 2D Hatchback, Automatic, 3T40/MD9
Testing Compression
1991 Pontiac LeMans LE, 2D Hatchback, Automatic, 3T40/MD9SECTION Testing Compression
- Disconnect the BAT terminal from the High Energy Ignition (HEI) distributor or module. Check compression pressure with engine at normal operating temperature, all spark plugs removed, and throttle valve wide open.
- Insert a recording-type compression gauge and crank each cylinder through 4 or 5 compression strokes. The lowest reading should not be less than 100 psi (7.03 Kg/cm2 ). If compression builds up quickly and evenly to the specified range on each cylinder, compression is normal.
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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.