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 MANUALSGMC1991FORWARD CONTROL 3.9 L, SM465/M20REPAIR AND DIAGNOSISENGINE PERFORMANCEENGINE CONTROLS - SPECIFICATIONSMECHANICAL CHECKSENGINE COMPRESSION
1991 GMC Forward Control 3.9 L, SM465/M20
Engine Compression
1991 GMC Forward Control 3.9 L, SM465/M20SECTION Engine Compression
COMPRESSION SPECIFICATIONS
| Application | Compression Ratio | ||||
|---|---|---|---|---|---|
| 2.5L (VIN E) | 8.3:1 | ||||
| 2.8L (VIN R) | 8.9:1 | ||||
| 3.1L (VIN D) | 8.9:1 | ||||
| 4.3L (VIN Z) | (1) 9.3:1 | ||||
| 5.0L (VIN H) | 9.3:1 | ||||
| 5.7L (VIN K) | (2) 9.1:1 | ||||
| 6.2L Diesel (VIN C & J) | 21.3:1 | ||||
| 7.4L (VIN N) | 7.9:1 | ||||
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RENDER: 1.0x
NO RELATED
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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.