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 MANUALSCHEVROLET1987FORWARD CONTROL 4.8 T, AUTOMATIC, 400/M40REPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMAIR INJECTION SYSTEM - VEHICLES OVER 8500 GVWCOMPONENT TESTINGDECELERATION VALVE
1987 Chevrolet Forward Control 4.8 T, Automatic, 400/M40
Deceleration Valve
1987 Chevrolet Forward Control 4.8 T, Automatic, 400/M40SECTION Deceleration Valve
- Remove air cleaner. Plug air cleaner vacuum source. Connect tachometer. With engine running at specified idle speed, remove small deceleration valve signal hose from manifold vacuum source. Reconnect signal hose and listen for air flow through ventilation pipe and into deceleration valve. A drop in engine RPM will be noticed when signal hose is reconnected.
- If air flow does not continue for at least one second or engine RPM does not drop noticeably, check deceleration valve hoses for restrictions or leaks. If no restrictions or leaks are found, replace deceleration valve. See Figure , Figure and Figure .
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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.