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 PERFORMANCESYSTEMCCC TBI TESTS W/CODESTESTING & DIAGNOSISTROUBLE CODE DIAGNOSTIC MATERIALSFIELD SERVICE MODE CHECK
1987 Chevrolet Forward Control 4.8 T, Automatic, 400/M40
Field Service Mode Check
1987 Chevrolet Forward Control 4.8 T, Automatic, 400/M40SECTION Field Service Mode Check
- This test confirms proper operation of fuel system and verifies "Closed Loop" operation. Clear codes and perform this test after any repair is completed.
- When performing this check, always engage parking brake and block drive wheels.
- On some engines, the oxygen sensor will cool off after only a short period of time while engine is idling. This will cause engine to go into open loop. To restore closed loop mode, run engine at part throttle for several minutes, accelerating from idle to part throttle several times.
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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.