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Home >> GMC >> 1990 >> Forward Control 4.3 Z, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Testing >> Fuel System Pressure Test >> Field Service Mode Check
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Field Service Mode Check

NOTE: Oxygen sensor may cool off after while engine idles causing system to go into open loop. To restore closed loop mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

This test confirms proper fuel system operation 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. Parking brake on FWD models does NOT hold drive wheels.

1) Start engine. With engine running, ground test terminal "B" of the ALDL. In closed loop mode, SERVICE ENGINE SOON light will flash at a rate of one flash per second.

2) In open loop, light will flash at a rate of 2.5 flashes per second. If light is off most of the time, a lean exhaust is indicated. If light is on most of the time, a rich exhaust is indicated.

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