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 MANUALSCHEVROLET1991R 30 P/U 2WD V8-350 5.7LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTRELAYS AND MODULES - POWERTRAIN MANAGEMENTRELAYS AND MODULES - COMPUTERS AND CONTROL SYSTEMSENGINE CONTROL MODULETESTING AND INSPECTIONELECTRONIC CONTROL MODULE (ECM) TESTS - TBIMANIFOLD ABSOLUTE PRESSURE (MAP) OUTPUT CHECK
1991 Chevrolet R 30 P/U 2WD V8-350 5.7L
Manifold Absolute Pressure (MAP) Output Check
1991 Chevrolet R 30 P/U 2WD V8-350 5.7LSECTION Manifold Absolute Pressure (MAP) Output Check
Chart C-1D - Manifold Absolute Pressure (MAP) Output Check:
Wiring Diagram For Chart C-1D - Manifold Absolute Pressure (MAP) Output Check:
MANIFOLD ABSOLUTE PRESSURE (MAP) OUTPUT CHECK
CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor measures manifold pressure (vacuum) and sends that signal to the ECM. The MAP sensor is mainly used for fuel calculation, when the ECM is running in the throttle body backup mode. The MAP sensor is also used to determine the barometric pressure and to help calculate fuel delivery.
TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.
1. Checks MAP sensor output voltage to the ECM. This voltage, without engine running, represents a barometer reading to the ECM.
2. Applying 34 kPa (10" Hg.) vacuum to the MAP sensor should cause the voltage to be 1.2 - 2.3 volts less than the voltage at Step 1. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.
3. Check vacuum hose to sensor for leaking or restriction. Be sure no other vacuum devices are connected tot he MAP hose.
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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.