GM PFI Operation and Testing

Overview

Operating Modes Starting Clear Flood Run Acceleration Deceleration Battery Voltage Correction Fuel Cutoff

Starting Mode PCM runs fuel pump for 2 seconds PCM looks at Coolant Temp and TPS A/F Ratio varies from 1.5/1 (-32F) to 14.7/1 (201F) May have cold start injector Activated by cranking system Controlled by timer

Clear Flood Below 600 RPM 80% WOT or more A/F 20/1 or zero on 2.8-liter engine

Run Mode Above 400 RPM Open or closed loop Closed loop when O2S voltage is varying Coolant is above specified temp Time has elapsed Leaves open loop for WOT Acceleration and hard Deceleration

Acceleration Mode Rapid increase in manifold pressure or air flow Richer mixture Compensates for poor evaporation due to higher pressure.

Deceleration Mode Rapid decrease in throttle and manifold pressure Fuel is cut off completely momentarily

Battery Voltage Correction Mode Voltage drops below specified value Enrichens fuel Increases throttle opening if at idle Increases dwell to account for poor coil saturation

Fuel Cutoff Mode Ignition is turned off Distributor pulse stops Fuel is cut off Prevent dieseling Prevent fire in crash

Fuel Trim Before OBD II STFT was Block Learn LTFT was Integrator More in diagnostics

Fuel Trim

Inputs

Mass Airflow AC - Hot foil Bosch - Hot wire with burn off Hitachi - Heated thermistor Frequency out AC 30 Hz to 150 Hz

Hall Pickup

Hall Fast Start

Hall CMP Distributor

Reluctor

Opti-Spark

EGR Diagnostic Vacuum switch Temp Switch 5.0 and 5.7 engines 12v when EGR off 1v when EGR on

EGR Diagnostic

Outputs

Injector Circuit

SFI Injector Circuit

Pintle VS Multec Injectors

Cold Start Injector

Thermal Time Switch In water jacket 68F or below max 8 seconds Less time with more temperature Only while engine is cranking

Idle Air Control Stepper Motor Zero and count pulses Ingition off EFI 30 MPH on PFI from VSS

IAC

EGR Early EGR was controlled by ported vacuum Three types Standard ported Negative feedback pressure Positive feedback pressure Exhaust backpressure and vacuum

EGR EGR started in 1988 is Digital Controlled by 2 or 3 solenoids Solenoids control different size orifices

Digital EGR

Linear EGR One Solenoid PWM 1994

CMFI

CMFI Distribution

CMFI Injector

CMFT Poppet

Electronic Spark Control ESC Timing modified by KS

Conventional Injector Driver

Peak and Hold Injector Two Drivers

PWM Injector

Injector Cleaning Areas needing cleaning Spray surfaces Inlet screens Methods Fuel tank additives 3 part vacuum systems Pressurized fuel rail

Injector Testing NOID light Volt or ohm meter Scope Stethoscope Injector Balance Tester Scan tool Graduated cylinder