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
OptiSpark
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
Cold Start Injector
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
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
