ENGINE PERFORMANCE II

LESSON 4

ELECTRONIC FUEL INJECTION Lesson 5

Electronic fuel injection systems use solenoid-operated injectors that are pulse width modulated to meter the fuel entering the engine. The injector solenoid may control a pintle, ball, or disc valve. Ball and disc valve are more resistant to clogging. The PCM controls the ground side of the injector circuit. A zener diode in the PCM allows the injector to produce a voltage spike of 30v - 120v, which gives the injector a quicker response time without damaging the PCM or injector seat. Draw electrical circuit!

FUEL INJECTOR TYPES:

Bottom-fed > the fuel enters the injector near the bottom

and flows to the valve.

Top-fed > the fuel enters the injector at the top and flows through the body to the valve.

INJECTOR CONTROL CIRCUIT TYPES:

Saturation current > the current is limited by the injector winding resistance (10 - 20 ohms). The PCM only controls the “ON” time.

Peak-and-hold > the current is controlled by the PCM. The PCM lets the

opening current build to a peak and then reduces it to a

holding level until the injector is turned off. The injectors

used with this type of control have very little resistance

(1 - 3 ohms) and would burn out if the PCM did not limit

the current.

EFI has two basic forms: THROTTLE BODY INJECTION - TBI

MULTIPORT FUEL INJECTION - MFI

TBI - a fuel injector is mounted in the throttle body above the throttle plate.

Mounted to the throttle body are the TPS, fuel pressure regulator and idle control actuator. MAP or MAF sensors may also be attached.

There are two basic TBI units; single and dual throttle bodies. Single units are used on 4-cylinder engines, dual units are used on V-6 and V-8 engines. Single units have one bore, throttle plate and injector; dual units have two of each. Some Japanese designs have two injectors above one throttle plate. TBI injectors are normally pulsed once for each crankshaft reference pulse (cylinder ignition event).

Dual TBI injectors are pulsed alternately. Some dual injectors are not interchangeable side-to-side (check part numbers!!).

Typical TBI FUEL PRESSURE SPECS.

GM 9-13psi (Bottom-fed injector)

Ford - High pressure 39-41psi (Top-fed injector)

Low pressure 14.5psi (Bottom-fed injector)

Chrysler -

Pre-1986 36psi +/-2psi (Top-fed injector)

1986 & up 14.5psi (Bottom-fed injector)

MFI -Multiport fuel injection has an injector placed in each intake runner above the intake valve. The PCM pulses the injectors to deliver fuel.

The fuel sits on the top of the valve until it opens, and is pulled into the cylinder with the incoming air. Since the fuel is placed directly onto the hot intake valve, early fuel evaporation and heated air intake systems are not needed. Fuel control and throttle response are better, since fuel does not travel through the intake runners and condense out of the air

stream.

There are two basic types of MFI:

1. Multiport FI (MFI) - The injectors are pulsed in groups (banks), at

the same time, once for each crankshaft rotation. Half the fuel

required is delivered on the first pulse and the rest on the second

pulse. Simultaneous Double-Fire.

2. Sequential Fuel Injection (SFI)

Each injector is pulse in the firing order, once every other

crankshaft revolution. The intake valve opens immediately after the injector is pulsed.

* Cold-start injector > used on some systems to provide extra fuel for cold starting. Controlled by a thermo-time switch. On during

cranking when temp. is below 70^o F or less. Coolant temp determines length of on time, which is normally not more

than 8 seconds.

GM Central Port Injection - used on V-6 and V-8 Vortex truck engines. The injectors are poppet valves located in each intake port. The valves are fed fuel through nylon tubes from the injector located in the center of the intake manifold. When the injector valve or valves are energized the fuel pressure (54-64psi) opens the poppet valves delivering fuel to the

intake valve port. This system requires a minimum fuel pressure of 50psi for the engine to start.

Fuel pressure regulator operation -

The rail mounted fuel pressure regulator’s job is to maintain a constant pressure drop across the injector tip. Fuel pressure is maintained from the fuel pump outlet to the return end of the fuel rail, which the injectors are mounted in.

A diaphragm-controlled valve limits the fuel being returned to the tank.

Fuel pump pressure opens the valve when pressure is too high, lowering the pressure in the fuel rail. To maintain the correct pressure, the valve is closed by spring pressure with the help of intake manifold pressure sensed through a vacuum hose. As the manifold pressure changes, the fuel pressure changes to maintain a constant pressure drop across

the injector tip. MFI fuel pressure is typically 10psi higher at WOT

than at idle. Fuel pressure is held in the fuel lines by an outlet check valve in the fuel pump. The pressure should hold 2-5 minutes after engine shutdown.

Typical MFI fuel pressure specs.

GM - 34psi at idle <> 44psi WOT (54psi > w/turbo boost)

Ford - 30psi at idle <> 40psi WOT (50psi > w/turbo boost)

Chrysler -

42psi at idle <> 62psi WOT (4 cyl. w/turbo boost)

46psi at idle <> 56psi WOT (3.0L V-6)

43psi at idle <> 53psi WOT (3.3 & 3.8L V-6)

29psi at idle <> 39psi WOT (3.9L V-6 & 5.2L V-8 1992)

40.2psi constant (3.9L V-6 & 5.2L V-8 1993 & up , V-10) This is a returnless fuel system. The regulator is in the tank. The PCM adjusts injector pulse width to correct for manifold pressure differences.

All fuel pumps should flow one pint of fuel in 20 seconds!!

Injector Service:

Flow testing on a bench is the best way of injector testing.

We do not have this type of test equipment but we can flow test

on the vehicle.

1. Resistance testing

2. Injector Balance or Pressure Drop Testing

3. Injector Cleaning

o The best way is rail flushing.

o Pressurized Chemicals

o Tank additives (Techtron by Chevron)

Not only cleans the injector but also cleans the valves and piston rings.

ENGINE PERFORMANCE II

LESSON 5

FUEL INJECTION CONTROL

The PCM is programmed to provide the correct air/fuel ratio for each operating condition. Fuel economy, tailpipe emissions and driveability are all affected by the PCM fuel control decisions.

The PCM uses two basic modes for controlling the air/fuel mixture:

OPEN LOOP and CLOSED LOOP

OPEN LOOP - the A/F ratio is controlled to provide the best driveability and minimal emissions under the following conditions:

A. Start-up - richer mixture

B. Cold engine operation - richer mixture

C. Acceleration - richer mixture

D. WOT - richer mixture

E. Deceleration - leaner mixture or complete fuel cut-off

F. Heavy engine load - determined from throttle position & air flow

CLOSED LOOP – during closed loop operation the A/F ratio is constantly swinging from slightly rich to slightly lean and back. The A/F ratio is maintained at an average of 14.7:1. This ratio allows for the best catalytic converter operation and the least amount of tailpipe emissions during the following conditions:

A. Idle

B. Steady cruise

Closed loop can only be attained when the engine has reached a minimum temperature, the O2S is hot and operating and the open loop timer has expired (time depends on start-up temp.).

During extended light load cruise conditions the PCM may enter a

semi-closed loop mode and command an A/F ratio of 16:1 or leaner to improve fuel economy.

BASIC PCM FUEL CONTROL STRATEGIES

1. Shut down – low system voltage or engine speed.

In this mode the computer does not control fuel.

2. Start-up Enrichment - provides a rich fuel mixture for a short time after any engine start. The length of time depends on coolant temperature.

3. Open loop - During engine warm-up the fuel mixture control is based on inputs from all sensors except the O2 sensor. The length

of time open loop lasts depends on coolant temp., O2 sensor temp. and the open loop timer. Open loop occurs at

every start-up.

4. Closed loop - After the timer has expired and the engine and O2 sensor have warmed up, the PCM uses O2 sensor input to fine tune fuel control. The system remains in closed loop until a change in load occurs or the O2 sensor cools off. If a change in load occurs the ECM adjusts fuel delivery accordingly, ignoring the O2 input. The O2 input is only used during idle and cruise conditions.

5. Enrichment or WOT - If the throttle is opened to near WOT the ECM controls fuel delivery richer to meet the need.

6. Deceleration – when a deceleration condition is detected the fuel

delivery is decreased or completely shut off.

7. Cranking - throttle less than 80% open. A/F ratio 1.5:1 at -33^oF,

14.7:1 at 202^oF

8. Clear Flood - throttle more than 80%, A/F ratio 20:1or leaner.

9. Battery voltage correction - if battery voltage gets too low the ECM will increase injector pulse width to make up for a slower injector and fuel pump. pg 229

10. Fuel cut-off - key off or no rpm signal to PCM.

11. Limp-in - uses back-up program stored in a RAM to keep the engine running in the case of an ECM failure.

FMEM - Failure Mode Effects Management - the PCM substitutes a working value if a sensor fails.

Most EFI systems have similar strategies.

FUEL INJECTION CONTROL PROGRAMS

The PCM uses two programs to control fuel delivery through the injector.

These programs are referred to as long term and short term fuel trims or adapts. Both programs' commands are represented by numbers on a scan tool.

0% is the mid-point or normal command. A (-X%) means fuel is being subtracted, a (+X%) means fuel is being added as compared to the normal fuel need.

A. Short-term fuel trim - a short term or "right now" fuel correction based on O2 sensor input.

( GM > Integrator or INT 128)

B. Long-term fuel trim - a long term or learned correction based on the short term command. It has 16 or

more different cells for different engine load and speed combinations.

Long-term fuel trim is the base fuel program that can adapt to changes in operating conditions.

(GM >Block Learn or BLM 128)

The long-term and short-term trims comprise the total fuel trim correction.

The long-term fuel trim’s task is to maintain the short-term at its

“zero correction” point during closed loop conditions.

A sustained high or low short-term command will cause the long-term to change the base fuel delivery program allowing the short-term to return to its mid-point. If the short-term number is high, the long-term increases allowing the short-term to return to mid-point and vice versa.

SHORT-TERM<>LONG-TERM CHART

EXHAUST CONDITION	SHORT-TERM COMMAND	LONG-TERM COMMAND	INJECTOR PULSE WIDTH

THE SHORT-TERM RESPONDS TO THE EXHAUST CONDITION,

THE LONG-TERM RESPONDS TO THE SHORT-TERM!

Abnormal LONG-TERM numbers with normal SHORT-TERM numbers indicate a problem with the fuel system or engine condition that the PCM is compensating for to maintain a 14.7:1 A/F ratio.

If both SHORT-TERM and the LONG-TERM are abnormal the problem is too great for the ECM to compensate for.

FEEDBACK CARBURETOR CONTROLS

The PCM controls the A/F ratio using a mixture control (M/C) solenoid or stepper motor in the carburetor or remotely mounted in the engine compartment . The solenoid controls both the idle circuit and the main metered/power circuit. Air bleeds or fuel jets are controlled to adjust air or fuel entering the carburetor.

SEE pg 94-5, 273-4, 319 & 321, 332-3, 362-4

The control signal to the M/C solenoid is a duty cycle signal.

Each cycle is 0.1 second long. When the M/C solenoid is ON it leans the mixture by allowing more air into the air bleed circuit or restricting fuel flow into the main well. When the solenoid is OFF it enriches the mixture in the opposite way.

The PCM controls A/F ratio by changing the ON time of the mixture control solenoid during each cycle. The ON time can range from 10% to 90% duty cycle. 10% duty cycle is a rich command, 90% duty cycle is a lean command.

During open loop the M/C command is based on all sensors except the O2 sensor. During closed loop the M/C command responds to the O2 sensor to maintain a 14.7 to 1 A/F ratio. A lean exhaust condition causes a rich M/C command and a rich exhaust condition causes a lean M/C command.

The M/C dwell command will vary 5^o to 10^o (15% to 30%) in closed loop to try and keep the 14.7 : 1 A/F ratio.

CONDITION - COMMAND CHART

O2 OUTPUT VOLTAGE	EXHAUST CONDITION	PCM FUEL COMMAND	M/C COMMAND
0 – 450mV	LEAN	RICH	10% -50% (6^o - 30^o)
450 -1000mV	RICH	LEAN	50% - 90% (30^o - 54^o)

The fuel command is a result of the exhaust condition!!

AS LONG AS THE M/C DWELL IS VARYING THE A/F RATIO IS 14.7:1 AVERAGE!!

GM uses a dwell meter to measure M/C duty cycle. Dwell is a measure a coil ON time, so the meter works well. M/C dwell is ALWAYS measured with the 6 cylinder scale! 6^o dwell (10%) is a rich command, 54^o dwell (90%) is a lean command. (Green conn. near carb or MAP)

GM carburetor SYSTEM PERFORMANCE CHECK

Tests the ability of the ECM and M/C solenoid to control A/F ratio

Steady 6^o indicates a lean fuel problem.

Steady 54^o indicates a rich fuel problem.

GM M/C Dwell Specifications

E2SE Varijet: Idle 25 degrees 3000rpm 35 - 42 degrees

E2ME Dualjet & E4ME Quadrajet: Idle & 3000rpm 35 - 42 degrees

M/C dwell MUST be adjusted after a carb. overhaul!