I took some time today to put together the information covering the different fuel delivery systems and how they work on the P-30 Series Chassis. There are always some variables due to supplier and run changes. So it is good to have the Vin number when you find things that are a little different, when working on the systems.
It is a little long and did not cover carburetor information. They are a subject in themselves.
Edited 2-8-2013 Vapor lock problems added at the end of article.
GM 454 Engine Fuel Systems:
The following information hopefully will help those that own and love the classic Coaches and wish to keep them running with the performance they were designed to provide. Covering fuel delivery, performance and starting in regards to the fuel system operation.
Ignition system information is covered in the electrical thread.
A M4MC or EM4MC carburetor was used on most of the 1980 series engines. This is an aspirated fuel delivery system with the addition of a mixture control system controlled by the ECM. It is not even close to the ECM used today. This fuel delivery system uses a mechanical fuel pump with the addition of an inline lift pump between this pump and gas tank on some systems to help prevent vapor-lock.
This carburetor featured tamper-resistant controls to discourage readjustment of factory settings, which could affect either or both emission control and throttle response.
This style carburetor uses a mixture control needle to control the fuel mixture with a dwell signal generated by the ECM. This dwell signal is set between 25 to 35 degrees; the optimal setting is 30 degrees.
Repair and adjustment requires the use of very special fixtures to set the needle height and travel. That has a very profound effect on performance and throttle response.
Throttle body injection (TBI) units.
The next generation of fuel delivers to the engines. The main control sensor is the oxygen (O-2) sensor, which is located in the exhaust pipe. The sensor tells the ECM the amount of oxygen in the exhaust gas, and the ECM changes the air/fuel ratio to the engine by controlling the fuel injector. A 14.7:1 air/fuel ratio is required for efficient catalytic convertor operation. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel system is called a “Closed Loop” system.
Modes of operation:
The ECM monitors voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions called “modes” All the modes are controlled by the ECM.
When the key is first turned “ON” the ECM turns on the fuel pump relay for two seconds and the fuel pump builds up pressure to the TBI unit. The ECM checks the coolant temperature sensor, throttles position sensor (TPS), manifold absolute pressure map sensor and crank signal, and then determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36 degrees C (-33F) to 14.7:1 at 94 degrees C(201F) running temperature.
The ECM controls the amount of fuel delivered in the starting mode by changing how long the injector is turned “On and Off” This is done by “pulsing” the injector for very short times.
Clear Flood Mode:
If the engine floods, clear it by pushing the accelerator pedal down all the way. The ECM then pulses the injector at a 20:1 air/fuel mixture ratio and holds this injector rate as long as the throttle stays open, and the engine is below 600 RPM. If the throttle position becomes less than 80%, the ECM returns to the starting mode.
The run mode has two conditions called “open loop” and “closed loop”
When the engine is first started and it is above 400 rpm, the system goes into “Open Loop”.) The ECM ignores the signal from the 0-2 sensor and calculates the air/fuel ratio based on the from the coolant temperature and MAP sensors.
The system stays in “Open Loop” until the fallowing conditions are met:
1. The 0-2 sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature.)
2. The coolant temperature is above is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.
The 7.4 L engine is designed to operate “Open Loop” at idle. The system will go to “Closed Loop” when the rpm is increased and all the above conditions are met.
The specific values for the above conditions vary with different engines and are stored in the programmable read only memory (PROM). When these conditions are met, the systems go into “Closed Loop” operation. In “Closed Loop” the ECM calculates the air/fuel ratio (injector time on) based on the signal from the 0-2 sensor. This allows the air/fuel ratio to stay very close to 14.7:1.
The ECM looks at rapid changes in throttle position and manifold pressure and provides extra fuel.
When deceleration occurs, the fuel remaining in the intake manifold can cause excessive emissions and backfiring. Again, the ECM looks at changes in throttle position and manifold pressure and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel for short periods.
Battery Voltage Correction Mode:
When the battery voltage is low the ECM can compensate for a week spark delivered by the distributor by:
1. Increasing injector on time of fuel delivery.
2. Increasing the idle rpm.
3. Increasing ignition dwell time.
Fuel Cutoff Mode:
No fuel is delivered by the injectors when the ignition is “OFF”. This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running. Fuel cutoff also occurs at high rpm to protect internal engine components from damage.
Direct fuel injection systems work in much the same way.
FUEL CONTROL OPERATION:
The fuel control system consists of the fallowing components.
1. Throttle body injector (TBI) unit.
2. Fuel pump in tank (no engine pump).
3. Fuel pump relay.
4. Fuel tank
5. Accelerator control
6. Fuel lines.
7. Fuel filters.
8. Evaporation emission control system.
The injector fuel control systems have a electric fuel pump, located in the fuel tank on the gauge sending unit. It pumps fuel to the throttle body and the direct fuel injector system through the fuel supply line and then through an in-line fuel filter. The pump is designed to provide pressurized fuel at 125 kPa (18psi.)
A pressure regulator in the TBI keeps fuel available to the injectors at a constant pressure between 62 and 90 kpa. (9 to 13psi.) Fuel in excess of injector needs is returned to the fuel tank by a separate line.
The ECM controls the injectors that are located in the fuel meter body assembly of the TBI. The injectors deliver the fuel in one of several modes.
In order to properly control the fuel supply, the fuel pump is operated by the ECM through the fuel pump relay and oil pressure switch.
Model 220 TBI unit:
A model 220 consists of three major casting assemblies:
1. Fuel meter cover with: Pressure regulator.
2. Fuel meter body with: Fuel injectors.
3. Throttle body with: Idle air control valve (IAC) and the Throttle position sensor. (TPS)
The throttle portion of both the TBI units may contain ports located above, or below the throttle valve. These ports generate the vacuum signals for the exhaust gas recirculation (EGR) valve, MAP sensor and the canister purger purge system.
The fuel injectors are solenoid-operated device, controlled by the ECM. The ECM turns on solenoid, which lifts a normally closed ball valve off a seat. Fuel under pressure is injected in a conical spray pattern at the walls of the throttle body bore above the throttle valve, or into the manifold or engine cylinder.
The fuel which is not used by the injectors passes through the pressure regulator before being returned to the fuel tank.
The pressure regulator is a Diaphragm operated relief valve with injector fuel pressure on one side and air cleaner pressure on the other. The function of the regulator is to maintain a constant pressure at the injectors at all times, by controlling the flow in the return line (by means of a calibrated bypass)
The pressure regulator on a TBI 220 unit is serviced as a part of the meter cover and should not be disassembled.
Idle Air Control System:
All engine idle speeds are controlled by the ECM through the idle air control (IAC) valve mounted on the throttle body. The ECM sends voltage pulses to the IEC motor windings causing the IAC motor shaft to move in or out a given distance a given step for each pulse often referred to as counts.
This movement controls airflow around the throttle plate which in turn controls engine idle speed, either hot or cold. The IAC position counts can be seen using a scan tool. 0 counts correspond to a fully closed passage and 140 counts or more corresponds to full flow. This can change depending on the application.
Actual or controlled idle speed is obtained by the ECM positioning of the IAC valve pintle. The resulting idle speed is generated from the total idle air flow passage + PVC+ throttle valve + vacuum leaks.
Controlled speed is always specified for normal operating conditions. Normal operating condition is coolant temperature in operating range, the A/C is off, automatic in drive with proper Park/Neutral switch adjustment. A high or low coolant temperature, an A/C clutch engaged may signal the ECM to change the IAC counts.
The minimum idle air rate is set at the factory with a stop screw. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts) from the seat during normal controlled idle operation. The IAC counts will be higher than normal on an engine operating at high altitude or an engine with an accessory load such as the alternator, A/C, power steering or hydra-boost brakes activated.
Throttle Position Sensor: (TPS)
The throttle position sensor (TPS) is mounted on the side of the throttle body opposite the throttle lever assembly. It's function is to sense the current throttle valve position and relay that information to the ECM. Throttle position information allows the ECM to generate the required injection control signals.
If the TPS senses a wide open throttle a voltage signal indicating this condition is sent to the ECM. The ECM then increases the injector base pulse width, permitting increased fuel flow.
As the throttle valve rotates in response to movement of the accelerator pedal the throttle shaft transfers this rotation movement to the TPS. A potentiometer within the TPS assembly changes its resistance (and voltage drop) in proportion to throttle movement.
By applying a reference voltage of 5 volts to the TPS input, a varying voltage is applied to the TPS output. Approximately 2.5 volt results to a 50% throttle opening (depending on TPS Calibration).
Fuel Pump Circuit:
The fuel pump is a turbine type (1994), low pressure electric pump, mounted in the fuel tank. Fuel is pumped at a positive pressure ( 83kpa or 12 psi) from the through the in line filter to the pressure regulator in the TBI unit. Excess fuel is returned to the fuel tank through the fuel return line. 1994 and 95 years use a roller vain type pump rated at 210kpa (30psi).
The fuel pump is attached to the fuel gauge sender assembly. A fuel strainer is attached to the fuel pump inlet line and prevents dirt particles from entering the fuel line and tends to separate water from the fuel.
Vapor lock problems are reduced when using an electric pump because the fuel is pushed from the tank under pressure rather than being pulled under vacuum, a condition that produces vapor.
When the key is first turned on without the engine running, the ECM turns a fuel pump relay on for two seconds. This builds up fuel pressure quickly. If the engine is not started within two seconds, the ECM shuts off the fuel pump and waits for the engine to start. As soon as the engine cranks the ECM turns the relay on and runs the fuel pump.
On the 5.7 G van and all other 5.7 or 7.4 engines in vehicles over 8500 GVW, a fuel module will override the ECM and the fuel pump will for approximately twenty seconds. The fuel module corrects for a hot restart (Vapor Lock) during a high ambient condition.
When the engine is cranking or running the ECM receives distributor reference pulses which in turn energizes the fuel injectors. As a backup system to the fuel pump relay, the fuel pump can be turned on by an oil pressure switch. When the oil pressure reaches about 28kpa (4psi) through the cranking and the fuel pump relay does not complete the circuit, the oil pressure switch will close to complete the circuit to run the fuel pump.
An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn on the fuel pump as soon as oil pressure reaches about 28kpa (4psi).
Fuel Filter Part Number:
The fuel filter number is 25055052, AC GF 481 in line filter, this filter is used to remove dirt from entering the Injector units. Located inside the Right frame rail and is a threaded style filter. This filter is used on the in tank fuel pump systems, where the fuel supply lines are under pressure.
A woven plastic filter is located on the lower end of the fuel pickup tube in the fuel tank. The filter prevents dirt from entering the fuel line and also stops water unless the filter becomes completely submerged in water. This filter is self-cleaning and normally requires no maintenance. Fuel stoppage at this point indicates that the fuel tank contains an abnormal of sediment or water; the tank should be thoroughly cleaned.
High Altitude and temperature Vapor Lock:
Results of Vapor Lock Investigation
During the week of October 17, 1983, Chevrolet invited
all Class A Motor Home manufacturers to the GM Proving
Grounds in Phoenix, Arizona. The purpose of this meeting
was to discuss the findings of the investigation into the
cause. of vapor lock and to make specific corrective recommendations
for current and future production vehicles
(including changes in truck emissions) . The meeting was
attended by 34 of 36 RV manufacturers . Chevrolet suggested
that the RV manufacturers incorporate the following
recommendations into current and future vehicle
building, as well as develop some adaptation for problem
units already existing in the field. The following recommendations
were presented :
A one-half inch steel fuel line mounted on the outside
of the frame rail, protected against rub and
" Also recommended is a pump bypass line along
with a check valve. The 12-801 is a positive displacement
pump and will not allow fuel flow if it stops
running . The bypass, which closes under fuel
pressure, will allow the engine mounted mechanical
pump to pull fuel from the tank in the event of an
electric pump failure.
" The fuel requirements for the 454 engine at wideopen
throttle are 25 gallons per hour at 2 PSI
minimum and 3 PSI maximum. If fuel line lengths or
routings create a situation where this cannot be
met, an electric pump should be added at the fuel
tank to supply fuel to the mechanical pump on the
engine. Pressure in the supply line will further
reduce the chance of bubbles forming versus a
negative pressure situation with a mechanical