![]() The 7.4 L engine is designed to operate “Open Loop” at idle. A specific amount of time has elapsed after starting the engine. The coolant temperature is above is above a specified temperature. The 0-2 sensor has a varying voltage output, showing that it is hot enough to operate properly. ![]() The system stays in “Open Loop” until the fallowing conditions are met: 1. Run Mode: The run mode has two conditions called “open loop” and “closed loop” Open 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. If the throttle position becomes less than 80%, the ECM returns to the starting mode. 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. Clear Flood Mode: If the engine floods, clear it by pushing the accelerator pedal down all the way. 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. This ranges from 1.5:1 at -36 degrees C (-33F) to 14.7:1 at 94 degrees C(201F) running temperature. 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. ![]() Starting Mode: 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 fuel is delivered under several conditions called “modes” All the modes are controlled by the ECM. Modes of operation: The ECM monitors voltages from several sensors to determine how much fuel to give the engine. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel system is called a “Closed Loop” system. A 14.7:1 air/fuel ratio is required for efficient catalytic convertor operation. 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. The main control sensor is the oxygen (O-2) sensor, which is located in the exhaust pipe. The next generation of fuel delivers to the engines. ![]() That has a very profound effect on performance and throttle response. Repair and adjustment requires the use of very special fixtures to set the needle height and travel. This dwell signal is set between 25 to 35 degrees the optimal setting is 30 degrees. This style carburetor uses a mixture control needle to control the fuel mixture with a dwell signal generated by the ECM. This carburetor featured tamper-resistant controls to discourage readjustment of factory settings, which could affect either or both emission control and throttle response. 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. It is not even close to the ECM used today. This is an aspirated fuel delivery system with the addition of a mixture control system controlled by the ECM. A M4MC or EM4MC carburetor was used on most of the 1980 series engines. Ignition system information is covered in the electrical thread. Covering fuel delivery, performance and starting in regards to the fuel system operation. 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. Rich.Įdited 2-8-2013 Vapor lock problems added at the end of article. It is a little long and did not cover carburetor information. So it is good to have the Vin number when you find things that are a little different, when working on the systems. There are always some variables due to supplier and run changes. 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.
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