Electronic Fuel Injection
The Answer to Modern Fuel in Our 25 Year-old GMC Motorhomes?
 by Bill Harvey

 December 2000


Twenty-five years or so ago, the Rochester Quadrajet Carburetor performed very adequately in moving our 6 tons of motor home and passengers down the highway and up hills at reasonable speeds. Now however, with pollution controls and reformulated gasoline mandated by these controls, we are forced to look at other means of deriving better fuel efficiency as well as performance.

Performance may be enhanced by changing the final drive to a higher gear ratio. This will move engine rpm up on the power curve. Rebuilding and recalibrating the Quadrajet may also restore performance, but may not provide much in the way of improved performance or mileage..

Is there another answer? More and more folks are looking at Electronic Fuel Injection.

It must be noted at the beginning that this may not be practical, particularly in areas with tight emission ("smogging") inspections. A possible solution will be discussed later. In the meantime, let's look at the basic principle of EFI as it may be applied to our GMCs.

EFI Benefits

The purpose of EFI is to match the amount of fuel burned to the amount of oxygen present under all operating conditions and reduce emissions. Maximum power is generated with an air/fuel ratio by weight of 12.5 to 1 while normal cruising the ration should be 15 or 16 to 1 for maximum fuel economy and the engine runs neither too rich or too lean. The following figure shows what the goal is and compares rich and lean burning.

Figure 1

The carburetor will work fine when set for operation at a particular atmospheric pressure. The problem with the carburetor is that it cannot be easily adjusted for different elevations. A carburetor calibrated for operation at sea level will not permit the engine to run as efficiently as possible at higher elevations as shown by the middle line above.

Unfortunately no carburetor or EFI system can run as efficiently at altitude as it can at sea level. If for example, I drive around my home at 6400' above sea level where atmospheric pressure is only 11.6 psi and the best that I can achieve is 79% efficiency. If I drive over Donner Summit at about 8400' the efficiency drops to 74% due to reduced combustion pressure. We won't talk about the high passes in Colorado such as Loveland, Monarch Passes or Trail Ridge Road at 12,000' where efficiency drops to 65%.

Running lean (bottom line above) also creates problems with knocking and overheating plus potential damage to the pistons, valves and exhaust manifolds.

In addition to less efficient operation with either a carburetor or EFI at altitude, the carbureted engine looses suffers a further loss of efficiency by not burning all the fuel that my lead foot provides. Mileage suffers accordingly and pollution increases.

The EFI system provides the compensation required for operation at different altitudes as well as reducing carbon monoxide .(CO) and hydrocarbon (HC) emissions.


EFI Operation

Since EFI attempts to achieve more efficient fuel combustion regardless of ambient atmospheric pressure, just how is this accomplished?

EFI controls the amount of fuel such that no more fuel is provided than usable oxygen is present. This amount of fuel is injected either through ports for each cylinder (Port Fuel Injection - PFI) or directly into the intake manifold (Throttle Body Injection - TBI). Most of our modern vehicles use PFI. However, unless you want to take the time and expense of replacing or modifying the intake manifold with a "rail" system for port injection, you will want to use a single throttle body injection (TBI) system.


TBI System Components
The following describes the functions of the essential parts of the TBI system:

Throttle Body Injector
Very simply the TBI replaces the carburetor and consists of one or more solenoid valve injectors which inject fuel when pulsed by a signal from the Electronic Control Module (ECM) a rate which assures maintenance of the correct air/fuel ratio. The TBI incorporates a Throttle Position Sensor to provide a signal to the ECM and a Start Solenoid Valve to provide extra fuel for starting.

Manifold Absolute Pressure Sensor
The MAP samples the air pressure within the manifold as a means of determining the volume and flow of air. Manifold air pressure varies with engine load just as manifold vacuum varies with engine load. In other words, MAP is a measure of manifold vacuum (really reduced air pressure). This information is sent to the ECM where it is used to help determine fuel requirements for the engine load, speed and altitude.

Electronic Coolant Temperature Sensor
The ECT functions like a choke in a carbureted system. Its output is used by the ECM to provide an enriched fuel mixture for starting and cold running.

Oxygen Sensor
The oxygen sensor, mounted in the exhaust, is used in "Closed Loop" operation to determine that combustion is complete. This is another parameter utilized by the ECM. It works in conjunction with the MAP to compensate for altitude and/or barometric pressure. An "Open Loop" system does not use an oxygen sensor and fuel injection is based solely on the MAP and distributor information.

Fuel Pump, Filter, Regulator and Line
A higher output pressure fuel pump is necessary to provide the injection pressure necessary for proper operation. A pressure regulator maintains the correct injector pressure. A special fuel line filter should be incorporated in the fuel delivery line as well.

Electronic Control Module
The ECM is the computer brains of the EFI system. It uses a programmable-read-only-memory (PROM), read-only-memory (ROM) and random-access-memory (RAM) to accomplish a variety of functions. The PROM is the so called "computer chip" which provides the specific information for the engine. The ROM has the general information required for system operation. The RAM provides a "scratch pad" for ECM calculations, "learning" capability and diagnostic information.

Wiring Harness is used to connect everything together.

While not an actual part of the EFI system the distributor provides engine rpm information to the ECM. Full distributor control may be provided by the ECM with some systems as well. The HEI system that most of our Motorhomes use works well with the EFI system but does not permit timing adjustment.

Figure 2
Components of a typical after-market kit.


EFI Operating Modes

Start Up
Timing is locked in and the air-fuel mixture is slightly enriched by the Start Solenoid Valve.

Open Loop
Even if the system is using an oxygen sensor, the system will run open loop with a slightly enriched mixture until the ECT senses the proper operating temperature. Then if an oxygen sensor is present the system will go to closed loop.

Closed Loop
In the closed loop mode, the oxygen sensor provides the necessary signal to the ECM to maintain the air-fuel ratio at 14.7 to 1.

In the enrichment mode the ECM will ignore the oxygen sensor even if running in closed loop. This may occur with: 1) a wide open throttle to provide more power; 2) high engine load; 3) high engine rpm where the air-fuel mixture tends to lean out; or 4) engine over heating.

When the engine is operating under little or no load, the ECM will lean out the air-fuel mixture. This provides both fuel economy and reduced carbon monoxide (CO) and hydrocarbon (HC) emissions.

Fuel Cutoff
This occurs when the throttle is closed and the MAP indicates a negative load as in heavy deceleration. The ECM minimizes fuel at the injectors resulting in reduced CO and HC emissions.

Clear Flood
This works just like the old technique of opening the throttle wide while cranking the engine to clear flooded fuel. The ECM can sense this condition and shut down the injectors completely or reduce the air/fuel mixture.

Run Mode
This is where the ECM senses a number of different signals and analyzes them to adjust both the fuel injection period and the spark timing. The fuel injection period is based upon the position of the throttle, signals from the MAP and the Oxygen Sensor when running in the closed loop mode. Figure 3 shows normally running wherein oxygen, MAP, coolant temperature and engine rpm are monitored and injection pulses are then sent to the TBI..

Figure 3

Figure 4 is a schematic showing how electronic spark timing is controlled by monitored advances of rpm, coolant temperature and manifold pressure (reduction) less any imbalanced mechanical advance to result in total spark advance to the distributor.


Figure 4

Electronic Spark Timing (EST)/Electronic Spark Control (ESC)
Modern vehicles have a distributor that can respond to ECM control such that spark timing may be adjusted based upon the output of the MAP. When the MAP senses an increase in manifold vacuum at high rpm, the ECM electronically advances the spark. Similarly a reduction of manifold vacuum under heavy load causes the ECM to retard the spark. In this situation the MAP and ECM work just like a standard distributor.

Since our distributors do not have the ability to respond to ESC, we can use something like the manual MSD spark adjustment and the MSD knock sensor. (MSD devices are used to permit electronic spark advance and retard as a means of providing appropriate timing for engine loads, altitude and similar conditions where it is desirable to be able to adjust the timing without manually rotating the distributor.)

Note: Most of the foregoing information was taken from Chilton's Guides.


TBI for GMC Motorhomes

Several manufacturers are now providing after-market TBI kits for our GMCs. Most of these are based upon the Holley 2bbl 670 cfm TBI unit and readily available GM parts.

(In case you are wondering, while our engines have 4 barrels, the two rear barrels are blocked off as unneeded to maintain the necessary air flow with acceleration.)

These systems vary in fuel pump output, open loop or closed loop operation, specialized computer "chip" and, of course, price.

The following, in no particular order, summarizes the features of each EFI system that appears to be suitable for our use:
Please note that either an Internet address or phone number or both is provided.

Howell Engine Developments, Inc.
Holley 670 cfm or GM 454 2 bbl TBI.
· GM ECM used in 1987-92 trucks.
· New ECM wiring harness.
· GM diagnostic connector and "check engine" warning light wiring.
· Calibrated PROM exclusively for 455 or 403 engines.
· Airtex in-line high pressure (12-15 psi) fuel pump and filter.
· GM single wire Oxygen Sensor (3 wire heated sensor available for headers).
· GM coolant sensor.
· Parts kit.
· Price: $1200

· Holley 670 cfm 2 bbl TBI.
· User adjustable Digital Electronic Control Unit.
· New wiring harness.
· 12-22 psi Adjustable Fuel Pressure Regulator.
· In-line 300 pph Fuel Pump.
· Adjustable Fast Idle Solenoid.
· Price: $879.99.
· Closed Loop Kit extra.
· Specific PROM info from Hypertech Inc. at 1-800-532-3351

1-714-639-4933 or www.turbocity.com
· Similar to the above with options available for customized engines including specialized PROMs.
· Prices from $1295.
Note: TurboCity provides and uses the ESC feature of the ECM and a GM throttle body (TB). The Holley TB is 40%larger than the GM unit which may not make much difference to our engines.

Affordable Fuel Injection
· Full function, closed loop TBI system includes:
· New 2 bbl TBI unit.
· Wiring harness.
· ECT, MAP, Oxygen sensor
· Fuel Pump and Filter.
· Fuel Pump Relay
· ECM with AFI specially calibrated chip.
· Price $995
· Open loop system price $945
· Remanufactured closed loop system price $845
· Remanufactured open loop system price $775

It should be noted in comparing these systems that it is very important to determine if the available PROM is suitable for either the GMC 455 or 403 and Motorhome use. A correct PROM will consider, compression ratio, cam profile, transmission type, vehicle weight, etc. In all probability a PROM from a junk yard may not work as well as you might like.



Most of the foregoing information was derived over the Internet and I assume no responsibility for the accuracy or the prices noted. I have tried to analyze the various materials available and condense this information into a relatively understandable form. However, it is strongly suggested that anyone contemplating installing an EFI system conduct similar research and decide for themselves just what the expect from the conversion to EFI and proceed accordingly.

Most of the kit suppliers note that it will take a reasonably competent mechanic a full day to install the system including making some modifications to the stock air cleaner and routing the wiring harness as well as doing the strictly mechanical job of removing the carburetor and installing the TBI.


Further Disclaimer

As noted at the beginning, California and certain other states do not permit EFI on our coaches later than 1973 or 1975. If you license in one of these states and choose to install EFI, you may be performing AN ILLEGAL ACT and be SUBJECT TO A FINE!
You must be prepared when it comes time to "smog". This may include reinstalling the old carburetor.