Page 16 - GMC Vintage RVing - Winter 2018

Technical Corner MAINTENANCE, PARTS, TIPS, INNOVATIONS AND MOREToday’s Self-learning Electronic Fuel Injection (EFI) SystemsJerry Work | former owner 1977 Clasco and 1978 Royale | Kerby, OregonEFI has become a hot topic among GMC owners in recent years, and for good reason. While the fuels available 40 years ago were well suited to atomize in the vacuum, springs, and Venturi world of carburetors, today’s fuels are formulated to atomize properly only while being squirted through solenoid operated fuel injectors via high pressure fuel pumps. Quite a different environment!Today’s fuel will run in yesterday’s carburetor, just not as well. Common problems are a significant tendency to vapor lock, formation of sludge and varnish while sitting between uses, and less reliable maintenance of the proper air fuel ratio while under load. While the first two are irritants, and certainly can land you on the side of the road, if the air fuel ratio moves too far out of a fairly narrow range required for engine longevity you can find yourself on the side of the road with an internally damaged engine.Most of us have a basic understanding of the notion of a stoichiometric air/fuel ratio being that ratio of the amount of air to the amount of fuel that will result in full combustion of the fuel when ignited. Different kinds of fuels require different amounts of air to fully combust.For unadulterated gasoline that ratio is 14.7 to 1. For ethanol it is9 to 1. Mix 85 percent ethanol to 15 percent gasoline (so called E85) and you get 9.87 to 1. So, the actual stoichiometric value is dependent on what all is added to or taken away from the gasoline.Forty years ago when our GMCs were designed, gasoline was mixed with octane and volatile organics to produce more performance.The carburetor of that day also had to be jetted for the altitudewhere the vehicle would be driven most. If the carburetor was jetted for use at low altitudes where the oxygen content of the atmosphere was higher, then when the vehicle was driven to altitudes over 5000-6000 feet where the oxygen content of the atmosphere is lower, then the resulting air/fuel ratio would become too “rich” to burn fully in one ignition event. The result was black smoke (unburned gasoline) from the exhaust pipe and a low performance, almostfluffy feeling in the vehicle.If the carburetor was jetted for use at higher altitudes then when driven at lower altitudes the air fuel ratio would become too “lean” resulting in hotter operation. Long term exposure to these lean burn conditions could result in internal engine damage like burnedvalves or pistons requiring a major overhaul to correct.Today’s “gasoline” has far less of the octane additive and instead alcohol in the form of ethanol has been added at rates of up to10 percent to produce a fuel that has a much lower air/fuel ratio to achieve complete combustion (more of this fuel has to be added toa given amount of air). Ethanol has a much lower stoichiometric air/fuel ratio than unadulterated gasoline so it takes more of it to make the same amount of power/performance in our old engine/ignition designs as it would running yesterday’s fuel. This change was made to “oxygenate” the fuel allowing the engine to produce less in the wayof unwanted exhaust emissions and to allow vehicles with a catalytic converter to reduce emissions still further. The trade off was greater fuel consumption so the engines had to be redesigned to regain the lost fuel economy while still achieving the lower exhaust emissions. Our old GMC motorhome engines can’t do that running this fuel through a carburetor so performance, mileage and emissions all three suffer.Today’s fuel also is formulated to be much less volatile than yesterday’s fuel. That means that it will not vaporize as easily when sitting in an open container or during fill ups at the pumps, nor will it atomizewell simply being drawn into the air stream through the Venturi inthe carburetor. This combination of lower octane content and lower volatility along with the addition of alcohol to “oxygenate” the gasoline has drastically changed the nature of the fuel itself and renders itfar less suitable for use with a carburetor. It simply will not atomize properly, will vapor lock far more easily while being drawn the long distance from the tanks in our GMCs to the mechanical fuel pump and on to the carburetor and this fuel makes altitude changes even more problematic. None of which sounds like a very good situation, does it?Changing from a carburetor to an electronic fuel injection system for which today’s fuel is formulated will dramatically alter the drivability of most GMCs. The tighter the engine, the more change you will notice. The one thing you will not find is very much difference in gas mileage. It simply takes a certain amount of fuel to move our GMCs through the air at speed. The higher the speed, the more the air resistance. So, if you want better mileage, slow down! As a rough rule of thumb, for every five miles an hour you slow down on average you will see something around a half of a mile per gallon gain.Since most all of today’s add on fuel injection systems operate ona similar principle, the brand doesn’t make nearly as much of a difference as it did just a few years ago. All of the newer offerings from the various manufacturers employ self-learning techniques that makes them plug-and-play in our GMCs. Also, most now employ the latest electronics which are far more resistant to heat and vibration than the older units they have replaced. In the older EFI units the electronics had to be sequestered away at the end of a long multi-strand wire stuffed up under a drivers or passenger seat away from the heat and vibration found in the engine compartment. Now those electronic components can be mounted inside the throttle body itself.Another major change in these newer self-learning EFI units is they have far fewer external sensors to mount and wire. Most now have only two external sensors – a water temperature sender and a sensor to measure the relative amount of oxygen left in the exhaust stream, a measure of how complete the combustion was during the last ignition event.In actual practice these new self-learning EFI systems do all the hard work for you without the need for your intervention. They do this by estimating the amount of oxygen in the air (the air density),16 GMC VINTAGE RVING MAGAZINE | GMCMI.COM