One of the more subtle aspects of electronic fuel injection is the relationship between electrical system voltage and fuel delivery. As electrical system voltage varies, so does the volume of fuel discharged through the injectors. In most cases, discrepancies between commanded and actual fuel delivery go by unnoticed because they’re often transient and even if they’re constant, the effects of voltage variation on fuel delivery are usually well within the self-correcting capabilities of the ECM or PCM.
Virtual electronic fuel injection control systems incorporate a table of values used to adjust fuel injector pulse width in response to changes in electrical system voltage. Consequently, the volume of fuel discharged through an engine’s injectors should remain consistent across the entire range of normal system voltage variation. But on occasion, fuel mixture will be too rich or too lean for no apparent reason.
One relatively common cause that’s not readily apparent is excess voltage variation. Even though the instrument panel gauge may indicate system voltage is normal, electrical power transmission losses (through the wires that bring power to specific components) may result in actual levels at the fuel pump, or injectors being substantially lower.
For a variety of reasons, insufficient voltage at a vehicle’s fuel pump has been a problem since the first use of fuel pumps powered by an electric motor as opposed to mechanical arm. This isn’t meant to imply that there’s an inherent problem with electric fuel pumps, only that their proper operation is predicated on operating voltage being adequate and consistent. That’s typically not a problem with young and “middle-aged” pumps and control systems. But as these components enter their “golden years” (aka old age) resistance in the wiring and connectors that supply electrical power can increase sufficiently to result in measurable voltage reductions. And even if the wiring is in excellent condition, it may not be sufficient to deliver the increased current demanded by a high-output fuel pump.
Low voltage-induced fuel supply problems aren’t necessarily an everyday occurrence, but Chuck Leeper of Cody Motorsports has noticed that they’re becoming more common, especially with owners of extensively modified vehicles. Chuck has been cleaning, flow testing and reconditioning electronic fuel injectors for more than 15 years. During that time, he has had a number of customers with “injector problems” that didn’t show up on his flow bench. According to Chuck, “The most common situation is that a customer will send me a set of injectors to clean and flow check. If they aren’t too nasty, I’ll usually test them first, and sometimes I find there’s nothing wrong with them. When I tell my customer his injectors are fine, he can’t believe it. Then I go through a trouble-shooting checklist with him, and that’s when we get to the real source of the problem. It usually turns out to be wiring or some other electrical problem that was dropping voltage at the fuel pump.”
Then there’s the question of the effect of system voltage variations on the injectors themselves. An electronic fuel injector is essentially a solenoid valve that opens when electrical current is applied, and closes when that current is removed. So it seems reasonable that system voltage levels would have an effect on injector operation.
Chuck couldn’t simulate vehicle-operating conditions with a standard flow bench, so he enlisted electronics specialist Wes Burch to put together a suitable test device. Wes came up with the electronics necessary to control the injectors with a GM LS1 PCM while using the flow bench fuel pump and hardware to pump test fluid through the injectors. He also built a fixture that enabled us to install the injectors in a standard LS engine fuel rail, thereby bringing the test configuration closer to a real-world vehicle fuel system.
Stay tuned for the testing portion of this tech special!
Text and Photos by Dave Emanuel