Dale Amy
October 1, 2003

Horse Sense:
Somewhat surprisingly, Pro-M says bad grounds are perhaps the most common culprit in diagnosing mass air meter problems. Not grounds within the meter itself, but poor vehicle grounds, such as failure to route a proper ground wire to the factory location when relocating a battery to the trunk. Residue buildup on the mass air meter’s thermistor and hot-wire element from oiled air filters is the number-two problem, causing lean conditions. Clean them carefully with a pipe cleaner or a Q-Tip.

To a great extent, we have the mass air meter to thank for the electronically fuel-injected Mustang's broad-minded adaptability when it comes to engine modifications. Were it not for Ford's 50-state adoption of mass air in 1989, the Mustang performance aftermarket-and indeed the readership of this magazine-might well be a lot smaller than the thriving and vibrant entity it is today. This is because, for high-performance street use, the mass air meter helps us enjoy a combination of power, economy, driveability, and emissions cleanliness that was once only a pipe dream.

The entire selling point of electronic fuel injection is its computer-controlled, unmatched ability to introduce the exact amount of fuel necessary for proper combustion, and to adjust this supply nearly instantaneously as driving conditions demand. Natu- rally, the amount of air headed into the engine at any given time is the main factor in determining how much fuel is required. Correct air/fuel ratios deliver combustion that is both powerful and clean, making everybody-from gearheads to Greenpeace-happy. The point is, an EFI system relies on having an accurate and real-time means of figuring out how much air is inbound to the combustion chambers.

It's sometimes easy to forget that fuel-injected Mustangs haven't always used a mass air meter to provide the EEC processor information on induc-tion airflow. From 1984 through 1988, that information was instead provided by speed-density calculations, a process that, although extremely accurate in stock appli-cations, did not have sufficient flexibility to adapt to the heavy-breathing modifications that have since become commonplace.

What's the Difference?
Before turning our complete attention to mass air meter systems, let's first be clear on how they differ from a speed-density configuration. With mass air, a metering device is placed smack in the induction inlet path, upstream of the throttle body. This meter measures airflow, while a speed-density system has no such meter and instead merely estimates airflow by monitoring various engine sensor inputs-including manifold absolute-pressure and temperature sensors, engine rpm, throttle-position and oxygen sensors, and coolant temperature-and comparing them to preprogrammed tables within the EEC processor.

Among other factors, the speed-density EEC's programmed fuel (and spark, and EGR) responses to these inputs were predicated on the known volumetric efficiency of a purely stock engine. Change that volumetric efficiency through significant modifications and the programmed assumptions would no longer be correct. It's this lack of latitude in that programming/sensor relationship that limits a factory speed-density system's ability to respond to major modifications such as free-breathing heads, a cam, or supercharging. Let's be clear, though, that these comments relate to a stock speed-density system, not the aftermarket digital fuel-injection systems specifically developed for extreme performance applications. But that's a discussion for another issue.

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