Air/Fuel Metering

The computer can only fuel the air it knows about. MAF and MAP sensors measure it; the throttle body admits it; gaskets and hoses are supposed to contain it. Air that sneaks in uncounted leans everything out — and air the MAF over- or under-reports sends fueling wrong at every rpm. Most "mystery lean" tickets end here.

What the symptoms mean

The diagnostic order

Work top to bottom — the order is the point. Each step rules out the cheapest, most common causes before anything expensive gets blamed.

1. Fuel trims at idle vs 2500 rpm. Leak-type leans improve with rpm (leak becomes a smaller share of airflow); MAF-type leans often get worse.
2. MAF reality check: grams/second at idle roughly equals engine liters (3.5L ≈ 3.5 g/s), and a WOT pull should hit roughly 0.8 × horsepower in g/s. Way short = dirty or dying MAF.
3. Smoke the intake, and wiggle the intake boot with the engine running — cracks hide in the folds and only open under movement.
4. MAF cleaner (never carb cleaner) on the element, then retest before replacing anything.
5. Throttle body: inspect and clean the blade edges; relearn idle where the platform requires it.

Tools that matter here

The classic mistakes

• Cleaning the MAF with the wrong chemical and finishing off a sensor that was savable.
• Skipping the throttle relearn after cleaning and returning a car that idles at 1500 rpm.
• Replacing a MAF for a lean condition the smoke machine would have found in five minutes.

Air/Fuel Metering trouble codes (44)