It’s tempting to think these symptoms indicate a vacuum leak or low fuel pressure, but this is an article about mass air flow sensors (MAF), so at this point you already know that’s the problem to be considered. Both fuel trims are now positive, but the O2 sensor shows a lean air/fuel ratio. After a few minutes at steady cruise, the engine seems smoother but throttle response is still lacking and power is definitely down, especially at wide-open throttle (WOT), almost like the engine has a governor. There’s a hesitation and not much power off the line, so it certainly feels like it’s not getting enough fuel. The idle is a bit rough long-term fuel trim is way negative but short-term is making up for it and the oxygen sensor trace looks normal. OK, there’s nothing obvious, so now it’s time for the scan tool and a test drive.
Once you get snookered by something as simple as a split vacuum hose or a loose ground wire, you’ll start every diagnosis with a quick visual/touch inspection. Most techs will grab a scan tool right away, but some of us older guys are more likely to start by looking under the hood for something obvious, like a broken wire or severe neglect or (aha!) signs of a recent repair. It’s tough enough when the trouble codes indicate something vague like “random misfire” or “system too lean,” but it can be even worse when there are no codes at all, with only a driver’s complaint about stalling or sluggish performance. Sometimes the hardest part of chasing driveability problems is knowing where to start. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
His began his writing career writing service manuals at Chilton Book Co. Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer.
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