Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/660061
16 | April 2016 Failure in spec What happens when a mass airfow sensor lies By Jacques Gordon Jacques Gordon has worked in the auto- motive industry for 40 years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifcations and has participated in ASE test writing workshops. S ometimes the hardest part of chasing driveability problems is knowing where to start. It's tough enough when the trouble codes indicate something vague like "random misfre" 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. What do you look at frst? 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. Once you get snookered by some- thing as simple as a split vacuum hose or a loose ground wire, you'll start every diagno- sis with a quick visual/touch inspection. OK, there's nothing obvious, so now it's time for the scan tool and a test drive. 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. There's a hesitation and not much power off the line, so it certainly feels like it's not getting enough fuel. After a few minutes at steady cruise, the engine seems smoother but throttle response is still lack- ing and power is defnitely down, especially at wide-open throttle (WOT), almost like the engine has a governor. Both fuel trims are now positive, but the O2 sensor shows a lean air/fuel ratio. Powerplant Photo courtesy of Snap-on Solus This Acura engine has both a MAF and a manifold absolute pressure (MAP) sensor. Notice how closely they follow each other. If the MAF sensor stops working completely, the PCM will use the MAP sensor to operate as a speed/density engine management system.