Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/636156
56 | February 2016 My initial road test feeling was that this truck had a partially plugged exhaust. I returned to the shop, scanned it and, yes, it had a P0302. I looked at the freeze frame data and fuel trims. Fuel pressure and temps were all in the normal range. My initial suspicion was still a plugged exhaust, so I checked the back pressure by removing the pre-cat O2 sensors and it had a reading of less than 1/4 of a psi on both the left and right. That value for back pres- sure is well within spec, so that blew up my theory of a plugged exhaust. Next I hooked up my Ford scan tool and went to the power bal- ance display during the road test. It verifed that it was only cylinder Number 2 that was caus- ing the misfre. I went through all the basics, testing cylin- der compression both running and static, a cylinder leak down and all with normal results. We all know that a Ford misfre can be a bit of a bear to diagnose, due to the fact that the misfre monitor may initiate failure mode effects management (FMEM) to prevent catalyst damage. When a Ford enters FMEM, it will shut off the fuel injector to prevent raw fuel in the misfring cylinder form entering the catalytic conver- tor and overheating it. The FMEM will keep that injector shut off for 30 seconds on a 2009 model, and after those 30 sec- onds it looks to see if the misfre is still occurring. If it is, it repeats this cycle until the misfre stops. Knowing this I hooked up my small two-channel scope to the Number 2 injector and Number 5 injector on the opposite engine bank as a reference. Performing a road test quickly verifed that the engine was going into FMEM and shut- ting off the control circuit to the Number 2 fuel injector. The next step was to scope the cam and crank sensors to see if there were any anomalies that could cause a miss, and the patterns showed me nothing out of the ordinary (I was grasping but didn't want to miss anything). I knew that the injec- tor was OK; it was new and had even been moved to another cylinder just to verify it The Tech Bench A misfre on a Ford is usually a pretty simple repair of either a spark plug or the more common ignition coil issue.