The Four Year MIL Case Study - 2011 Ford F250 - Intake Air Measurement
A high mileage but well cared for F-250 came in with a complaint of low power. The technician noticed that the MIL was on and asked the customer how long it had been on. The customer stated that it had been on for four years, although the loss of power was gradual and more recent. The IDS capture below shows four codes, with three of them also setting as Pending codes, meaning they failed recently (during the last trip).
Our favorite 6.7L breathing codes are P2073 and P2074, because they usually set for a clogged, missing, or sucked-in air filter. P1247 sets for low boost pressure, and P1548 sets for an air filter restriction. There’s no air filter restriction sensor, so the P1548 sets using similar logic to the P2073 and P2074, and is based primarily on the MAF sensor.
Based on experience and this combination of codes, the technician first checked the air filter. It looked clean, but was an aftermarket (K&N) filter and housing. This appeared to be the only modification on the vehicle, and seemed to be properly mounted and in good shape.
There are a number of good tests later for low boost, and a long list of possible causes could be built for this combination of codes. The technician could have spent hours verifying VGT and wastegate operation, checked for exhaust restrictions, and smoked the intake and ducting for leaks. However, it’s a lot easier to just ask the customer when the K&N filter was installed. “Four years ago.” The MIL has been on since the aftermarket filter was installed. Huh.
Anyone for a MAF Reset?
Given that the low boost has been progressive and more recent, it’s unlikely that resetting the MAF will fix the low boost codes. However, it’s silly to continue diagnosing the drivability complaint when it’s already clear that there are unresolved breathing codes. The capture below shows the reset function being performed using the IDS Scan Tool.
The truck was taken on a test drive after the MAF Reset (and codes were cleared). Surprisingly, not only did none of the codes reset, but the power was restored as well! Hours of testing were avoided. The Scan Tool revealed that only permanent codes remain.
These codes will clear themselves over time, so they really mean “hasn’t passed or failed yet.” In the end, there was nothing wrong with the K&N filter, the installation, the MAF or MAP sensors or the throttle plate, and there were no leaks or restrictions. In fact, the flow chart would never have repaired this vehicle. Only an understanding of the sensitive nature of the PCM’s MAF expectations could result in a fix. Any change in the intake system requires a MAF Reset, even if there was nothing wrong before or after the change.
Nice one Tim, and yes...MAF calibrations can directly affect boost issues.
Hello Tim thanks for the write up. I have a question about the MAF reset. Since this is a diesel, the IDS can't reset fuel trim because we don't have it. So would I be wrong if I assumed that the MAF reset has to do more with the software reaction to the sensor output? Like a crank sensor relearn. General idea is replacing the component warrants a reset of look up tables within the program, or put shortly a calibration takes place. I don't have much experience with the IDS or newer diesel trucks. I appreciate your post.
Hi Joe. That's about right. The 6.7L Powerstroke is very sensitive to MAF readings, but the expected value is a very complicated model that is hard for us to understand. Gasoline engines are air-controlled, so MAF is a very predictable value at idle and we can use it in VE calculations. Diesels are fuel-driven, and MAF isn't really even needed for fuel control - it's primarily for boost, EGR and aftertreatment systems. Based on turbo vane-induced backpressure and varying (and sometimes very high) EGR flow, the actual MAF value is all over the place. As such, it's hard to just look at the value and say "Yup, that's bad". The PCM keeps the expected value a secret, but it's pretty good about setting codes when actual MAF doesn't agree with the software model. Now that we have that out of the way, the bad news is that the PCM doesn't adapt to new values very well, so if the actual MAF changes (via an aftermarket filter in this case), the model will never match.
As for Fuel Trim, the 6.7L Powerstroke does have it (sort of) but it's adjusted per injector. You can see it in the Cylinder Balance PIDs, which are like short term adaptation. It's a closed loop, but instead of oxygen sensors, the feedback is crankshaft velocity. The PCM adjusts fuel volume up or down to match each cylinder's contribution. It compensates for varying fueling rates that change over time due to resistance, deposits, and wear. However, it also compensates for compression loss (to a point). The values can be cleared from the IDS and aftermarket tools. Even though it's a short term adaptation, the engine will be imbalanced after a repair until the values are cleared (even if you properly enter the new injector IQA flow code). I have attached a Snap-on capture showing the clearing selection from the Function Tests menu.
Wow, that was very helpful Tim. Thank you for taking the time and offer offering help. I very much appreciate your write up.
Thanks. We appreciate the partnership. If you didn't come to the classes, we couldn't afford to figure out how this stuff really works!
It doesn't apply to your 2011, but look what just came out today: