A 2000 Nissan Maxima came in with a complaint of a slipping transmission, but it was obvious driving into the service bay that the customer’s best guess was nowhere close. The idle was so rough that I almost made an equally rash assumption that the 3.0L V6 needed the all-too-common ignition coil replacement. But a quick check of the stored codes revealed a P0171 & P0174 for a lean condition on both banks.
These codes are common for all manufacturers, and they are not component-specific. Could these codes be set by a coil failure? Yes, but Nissans reliably set actual coil codes for coil failures. Still, the list of possibilities is endless. At ATG, we try to choose the tests that return the most information with the least effort to not only fix the car correctly the first time, but to do it in a fraction of the time compared to a traditional flowchart diagnostic path.
The ATG approach asks what every one of the possible faults has in common, and for these codes the only possible answers are that the engine is breathing wrong or that it’s fueling wrong. A simple Scan Tool test drive can zero in on which of those two areas need more focus. No fuel pressure check? No inspection for vacuum leaks? Until the Scan Tool says one of those tests are needed, they’re just a waste of effort.
Can this engine breathe? Can it fuel? A simple Volumetric Efficiency (VE) test determines engine breathing as a percentage of engine displacement, and Fuel Trim determines how hard the closed loop engine management system is struggling to keep the mixture correct. Both of these topics are discussed in great depth in other ATG manuals, seminars and training videos, but for this case I’ll just give you the results:
The capture to the right shows the MAF sensor grams per second, engine speed, and intake air temperature (IAT) during full-throttle acceleration. The IAT and peak RPM and MAF values were entered into a VE calculator (Google “volumetric efficiency calculator” and download or use a free one online). In this case the VE was 26%! So this 3.0L engine was only flowing 0.78 Liters. This engine should have a VE over 90%, so that clearly explains why it’s running poorly. The ATG drivability strategy indicates that low VE should be compared to Fuel Trim to see if the breathing fault is real or fake. By that I mean that an engine breathing poorly would have a low MAF signal and deliver appropriately low fuel quantities. The end result would be a somewhat normal Fuel Trim for a ‘real’ breathing fault. But if the engine is breathing correctly and only thinks it’s not, then Fuel Trim would be very high. This can happen if the MAF has failed or there’s unmeasured air getting into the engine.
In the example to the right, the Short Term and Long Term Fuel Trim added together at high RPM was 57.8%, so this is clearly a ‘fake’ breathing fault. No disconnected or torn intake duct between the MAF and throttle plate? Then it needs a MAF sensor. It just does. Sure, unmeasured air can also occur with lower intake manifold leaks (not on these engines, though), or PCV faults, but neither of these faults would affect VE at full throttle when there’s no vacuum. Back to the MAF – get one!
The point of this case study isn’t that it was difficult. The point is that it was a 100% reliable diagnosis in 10 minutes, including a quick visual of the intake duct, and a check of the MAF for debris in the air path (no dead bugs here). Can you make money being that sure in 10 minutes? We think so. And this isn’t just about MAF sensors – it’s about asking the right questions up front to best categorize the fault, so that every test decision is more focused than the one before. Remember, time is your money.