If you have been in this profession long, one thing is certain, you have likely gotten fooled a time or two when it comes to what is wrong with the vehicle you are trying to diagnose.
Diagnosis is a tricky thing. Sometimes it seems like the problem is right there in front of you, and sometimes you really have to work at finding the actual issue.
About a month ago, a local shop contacted me about a problem child, a 2004 Silverado 3500 equipped with a 6.0L gas engine. Like most things that are a problem, this one was intermittently setting DTC’s and the SES light, which led the customer to bring it to the shop. In addition, sometimes, when there were no DTC’s set, the engine and transmission simply did not perform as they should, while other times, the truck ran like a brand-new vehicle.
The truck would randomly set a P0101 (MAF sensor performance), and sometimes bank 1 and bank 2 lean DTC’s when things really acted up. But most of the time, the problem was not bad enough to set the P0101 or the lean codes. The shop had spent a lot of time and effort on this thing; cleaning the MAF, a new MAF sensor, a new ECM, new air cleaner, wiring bypassed, power and grounds checked, and inspecting the intake to MAF air connection. The GEN 3 small blocks have some inherent issues which could result in the problem, so we developed a strategy that we were going to follow to see if the problem could be isolated.
Looking at the scan data (even when the truck set a P0101) I could not see anything that popped out with the MAF sensor that made me suspicious. We felt that we needed to look elsewhere as the MAF and its circuits were not likely the guilty parties. On this application, to set a P0101, the following needs to happen:
- TP/APP, Map, Crank sensor, EGR, and Canister Purge DTC’s are not present
- TP angle is less than 95%
- TP angle change is less than 5%
- Map sensor value is greater than 17 kPa
- The change in Map is less than 3 kPa
- The calculated MAF airflow value is outside of the software calculation table for more than 40 of the last 200 samples. (The MAF reading and MAF calculation are performed during the same cylinder event every 100 ms.)
This is where I always start if I have a DTC setting, especially if it is intermittent, as was our case. Knowing what the computer is looking for will give you some major clues regarding where you need to look. So, what could be causing the DTC’s? Let’s pretend that no work had been performed on the truck. Even if you know the person working on the vehicle, it never hurts to start from. So, what areas should you look at:
- MAF sensor faulty (Dirty)
- MAF power or ground faulty (High resistance will set the DTC)
- MAF signal line issues
- MAF 5 Volt reference issues (High resistance will set the DTC)
- MAF wiring routed too close to the alternator or ignition wires
- Minimum air rate too low, although this was an electronic throttle control model
- Water intrusion into the MAF
- MAF air leaks
- Intake manifold air leak (very common on these engines)
- MAP sensor or circuit faulty (High resistance in the MAP sensor circuits will set the DTC)
We went about rechecking some of the items the shop had already checked, MAF/MAP reference, ground, and signal circuits. We ran voltage drop tests across the ignition feed circuits and an ohmmeter test of the sensor grounds. We graphed the MAF sensor frequency from idle to WOT (1000 HZ to about 10,000 HZ typically). We inspected the intake manifold for a vacuum leak. We inspected the MAF connections for leakage issues and the sensor circuits for proper wire routing. That left us with the MAP sensor. When we checked the MAP sensor for being skewed, we hit the jackpot. At certain engine loads, the MAP would not readily respond to the change in the manifold pressure.
At this point, you are probably wondering how a bad MAP sensor could cause a MAF sensor DTC? The answer is quite simple. As we discussed above, the P0101 is set based on the actual MAF reading varying from what the calculated MAF reading should be at a particular load. Well, the load is calculated using the MAP sensor values. Based on the MAP value, the ECM selects which MAF frequency table to use for the diagnostic at that particular load. This means the ECM has now selected the wrong table to calculate the MAF operation because the MAP sensor had a problem. Of course, the ECM does not know it has selected the wrong information; it simply knows the MAF value does not match what it thinks it should be, leading the ECM to set the MAF DTC (Figure 1).
The MAF DTC is not the only diagnostic to use other sensor inputs to determine if the component is operating correctly. There are lots of sensors that share their values for diagnostic purposes in today’s vehicles. What is the key then to diagnosis? Patience, an understanding of how the circuit/components operate, and a clear understanding of what the controller is looking at to set the DTC in question. If you are faced with a challenge like the problem with this truck, you now have the tools to be successful. Until next time remember, “The secret to success is to do common things uncommonly well.”