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July 2015 37 Using Critical Thinking When There Are No Codes 2003 Suzuki Vitara 2.0-liter By Duane Mariage our shop was to drive extremely slow with no acceleration. Once we were back at the shop, we turned the key off and back on to verify that the bulb check indicated that the check-engine bulb was good. We connected our scanner and were sur- prised to find that there was no current or pending codes. Since the powertrain control module (PCM) was not going to be much help, it was time to do some "critical thinking." On the test drive, we could du- plicate the driver's complaint, and our sense was that it was possibly a spark or fuel problem. We didn't want to guess, though, so we at- tached a vacuum gauge and found that at idle it was steady and nor- mal. Of course, when it was buck- ing and jerking, the needle would bounce around. Critical thinking is the process of analyzing the information gath- ered from observations and testing and letting it lead us to the prob- lem. It's like putting a jigsaw puz- zle together. We understood what the finished picture should look like; now all we had to do was put the pieces of the puzzle together and see if there were any missing pieces. We figured it was not a com- pression or mechanical problem because, at idle, it ran normally and the vacuum gauge was steady. We also remember watch- ing the rpm gauge when we were cranking the engine but it wouldn't start. A normal rpm nee- dle will display a reading of 200 or so when cranking and as soon as it starts the rpm will move to about 1,500 and then settle down in the F or diagnostic technicians, it's always nice when the PCM sets a code because it gives us a start- ing point. Yet what do we do when there are no codes but there is a clear and distinct problem with how the vehicle runs? Let's look at a 2003 Suzuki Vitara 2.0-liter (Figure 1) that won't hardly pull itself, but there are no codes. The customer says that it usual- ly starts OK when it's cold, but after driving only a few blocks, the engine bogs down and won't hard- ly run. The car was dropped off at our shop late in the day. We let it sit overnight so we could duplicate the problem from a cold start. The next morning, it was raining and about 50°F. It started, idled and ran normally at first. We drove a few blocks and then the problem was very clear. It began by buck- ing and jerking. When we acceler- ated, it simply did not want to do it. When we came to a stop, it would idle down and run OK until we accelerated again. A few mo- ments later, the engine stalled with us in the middle of the road. We pulled over into the turn lane and attempted to start it for several minutes. It seemed to crank OK, but it coughed and sputtered and would not actually start. Eventually, it did start and idled just fine. But as soon as we acceler- ated it would buck and jerk. The only way we could get it back to 700-800 range. Our needle would bounce around erratically between zero and 2,000 (Figure 2). Understanding that during cranking the starter is the only force turning the engine, it could never turn it at 2,000 rpm. The starter alone just doesn't have that kind of power. So, there was some- thing else causing the gauge to bound up to 2,000. What could that be? Reviewing the service manual, we found that the crank sensor and the cam sensor are inputs that drive the rpm gauge. The crank sensor is responsible mostly for misfire identification. It is the cam sensor that controls the ignition. So, when we saw the rpm gauge bouncing erratically, was one or both of those sensors acting the same way? The quickest way to find out is to use the live scan data. We accessed the data and limited our performance information data (PID) (Figure 3) selection to "en- Figure 1 Figure 2 Figure 3 continues next page