Had a fun one today. An '02 2WD Toyota Highlander with a 2AZ-FE 2.7l 4 cylinder, with an automatic was giving us some trouble with diagnosis. This vehicle came in with a blown head gasket. And I mean blown. I'd never seen one where it was blown so well gases were coming out the side of the block where the head and block surfaces met. All the threads for the head bolts had been pulled out. A common failure on this motor, but I'd never seen one this bad. Usually we can just re-tap the threads and all is good. The head went out to the machinist who confirmed this engine was going to be un-repairable. A Reman. engine is installed. We find two of the ignition coils had been damaged in the overheat, and so replace all four coils. The vehicle is now running very well in the stall, and on the road. Only problem we now have is a MIL illuminated with a code P0340 - Issue with Camshaft position sensor. We decide to just put a new Cam sensor in which sits at the back of the head figuring it would likely have been just as easily damaged as the Ignition coils. No dice. Still an MIL. We then went to Identifix to see what was common as a failure with this code, and found that more often the problem was a bad crank sensor and not the cam sensor. The Crank Sensor was replaced with the same idea as when we had changed the Cam sensor thinking the heat could have damaged it as well. Still in the same place. MIL illuminated and no closer to a fix.
We finally hook up the PicoScope to take a look at the two patterns, thinking perhaps there was a timing issue between the cam and crank sensors.
I have to be honest. I couldn't see the problem with the pattern right away. We didn't have a known good pattern to use as reference. I finally found an image to use as a baseline this morning. https://www.youtube.com/watch?v=vrmuhgQteCU
We had a missing (weak) signal. We then pulled the Valve cover to find on the end of the camshaft there are three dowel pins that make up the three humps of the camshaft wave pattern.
You can see the three dowel pins. One of the pins had been pressed in to far creating an excessive air gap! We then measured all the pins on the old cam, which we confirmed were all of equal height. Next we reset the heights of all the dowel pins on the new cam. Put it all back together, and presto we got a good wave pattern and no more MIL.
Notice the amplitude on all three humps of the Cam sensor wave pattern match.
I'm really wondering what we would have done if we didn't have the scope. I am still thinking how we could have caught this one without it. a DVOM would not have helped. And to be honest I don't think a cursory look of the dowel pins would have stood out either. You had to first know there was a problem with the waveform to go looking for an issue. The difference between right and wrong just wasn't that much. And so ends my first Case study.