Rabbit hole diagnostics

This weeks tech tip is about getting sucked in to the dreaded rabbit hole. When you start a diagnostic approach you need to do a funnel style diagnostic approach. Do wide range testing and narrow down the possibilities as you go. This past week I was called to a shop for a misfire diagnoses. They had recently performed a head gasket repair on a Subaru, during the repair they sent the head to a machine shop to verify cylinder head integrity. While at the machine shop they resurfaced the cylinder head. After the vehicle was reassembled the entire bank was misfiring. The misfire was a constant, not matter the operating conditions. The shop had already verified spark, injector pulse, and fuel pressure. After several days of diagnosing the issue, they called me in to do an in cylinder test to check for possible cylinder head issues, to verify the machine shop did their job properly. When I got there I verified the concern, and performed my capture. During my test I noticed that my spark was happening close to 80* before and after top dead center (this vehicle has a waste spark system). And here is where this post comes in to play, rabbit hole diagnostics. Sometimes you just need to take a step back, and double check everything especially if you just performed a large job such as this. The issue with this vehicle is they switched the plug wires on the bank, and the vehicle was firing the spark at the wrong time causing the whole bank misfire. We have all done this at least once, but sometimes us as technicians get it in our heads that it has to be something else besides what we may have done. So just take a step back think about what the issue at hand, and go back in to your diagnostic approach with a clear head. Could they have figured this out with out an in cylinder test? The answer to that is absolutely, I ran in to something very similar to this about a month ago, and a simple relative compression test and cylinder sync allowed me to verify that cylinders were firing past TDC.