Engines have become very reliable in recent years, many reaching over 200,000 miles without major issues. As technicians, we replace a lot more ignition coils than timing chains. When that vehicle ends up in our bay with a mechanical issue though, the diagnosis can be tough. In the January 2017 article, “Efficient Engine Mechanical Diagnostics”, we looked at a truck with a jumped timing chain. Utilizing a DSO, I was able to be confident in my diagnosis. There are many techs that have taken an engine apart only to find nothing broken. Then there is the opposite of replacing many parts before finally realizing there is a mechanical issue. While mechanical issues are not the most common, they can be the toughest on technicians.
There are many Honda engines on the road today. Overall, they are some of the most reliable. A 2007 Honda Accord 2.4L came to me with a misfire complaint. About every 100,000 miles we should be checking the valve clearance, and adjusting as needed. Most technicians think about valves becoming “loose” over time and adjusting them. Many technicians think that adjustment is not needed because there is not any valve train noise. This is not the case on a Honda, because their valves get “tight” over time. Eventually this can cause a valve to stay open, causing a leak.
After checking the basics, ignition system, and fuel system I determined there might be an engine mechanical failure. My go-to first test for engines is the relative compression test. I performed a relative compression test with ignition sync and found low compression on cylinder 1. Case solved, sell them an engine, right? At this point, there is still more testing to do. The lower end of the engine is most likely fine, time to determine the real fault.
A great second step after a relative compression test fails is an intake vacuum test. Just like relative compression, the fuel or ignition system is disabled and the engine gets cranked over. Using a pressure transducer in the intake system we will be able to see the individual intake pulls of each cylinder. The transducer I am using is called a First Look Sensor. It is a piezo sensor that does not directly measure pressure or vacuum, instead it measures differences in pressure.
I cannot take credit for this setup, a very well respected trainer taught me how to place this sensor on the intake. You want to find an intake port that is fairly central if possible, not too big and not too small. This has become more and more difficult lately, but the Honda has a great port to access. The last tip I learned from this trainer is to use a spark plug boot to connect to the car. (Fig. 3)