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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.

(Fig. 1)
I scanned the vehicle and found a P0301: misfire number 1 cylinder. (Fig. 1)
(Fig. 2)
To confirm that there is a misfire and to see what cylinder, I went to the data list. Cylinder number 1 has a constant misfire at all the various RPM levels. It feels slightly better at higher RPM, but there is always a misfire. (Fig. 2)

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)

(Fig. 3)
The waveform (Fig. 4) is what I captured. Channel A (blue) is the trigger signal to the cylinder 1 ignition coil. The downward slope of the pulse is when the spark plug would normally fire. Channel B (red) is the relative compression waveform using a high-amp current probe around the battery cable. Channel C (green) is the First Look Sensor in the intake manifold. The red arrow shows the missing compression from cylinder 1. When analyzing vacuum waveforms, the voltage will go low (towards bottom of the screen) whenever there is a vacuum pull. The green arrow shows a missing vacuum pull.
(Fig. 4)
(Fig. 5)
The missing vacuum pull does not necessarily line up with the cylinder 1 compression stroke. To diagnose this car, we have to think back to our basic engines training. The intake stroke is 360 degrees after the spark plug fires. In the below screen shot, I have utilized a piston position chart. This chart will allow me to see what cylinder is on what stroke at any part of the pattern. (Fig. 5)
(Fig. 6)
After overlaying the piston position chart, it is clear that cylinder 1 is missing an intake pull. I decided to check valve adjustment. This decision was based on my past experiences, the fact that it was overdue, and it is not too labor intensive. After removing the valve cover I found that cylinder 1 was too tight. I could not fit a feeler gauge in, and the valve was actually staying slightly open as a result. I adjusted all of the valves and decided to re-test. (Fig. 6)
(Fig. 7)
After adjusting the valves the misfire went away. During the repair, I left all of my diagnostic equipment hooked up. I always like to confirm my repairs with the scope again. I also like to build a database of known-good patterns. As you can see in the pattern below the relative compression looks good again. The intake pulls are also perfectly even. (Fig. 7)
Time is money in the shop. This vehicle could have been diagnosed without a DSO. But could it have been diagnosed as accurately and quickly without? Sometimes a DSO does not add capability, but it improves accuracy and speed. Engine mechanical issues do not have to be difficult. Using some simple tests and techniques can build confidence in our diagnosis.

Article By:

Matthew Shanahan

Listen to his Technician.Academy podcast here.

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