In the past, I spent several years chasing down a Chrysler van that was missing at idle. This was only a slight miss at idle. The problem disappeared once the throttle started to be opened. I first checked the fuel, ignition, and mechanical integrity of the engine. Visit our website and learn more about automotive lab scope.

After a vacuum gauge test showed no results, it was time for me to perform compression tests on this transverse 6-cylinder engine. Unsurprisingly, it was the front, rear cylinder that tested low. After a cylinder test to determine if the exhaust valve of that particular cylinder leaked, I confirmed it.

Two problems were at play. In the first place, I did not test this possibility as early in my diagnosis process, and I wasted valuable time hoping to find the issue in something simpler. Two, I spent a great deal of time on the leakdown testing and compression testing. Transverse engine designs can be difficult to get at the rear banks. It was fortunate that this vehicle still had factory warranties and low mileage. Therefore, the customer wasn’t required to pay.

Then what? Was it worth the extra 2 hours of testing if these results had been good? When I realized I could do a compression test relative with the digital storage oscilloscope I did it in a few minutes.

It is one of my first steps when I am diagnosing issues with driving. In order to rule out the possibility that a mechanical issue is causing the problem, I need to do this quickly.

Comparative Compression and the Starter’s Draw

It was through starter draws that I first learned this method. It takes current for the starter to spin, so the higher current drawn by the system the more resistance it has to overcome. In our early days as technicians we learned to gauge starter draws. Now, if the system is tested using a scope that can display the current graph while changing the timebase, then it’s possible to observe the effects of the cylinders on this current draw.

This test is performed by using your high amp clamp. Simply wrap it around the positive lead of the battery that leads to the starter. Set your time base on 100ms. The pattern can be clarified by making finer adjustments. To prevent your engine from starting up, you should make sure you have a strong battery and disable the fuel and/or the ignition systems.