Larry Johnson's Buckhead Auto Repair Center ASE.Mechanics

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Several years ago, I was chasing an elusive miss at idle on a Chrysler minivan. The miss was just barely noticeable at idle, and cleared up as soon as the throttle began to open. I checked the fuel system, I checked the ignition system, and finally started looking at the engine's mechanical health.

A vacuum gauge reading on the Chrysler showed me nothing, so it was time to test compression on this transverse V6. Sure enough, the rear bank, front cylinder tested a little low. My next step was a cylinder leakdown test, and I then confirmed that the exhaust valve on that cylinder was leaking...just enough to cause the idle misfire.

The Chrysler problem was twofold. First, I didn't test that possibility early in my diagnostic process and wasted a lot of precious time, hoping I'd find the problem in something easier. Second, the compression and leakdown tests were time consuming...many transverse engine designs aren't exactly the easiest when it comes to accessing that rear bank. Luckily, this vehicle was low mileage and still under factory warranty, so the customer didn't have to pay for it.

But what if he did? Could I justify the 2 hours plus it takes to perform these tests if they had proven to be OK? Then I learned I could quickly perform a relative compression test with digital storage oscilloscope (DSO), and in a matter of minutes, not hours.

Now, this is one of the first tests I perform when diagnosing driveability issues. I want to quickly eliminate the possibility that an engine mechanical failure is the cause of the problem.

Relative Compression and Starter Draw

The first method I ever learned involves starter draw. Think about it a minute...it takes current to turn over the starter and the more resistance to turning the starter has to deal with, the higher the current draw will be. We all learned as young technicians how to measure starter draw when testing that system, but when this test is powered by a scope, with its ability to graph the current while adjusting the time base, it allows us to see the impact of each cylinder on that current draw.

To perform this test, use your high amp clamp around the positive battery cable leading to the starter. Fire up your scope, and adjust the time base to 100ms per division. (You can make finer adjustments to clarify the pattern). Make sure the battery is strong and disable the fuel and/or ignition system to prevent the engine from starting, just as you would if you were performing a regular compression test.

My scope of choice is a PICO 3443. With it's ability to record, I can go back and review the pattern starting from the beginning. On my older UEI scope, the pattern wasn't as clear and would disappear from the screen if I tried to adjust the voltage divisions to make it clearer. That, however, is easily corrected by using the AC coupling feature on your scope. By removing the DC component, only the pattern remains...centered in your screen.

Remember, I'm looking at the relative amount of current that each cylinder is requiring to overcome the compression pressure and turn over. I don't really care about how much the current is...what I'm looking for is any "peaks" that are lower than the others. If I find one, I can then use my synch to trigger off of cylinder #1, then use the firing order to identify exactly which cylinder is at fault. In my experience, I can easily pick up on a compression variation of as little as 10% with this method.

But it is relative to all the cylinders. If there is a mechanical issue resulting in an equal loss to all, the pattern will look normal. However, this is a rare occurrence and the vacuum test would show this as a lowered intake manifold vacuum.