Looking For a Misfire? Use Your Scope to Check the Mechanical Health of the Engine!

Several years ago, I was chasing an elusive miss atto measure starter draw when testing that system,
idle on a Chrysler minivan. The miss was just barelybut when this test is powered by a scope, with its
noticeable at idle, and cleared up as soon as theability to graph the current while adjusting the time
throttle began to open. I checked the fuel system, Ibase, it allows us to see the impact of each cylinder
checked the ignition system, and finally startedon that current draw.
looking at the engine's mechanical health.To perform this test, use your high amp clamp
A vacuum gauge reading showed me nothing, so itaround the positive battery cable leading to the
was time to test compression on this transverse V6.starter. Fire up your scope, and adjust the time base
Sure enough, the rear bank, front cylinder tested ato 100ms per division. (You can make finer
little low. My next step was a cylinder leakdown test,adjustments to clarify the pattern). Make sure the
and I then confirmed that the exhaust valve on thatbattery is strong and disable the fuel and/or ignition
cylinder was leaking... just enough to cause the idlesystem to prevent the engine from starting, just as
misfire.you would if you were performing a regular
The problem was twofold. First, I didn't test thatcompression test.
possibility early in my diagnostic process and wastedMy scope of choice is a PICO 3443. With it's ability to
a lot of precious time, hoping I'd find the problem inrecord, I can go back and review the pattern starting
something easier. Second, the compression andfrom the beginning. On my older UEI scope, the
leakdown tests were time consuming...manypattern wasn't as clear and would disappear from the
transverse engine designs aren't exactly the easiestscreen if I tried to adjust the voltage divisions to
when it comes to accessing that rear bank. Luckily,make it clearer. That, however, is easily corrected by
this vehicle was low mileage and still under factoryusing the AC coupling feature on your scope. By
warranty, so the customer didn't have to pay for it.removing the DC component, only the pattern
But what if he did? Could I justify the 2 hours plus itremains...centered in your screen.
takes to perform these tests if they had proven toRemember, I'm looking at the relative amount of
be OK? Then I learned I could quickly perform acurrent that each cylinder is requiring to overcome
relative compression test with digital storagethe compression pressure and turn over. I don't really
oscilloscope (DSO), and in a matter of minutes, notcare about how much the current is...what I'm looking
hours.for is any "peaks" that are lower than the others. If I
Now, this is one of the first tests I perform whenfind one, I can then use my synch to trigger off of
diagnosing driveability issues. I want to quicklycylinder #1, then use the firing order to identify
eliminate the possiblity that an engne mechanicalexactly which cylinder is at fault. In my experience, I
failure is the cause of the problem.can easily pick up on a compression variation of as
Relative Compression and Starter Drawlittle as 10% with this method.
The first method I ever learned involves starterBut it is relative to all the cylinders. If there is a
draw. Think about it a minute...it takes current to turnmechanical issue resulting in an equal loss to all, the
over the starter and the more resistance to turningpattern will look normal. However, this is a rare
the starter has to deal with, the higher the currentoccurrence and the vacuum test would show this as
draw will be. We all learned as young technicians howa lowered intake manifold vacuum.