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 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 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 my recently acquired UEI scope, 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 possiblity that an engne mechanical failure is the cause of the problem.  Since that time, I have learned a few variations on this theme.  Allow me to share them with you.

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.  You should see something similar to the one shown above.

This capture is taken with 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...like the one shown below.

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.

A recent addition to my diagnostic arsenal is the FirstLook sensor.  This is a pressure transducer that will work with any scope and is useful for a number of engine tests.  But since I'm talking about overall engine health I'll focus on that for now.

(if you want to add the FirstLook to your toolbox, click here and visit our friends at AES!)

These pulses are caused by the piston movement and indicate the health of the piston and rings.  If pressure pulses here are equal, odds are good the loss is in the valvetrain or cylinder head area.  If not, then it's time to take a closer look at the bottom end...after a phone call to the customer.

The point here is, all this was done in minutes instead of hours.  If you make your living on flatrate, which method would you rather use?

The last method I will share here is the PICO diagnostic software program, available for download at the PICO website.  This actually can test both relative compression and running power balance, but there are a few bugs still in the latter.

Using this program is the easiest of the three...all you have to do is hook up to the battery!  It works similar to the starter draw test, but instead of current the software monitors the change in voltage drop as the engine cranks over. The PICO has such a fast sampling rate, it can detect these minor variations easily.  It is otherwise performed the same way...with the engine disabled and with a good, healthy battery.  I apologize for the photo...I'm not that computer literate and couldn't figure out how to convert the actual file to one that I could upload to this site!