Have you ever had a misfire where you wanted to do a compression test on a cylinder that was buried underneath an intake manifold? Or worse, gone through all that effort to perform a compression test, only to discover that the compression was fine? There’s an easier way.
In this issue, we’ll cover the use of a relative compression test to make a quick Go/No-Go decision on the mechanical condition of all the engine’s cylinders without any disassembly. The test is very easy to perform and is non-intrusive. Basically, it’s a slightly advanced version of the cranking current test used when checking a starting system.
If you’ve been around for a while, you may remember the Sun VAT-40. Yep, we’re going back a little way. For this check, you’d disable the ignition system, connect the VAT-40’s current clamp around a battery cable, and then crank the engine.
Initially the needle on the amperage gauge would spike at a couple hundred volts, depending on the starter motor and the starting circuit’s condition. Soon the amperage would drop, due to counter-electromotive force, and the current draw would stay at a lower value for the duration of the test. The relative compression test of today uses the same technique, using some newer tools. This test will allow you to analyze the cranking current in greater detail and for different purposes. The only major difference between the old test and the new one is that you’ll need to disable fuel instead of ignition. You’ll need to keep the ignition active for synchronization; we’ll see why in a bit.
Here’s what you’ll need to perform this test:
- digital storage oscilloscope
- high current probe
- ignition sync connection
The ignition connection can be a sync probe, a secondary ignition pickup, a low current probe, or a conventional voltage connection.
Your choice will depend on what you have available and the vehicle you’re working on.
For the first example (figure 1), we clamped a high current probe around a battery cable. The waveform shows the starter motor current with the engine cranking.
Just like the old VAT-40 test, you can see the initial current spike followed by the lower current during continued cranking. But this method provides detail that would never be visible on an analog, needle-style gauge. Notice the repetitive current fluctuations visible throughout the capture.
The theory behind the fluctuations goes like this: Work requires current to be accomplished. As the piston comes up on its compression stroke, the starter motor works harder to turn the crankshaft against the mechanical resistance of compression. The result is higher starter motor current draw during the compression stroke.
The current draw, or work, required between compression strokes is lower because the starter doesn’t have to work as hard to keep the crankshaft spinning. So each triangulated peak in the scope capture represents one compression stroke of the engine.
A quick examination of the waveform shows that all the cylinders are requiring the same amount of work to accomplish their respective compression strokes.
This particular test — the relative compression test — gets its name by observing that all of the cylinders’ compression strokes are equal and relative to one another. It doesn’t tell us what the actual compression is, but we can say that a single-cylinder misfire probably isn’t being caused by a mechanical issue.
If a cylinder had low compression, its current peak would be lower because the starter motor wouldn’t have to work as hard during that cylinder’s compression stroke.
If we were chasing a misfire on a car like the one mentioned earlier, we wouldn’t waste the time and effort to remove the intake manifold to perform a manual compression test. Our relative compression test registered a Go, so it’s time to move on to other potential causes for a misfire.
Now that we know what a good test result looks like, let’s examine a bad vehicle (figure 2). 
This vehicle had a four-cylinder engine with a single cylinder misfire. We’ve added the second channel of the scope to provide an ignition sync. In this case, we clamped a sync probe around cylinder one’s spark plug wire.
You can obtain the sync from any cylinder and with multiple methods, as long as you know which cylinder you’re connected to. The goal is to synchronize the waveform with the firing order.
In this waveform, it’s obvious that one of the four cylinders has low or no compression. The addition of the sync probe allows us to count, in firing order, and determine which cylinder is the culprit. The firing order for this vehicle is 1-3-4-2.
Next we zoomed in to isolate 720º of crankshaft rotation on the same vehicle (figure 3).
We needed the longer duration in the initial capture to get a better handle on a potential intermittent issue.
Since we clamped the sync probe around the number one spark plug wire, you can see the firing event for cylinder number one. The event should occur near top dead center of the compression stroke. This indicates that the rising current leading up to the ignition sync, marked by the green arrow, identifies cylinder number one’s compression stroke.
With this information, you can count across the screen (numbered in black) and identify the cylinder with poor mechanical contribution. In this case, cylinder number two will require further testing. The results of this mechanical test: No-Go.
I hope you see the value of this Go/No-Go test, especially regarding a misfire on a vehicle where using a compression gauge requires additional labor. But wait! There’s another valuable piece of this test:
Going back to the VAT-40 days again, we used to have this octopus looking thing that had a bunch of fat wires coming out of it. Yes, I’m referring to a distributor! In those days, improper ignition timing could cause a variety of driveability problems. A timing light, something most modern technicians may not even own, was used to make sure that the base timing was correct.
Jump forward to today and ignition timing isn’t adjustable. Modern vehicles don’t even have timing marks. The lack of timing marks renders the old timing light useless. But can a vehicle still have an ignition timing issue? Of course it can. Many technicians overlook this issue because we no longer have the “octopus.”
Ignition timing was critical on internal combustion engines when we had distributors and it’s still critical today. Most modern vehicles use the crankshaft position sensor to control base ignition timing and the technician has no way of adjusting it.
If the crankshaft position sensor or its reluctor were to have an issue, the ignition timing could be off. The test we just used for measuring relative compression is extremely valuable for determining whether ignition timing is correct.
The theory is that, especially during engine cranking, the ignition should fire very near top dead center of the compression stroke. If TDC can be seen in our cranking current, and an ignition sync indicates when the spark plug fires, can we determine whether the ignition timing is close to where it needs to be?
Figure 4 is from a Dodge Caravan with a 3.3-liter engine. The sync signal, in red, is obviously occurring after top dead center.
For this, consider what we’ll call the double-R rule: to the Right is Retarded. In the case of the Dodge Caravan, the ignition timing is retarded, due to a very common issue that most transmission shops are familiar with. The flywheel breaks and moves, causing the reluctor for the crankshaft position sensor, which is part of the flywheel, to be out of time.
Chrysler products aren’t the only vehicles that experience this issue. Another example are Fords with worn crankshaft pulleys or improperly installed crankshaft pulleys with diamond washers. In fact, any vehicle that’s assembled incorrectly during a regular service, such as a timing belt or timing chain, could be identified using this test.
The relative compression test is a great way to check engine mechanical condition and base ignition timing easily. Play with this test for a while on vehicles in your shop and become familiar with what known good looks like. Then you’ll be able to find some diagnostic direction quickly when a misfire or ignition timing problem rolls in the door.
More importantly, you may be able to avoid removing an intake manifold to perform an old-school compression test.
