The engine doctor will see you now

Jan. 1, 2020
The miss was just barely apparent at idle. As soon as the throttle plate was opened, even slightly, the miss went away. Out on the road, the engine performed perfectly at any speed, under any load. The culprit turned out to be a burnt exhaust valve o
The manifold vacuum gauge has been around for as long as I can remember. That’s because it is still a valuable, if not high tech, tool.

The miss was just barely apparent at idle. As soon as the throttle plate was opened, even slightly, the miss went away. Out on the road, the engine performed perfectly at any speed, under any load. The culprit turned out to be a burnt exhaust valve on the rear bank of this Chrysler minivan causing just a slight loss of compression, but I didn't find it until I had nearly exhausted all other possibilities.

Up until that moment, I had considered myself a competent technician, but this van taught me a few valuable lessons in basic troubleshooting and actually was instrumental in my becoming a student of diagnostic techniques and methods.

Now, verifying the engine's overall health is one of the first steps I take when tackling a misfire diagnosis or troubleshooting an engine performance problem that has no related codes recorded.

This is the cranking starter draw on a Ford Focus. The even peaks indicate that all four cylinders are creating the same amount of resistance to movement in the starter motor. This indicates that the compression of each is similar.

The Easy Way First
In the initial stages of my troubleshooting, I'm not looking to prove that the engine is in 100 percent prime condition. I am looking for diagnostic direction, though, and for any early indications that there is a problem in the engine's ability to compress the air/fuel mixture or in its ability to flow that air in and out of the combustion chamber.

Traditional compression tests and cylinder leakdown tests certainly have their place, but the layout of many engine designs make performing these tests a time consuming proposition. Being on flat rate, this is time that I'm not getting paid for unless the customer authorizes it. And I can't ask the customer to spend that additional money unless I can show justification for it.

Manifold Vacuum
One of the first indicators of engine health you can use is an oldie but a goodie — manifold vacuum. To check manifold vacuum, connect your vacuum gauge to an intake vacuum port as close to the intake manifold as you can. Long distance connections can dampen the vacuum pulses at the gauge and limit its diagnostic use. The vacuum brake booster feed line works well and is usually easy to get to. A running engine at idle should produce between 18 inches/Hg to 21 inches/Hg and the needle should remain steady. Significantly lower readings or a gauge needle that bounces around are signs that further, more detailed tests should be done.

Here’s a six-cylinder with a plug removed to show you the difference it causes in starter current draw.

While you've got the vacuum gauge connected, snap the throttle to wide open and release a few times while watching the gauge. The needle should drop to roughly 5 inches/Hg or less, rebound quickly and surge past the idle reading, then fall as quickly back to the idle reading. Any delay in needle response can indicate a problem with airflow through the engine, usually in the exhaust. Or, hold the throttle at a constant 2,500 rpm. The gauge should read near the idle reading and stay there. Lowering vacuum readings while maintaining a steady rpm are another indicator of a restricted exhaust.

Use of the manifold vacuum gauge as a troubleshooting tool has dwindled over the years. Current intake designs impact the vacuum pulses we used to rely on to diagnose all sorts of engine problems. But that doesn't make it any less useful. Weaknesses indicated by this simple test are, at the least, justification you can take to your customer when seeking authorization to dig deeper.

PAGE 2
Measuring exhaust pulses with a pressure transducer is another useful tool in testing engine mechanical health. Some exhaust designs can also affect this pattern, so look for anomalies rather than analyzing each hump and valley.

Relative Compression Testing
If you have a digital storage oscilloscope (DSO), then you have all kinds of choices for testing engine health. The first one I learned was the relative compression test, which is the same one I used to finally find that burnt exhaust valve on the Chrysler. This test uses the scope and a high amp clamp to measure starter amperage draw at a time base fast enough to see the impact each cylinder has on that draw.

A weak cylinder takes less effort to spin through than a strong one does, and that means it also takes less current, and that is displayed on the scope's screen. Even my old UEI scope could catch as little as a 10 percent compression difference between cylinders using this method. By synching the pattern to a given cylinder and using the firing order of the engine, the weak cylinder(s) can be identified and more detailed testing done.

Back to the Focus I showed you earlier. This is the pattern of the exhaust pulses while cranking, just like I did the starter current test. The pattern is even and repeating, so the compression between cylinders is just fine.

Modern pressure transducers allow relative compression tests to be done via the exhaust pressure pulses leaving the tail pipe. The sensor shown in the illustrations is the FirstLook® sensor by SenX. The idea here is that cylinders that are healthy will pass equal pressure pulses through the exhaust that the sensor can detect. A weak cylinder will show up as a weaker peak in the pattern.

As with the starter current draw test, the pattern can be synched to help identify the bad cylinder(s). Here, though, the pulse displayed will be offset from the originating cylinder. After all, it takes time for the pulse to leave the engine and travel through the plumbing. When viewing these patterns, look for anomalies rather than specifics. Some exhaust designs cause one bank to have a longer route to follow than the other, and this can reduce the amplitude of the pattern on all the cylinders for that bank.

Had the first Focus pattern shown a problem, I could then isolate the cause to the valve train or the pistons/rings by measuring crankcase pressure at the dipstick tube. As each piston heads for BDC, a pressure pulse is generated the sensor can detect. Even peaks here mean the rings and pistons are sealing well.

If either of these tests indicates a weakness, you can use the pressure transducer to determine if the fault is in the valvetrain or in the cylinder by measuring crankcase pressure. How? Remove the oil dipstick and put a rubber hose over the tube, connecting the sensor to the tube. Crank over the engine and watch the pattern. A pressure increase will be present each time a piston heads for bottom dead center. If the rings are sealing properly, the peaks will be uniform and the loss of compression you found has to be caused by the top end.

One additional note: When performing any of these tests, be sure to disable the fuel and ignition systems to avoid both over fueling the cylinders and accidental engine start.

A picture is worth a thousand words, and that is certainly true when viewing a secondary ignition pattern. One cylinder with a consistently lower firing demand could indicate a lower compression on that cylinder.PAGE 3

Secondary Ignition Pattern
The secondary ignition pattern can tell you a lot about what is happening in the engine, including the health of the engine itself. Reading this pattern is a learned skill, but there are a few lessons related to today's topic I can pass along here.

First, look at the average of the firing lines (firing voltage demand) for all the cylinders. It's normal for the firing line of an individual cylinder to vary as conditions vary in the combustion process. Because firing demand is, in part, a function of the pressure the plug gap is under, any cylinder that has a consistently lower firing line is suspect.

Second, watch the spark line closely. No compression in a cylinder will not only have a lower firing demand, but the spark (or burn) line will be flat and longer than the others. (Note: This pattern can also indicate that spark is occurring outside of the cylinder, as in a leaking plug boot.)

Many drivability issues today are related to intermittent engine sealing issues, usually caused by sticking valves or valves that are coated in carbon deposits. These typically effect engine performance at idle can be next to impossible to find with a conventional compression test or even a cylinder leakdown test. But it does show up in the secondary ignition pattern's spark line.

This PICO bar graph of relative compression is measured by monitoring the AC ripple from the alternator at the battery. While it can’t tell you which cylinder is low, it only requires the connection of two leads at the battery and a quick crank of the engine. All of the tests shared take less time than a normal compression test on a 4-cylinder engine.

When a valve doesn't seal, the pressures built up in the cylinder have a path to escape through. As it tries to rush out of the opening, it creates a lot of turbulence in the combustion chamber. This hurricane wind literally can blow the ignition spark between the plug’s electrodes in all sorts of directions, including blowing it out completely. It shows up in the pattern as a series of peaks in what should otherwise be a relatively smooth slope. Some turbulence at higher engine speeds is normal, though, so don't confuse the two.

And Then What?
These are a few of the methods available to you for verifying the integrity of the engine's mechanical health in the early stages of your troubleshooting. Should any of these tests indicate a problem, you then have the justification you need to return to your customer and explain the need for more involved, and conclusive, testing. These tests include the traditional compression test (wet and dry), cylinder leakdown test, and running compression test.

By verifying the engine's state first, you also avoid wasting needless time correcting problems on other engine systems, needed or not. How will you explain injector cleaning or replacement, new ignition coils, or the replacement of other components as needed when you finally figure out the engine requires major repair? It is best to eliminate that early in your diagnostic process, saving you time, money and frustration.

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

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