It seems that we as an industry overall have become dependent on computer-generated Diagnostic Trouble Codes (DTCs). When faced with a drivability complaint where no Malfunction Indicator Lamp (MIL) is lit, do you scribble a quick “No Problem Found” on the repair order and send the customer on his or her way? If you started your career in auto repair after the introduction of electronic control modules, you might be tempted to do exactly that.
DTCs never were intended to take the place of a logical diagnostic process. They are merely aids that inform you of what the Engine Control Module (ECM) saw that it didn’t like. Even the use of an OEM diagnostic flow chart is flawed if you don’t understand what it is you’re testing and, more importantly, why you are performing a given test. For that reason, facing a no code complaint could be considered a blessing. Let’s dive into why and talk about some techniques you can incorporate that will improve your troubleshooting skills, code or no code.
Why the Light Comes on
According to industry stats, the average age of cars and light trucks in the U.S. is approximately 11.5 years. That means that most of the cars on the road today are certified to OBDII standards (1996 MY to present). These standards, in part, require that the onboard engine management system be able to monitor all systems related to emissions and warn the driver should any system fail in a manner that would cause emissions to increase beyond what the vehicle originally was certified to. How does the ECM know if a failure has occurred that will result in increased emissions? By performing its own tests on all the emissions-related systems, that’s how.
Testing by the ECM can be grouped into three basic categories. The first is the circuit integrity test, which, as the name implies, tests the input/output circuits for shorts to ground, shorts to power and opens in the circuit path. The second is the rationality test. In this test method, the ECM compares the data from two different inputs to see if they are in agreement for the given operating condition. For example, if the Throttle Position Sensor (TPS) suddenly reported a huge change in throttle opening, but the ECM didn’t see a corresponding rapid change from the Manifold Absolute Pressure (MAP) sensor, it might trigger a code P0105 (MAP Circuit Malfunction).
The third type of test is the functionality test. The ECM monitors the actual performance of an output component or entire system and compares it to what it knows to be good. If the ECM conducts this type of test from the sidelines, it’s a non-intrusive test. An example would be an Exhaust Gas Recirculation (EGR) system test that uses the MAP signal as the feedback to the ECM. When the EGR valve opens when it is supposed to, the manifold absolute pressure will change and the ECM will see that in the changing MAP input. If not, the ECM will consider this as a failure and log the event. If the ECM actively alters the normal operating conditions and then looks for a corresponding change, it is an intrusive test. Still using EGR as the example, if the ECM commands the EGR to close when it is supposed to be open, the upstream oxygen sensor will respond by going momentarily lean. If the ECM sees the voltage shift from the O2 sensor, it knows the EGR did, in fact, close.
This is one method Chrysler uses, by the way, and sets a P0400 (EGR System Malfunction). It is a great example also of how a code description alone is not enough to troubleshoot from. Failure to understand the ECM’s testing methods often leads to unnecessary time spent chasing ghosts and unneeded parts replacements. For example, because of the method the ECM uses to set the P0105 code described earlier, the fault can be as much a problem with the TPS signal as it could be the MAP. If you didn’t understand how the ECM uses the two together, you may never find the problem.
What does this have to do with no code diagnostics?
Remember the tests you took in school? They all had a pass/fail line, didn’t they? The tests the ECM runs are no different. They all have parameters that are the allowable variances for any given test. Misfire monitors are one example. We’ve all had a misfiring engine we could feel on a test drive, yet the MIL light never came on. That’s because the amount of misfires per engine revolution did not exceed the programmed parameters contained in the ECM’s software. Input sensors are another example. They can read erratically, yet not be caught by the ECM because the range of readings is within normal specifications or happens too quickly for the ECM to catch in the act. A spiking TPS is an example of a fault that can result in a drivability complaint, yet not set a related DTC.
Going ‘Old School’ with a Twist
Even with all the developments in engine technology, one simple fact remains: The engine has to “suck, squeeze, bang and blow” to work. And the range of acceptable performance has gotten smaller as engineers squeeze as much efficiency as they can from the powerplant. That means that even small variances from the norm can result in a drivability complaint. And finding those small faults takes a logical approach, starting with broad general tests to home in on the specific sub-system causing the problem and then more precise testing to isolate the root cause.
One of the first steps I take when dealing with a no-code drivability complaint is to verify the integrity of the engine itself. Traditional cylinder testing (compression and leak down tests), though, is both time-consuming and inconclusive. Today, it seems, more and more drivability issues are being caused by intermittent cylinder sealing as a result of carbon and other deposit build up in the intake tract and combustion chamber. Catching these intermittents in the act becomes a challenge.
I start with a relative compression test using a Digital Storage Oscilloscope (DSO) and high amp current clamp. This is a two-minute test that monitors starter current draw while the disabled engine is cranked over. Electric motors require more current when a load is applied so a healthy cylinder will cause a higher current draw than a weak one will. By adding a second channel to trace a reference pattern (usually an ignition event), I easily can use the firing order to pinpoint the weak cylinder and move forward from there. For the scopes I have used this technique with, I have found that I can pick out a cylinder with as little as a 10 percent compression loss with consistency. Even that, though, might not catch those intermittent seals. For that, I’ll need a more sophisticated technique or two.
If the car’s engine management system is on a Controller Area Network (CAN) protocol, I’ll peek into Mode $06 to see if there are any misfires recorded there. Mode $06 contains the ECM’s test results for all the non-continuous monitors. Pre-CAN Ford ECMs have misfire data there and all CAN vehicles do. Those misfires might not happen often enough to trigger a MIL, but if any are happening, they will be counted and recorded here. Check your service information provider for the appropriate test identifiers and what cylinders they reference.
Watching the secondary ignition pattern on a scope can reveal a lot to the trained eye and is useful for not only checking the mechanical integrity of the engine, but the health of the ignition and fuel systems as well. I, for one, still am learning the art of secondary waveform analysis, but one area that I have become very familiar with is the concept of combustion chamber turbulence caused by poorly sealing valves.
The clue is in the ignition pattern’s “burn line,” or that portion of the pattern representing the time the spark is actually traveling across the plug gap. When a valve is sealing poorly (but enough to pass a static compression test), the high pressures present force themselves past the valve and cause a literal whirlwind in the combustion chamber that bends and bows the spark like a candle flame moves with the breeze. In effect, the spark distance is constantly changing and you’ll see it in the burn line at idle.
But the best method to incorporate into your diagnostic routine is the in-cylinder running pressure test. This test uses the DSO mated to a pressure transducer that is installed in place of the spark plug. The engine is then run and the varying pressure in the tested cylinder can be viewed on the scope’s display. Intermittently sealing cylinders can be identified by variations in peak pressure and irregularities in the portion of the pattern referred to as the “exhaust pocket.” Contributing Editor Bernie Thompson is an acknowledged expert in this technique and wrote a primer that we featured in our June 2013 issue. If you don’t have a copy, you can always read it online at MotorAge.com.
Engine’s Good!
Once I have verified that the engine itself is not to blame, I move on to what the engine needs to run and run properly. It needs air and gas and something to get the mixture to burn. And it all has to happen in just the right amount and at just the right time to insure the engine runs smoothly. The problem is, if the problem I’m hunting is not bad enough for the ECM to notice, odds are conventional testing methods won’t find it either.
Weaknesses in the ignition system, for the most part, are caused by lack of maintenance, so a visual inspection of the plug condition and gap is one of the first things I like to do. Ignition system integrity is best tested with a DSO and monitored under the same engine conditions where your customer is experiencing their complaint. Capturing a secondary ignition pattern need not be complicated on Coil-On-Plug (COP) ignitions, either.
On two-wire systems, you can place your scope lead on the ground side of the coil primary and get a mirror-image of the secondary. Just be sure to use an attenuator to avoid overloading your scope by exceeding its maximum input voltage. The primary can kick out a few hundred volts of its own. Another option is to use a secondary probe; a pick-up that is designed to capture the signal by simply placing the probe on top of the coil you want to test. I prefer a scope capable of storing a lot of data points when hunting intermittents because I can log a lot of information and then sit back with a cup of coffee and review the stored frames in detail. Odds are I’m not going to be watching the live capture when the culprit shows himself.
I can look for timing issues (ignition and cam/crank) easily by adding a second channel to the in-cylinder pressure test and using it to record that cylinder’s ignition event. The peak of the in-cylinder test will always be Top Dead Center (TDC) of the tested cylinder’s compression stroke and a pattern capture that starts and ends here gives me an accurate representation of the entire 720° cycle. This also is an excellent way to check the operation of variable valve timing systems. By adding a third channel to the ground side (control side) of the injector, I can monitor its timing and pulse width at the same time.
Just keep in mind that ignition timing and injector pulse width are determined by the ECM. The ECM, in turn, makes those decisions based on the information provided by a variety of sensors. And they don’t always tell the truth! Use the graphing and recording features of your scan tool to log key data at the time the concern is presenting itself and then review that data, looking for readings that just don’t seem to fit. How will you know what isn’t right? Many OEMs provide scan data range values in their service information but the best way to learn what isn’t right is to first learn what is. You do that by hooking up your scan tool to known good cars every chance you get and taking the time to review that recording. If you can, save them in a permanent file for future reference.
Many top drivability techs have a set test drive route and they test drive every car they work on while recording certain Parameter Identifiers (PIDs). They’ll record the same PIDs each and every time with an emphasis on fuel trim and the factors affecting fuel trim. The test drive will generally involve a period at idle, a steady state highway cruise and at least one rolling wide open throttle acceleration to gauge volumetric efficiency. Diagnostic direction for both code and no code complaints can be enhanced using this method.
Facing a no code complaint actually can be a blessing. It forces you to think about how the systems operate and interact, without the aid of a printed checklist. To become proficient, you have to take the time to master your troubleshooting techniques and diagnostic tools by practicing on cars that don’t have a problem. In the end, it can only make you a stronger (and ultimately more valuable) diagnostician.
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