For many diagnostic techs, nothing is more discouraging and time-consuming than dealing with a no-code driveability complaint.
Today’s second-generation On-Board Diagnostics II (OBD II) can be extremely sophisticated because the OBD II powertrain control modules (PCMs) in modern vehicles have far more computing capacity than did the older, pre-1996 OBD I vehicles. Nevertheless, we occasionally have to deal with a no-code performance complaint on a modern OBD II vehicle.
To better illustrate a few of the diagnostic processes used in locating a no-code powertrain performance fault, the focus vehicle in this month’s Diagnostic Dilemmas is a 2000 Ford Taurus referred to me by a local transmission shop.
The Taurus is equipped with the 3.0L, VIN “U” engine with an automatic transmission. At 105,000 miles, the vehicle had been well maintained and was used mainly for commuting.
Trouble Code Basics
Before getting into no-code diagnostics, let’s review how trouble codes are stored in the PCM’s diagnostic memory. At the most basic level, a PCM can monitor a sensor or actuator for an open, shorted or shorted-to-ground circuit. When the PCM senses high, low or no voltage at all in a circuit, it stores a diagnostic trouble code (DTC) describing the failure.
In other cases, the PCM might measure the amperage flowing through an actuator like a fuel injector. If a shorted injector winding draws excessive amperage, the PCM might react by turning off the fuel injector driver and storing the appropriate DTC.
These are generally called functionality codes because, as the name implies, each has to do with the functionality of a component or its circuit.
The functionality of a common three-wire throttle position (TP) sensor illustrates some of the logic a PCM uses to store a DTC. The TP is powered by a 5-volt input or reference voltage. Normal output voltage might be 0.8 volts at closed throttle and 4.5 volts at wide-open throttle.
The reason WOT output is limited to approximately 4.5 volts is to allow the PCM to differentiate between WOT at 4.5 volts and an open ground circuit that would drive the TP output to 5.0 volts.
In essence, the PCM should be able sense the difference between an open circuit in the TP output signal (TP low) and an open in the TP ground (TP high). Depending upon how the PCM is programmed to respond, different DTCs can be stored.
Another method, called rationalization, uses inputs from two or more sensors to monitor the performance of a third sensor. To illustrate, if the TP sensor input is 4.5 volts at WOT and the crankshaft position (CKP) sensor indicates an engine speed of 5,000 rpm, it’s obvious that the engine should be operating at peak air flow.
If the mass air flow (MAF) sensor indicates that the engine is moving less air flow than the TP and CKP sensors indicate, the PCM might store an MAF sensor trouble code.
Here again, how the data is interpreted depends upon how the PCM is programmed to react to specific types of data. Many experienced diagnostic techs know that an MAF failure in some vehicles is often indicated by a P0171 (system lean, bank one) and P0174 (system lean, bank two) rather than by a conventional P0100-104 mass air flow malfunction code.
Last, a PCM can use mathematical algorithms programmed into its on-board diagnostics to measure degradation in expendable parts like catalytic converters. In this case, the PCM compares the activity of the upstream oxygen sensor with the activity of the downstream oxygen sensor.
In addition, the PCM also monitors the switching rates and voltage outputs of the oxygen sensors to monitor their condition. In any case, the oxygen sensor must be functional before it can provide the data needed to measure degradation in the catalytic converter.
When diagnosing a no-code driveability complaint, it’s also important to make sure that all of the OBD II test monitors are ready and that all of the enabling criteria, such as engine operating temperature, have been met. Knowing the enabling criteria involved is critical for understanding why a DTC might not be stored in a no-code driveability complaint.
Continuous monitors, like the fuel trim and ignition misfire monitors, run constantly. Non-continuous monitors, such as the catalyst and evaporative emissions monitors, run only once per “trip.” The readiness state of these monitors is normally displayed on a professional-level scan tool.
All of the monitors are driven by enabling criteria describing a set of driving conditions that must be met before the monitor will run. In many cases, the engine has to be warmed to a specified operating temperature before a trouble code can be stored and, in the case of evaporative codes, ambient temperatures and barometric pressures also come into play.
Occasionally, a stuck-open thermostat will conceal several potential DTCs. With that said, let’s look at the driveability complaint on our 2000 Taurus.
Taking a Test Drive
A local transmission specialty shop referred the 2000 model-year Taurus to me. The vehicle owner had brought the vehicle to the shop complaining of a jerking sensation as the vehicle accelerated in passing gear.
The shop owner test-drove the vehicle and returned it to his customer, explaining that he thought it was an engine performance problem rather than a transmission problem.
At this point, it’s important to collect as much hard data as possible. To accomplish that, I used a scan tool to poll each
module for pending, current and continuous or history DTCs.
This is an important first step simply because DTCs can often erase themselves during start-up and it’s also useful to know if another module has developed a diagnostic issue of its own.
With no apparent DTCs stored in the PCM, I took the Taurus for a test drive on a deserted back road to verify the owner’s complaint. Initially, the Taurus accelerated perfectly during warm-up. But, as the engine warmed up, it began to exhibit a slight “jerk” as the engine speed peaked at about 60 mph in an intermediate gear.
Fortunately, I had my scan tool attached, ready to capture data when I duplicated the owner’s complaint. After about 15 minutes of driving, the jerking sensation became very pronounced, even to the point of occurring in first gear as the vehicle accelerated.
As for the jerking sensation itself, the engine acted like the fuel pump had suddenly quit and then restarted. That scenario seemed to be supported by the captured scan tool data indicating a positive fuel trim of +20% at high engine speeds.
Here again, it’s important to understand how DTCs are stored. The reason the Taurus wasn’t storing a P0171 or P0174 “system lean” code was that the fuel trim must generally exceed a 25% correction. At this point, I had only a 20% correction, which resulted in a no-code driveability complaint.
The Diagnostic Dilemma
I suspected that the +20% fuel trim indicated a lack of fuel at high engine speeds. The jerking resulted from the fuel being supplied in spurts rather than an insufficient volume as suggested by a worn fuel pump or clogged fuel filter.
Back at the shop, I removed the fuel filter and drained its contents in a clear plastic container for examination. The filter appeared nearly new and the fuel it contained appeared perfectly clear.
I remember that the first car I owned as a teenager had the same jerking sensation during acceleration. The cause turned out to be a collapsing flexible fuel hose located between the frame and the engine-mounted fuel pump.
Could I have a similar problem in the Taurus? Possibly, but let’s review the test drive. The engine performed perfectly until it had run for 15 minutes. After 15 minutes, the jerking sensation increased in severity. Any problem requiring a 15-20 minute warm-up before it occurs always suggests the problem might be found on the engine itself.
I’ve had many distributor pickup and module problems show up in that time span because it takes about 20 minutes for a distributor to reach engine temperature.
This Taurus was equipped with a “coil-pack” or “waste-spark” ignition system. Consequently, if the ignition quit completely at high engine speeds, I might possibly see a positive fuel trim similar to the one I had because the unburned oxygen from the engine cylinders will flow past the upstream oxygen sensors.
Although this diagnostic scenario is unlikely, I still left it as a possibility. So the Diagnostic Dilemma is that the positive fuel trim could be caused by — you guessed it — no ignition or no fuel.
The TSB Search
In any new or unusual diagnostic case, I use a well-known tech website to review anecdotal case studies, pattern failures, system wiring diagrams and technical service bulletins. Most of the time, this gives me a good picture of what I should be looking at.
Unfortunately, a TSB search involves reviewing hundreds of documents written for a single vehicle application.
Sometimes the TSB search is made easy and sometimes not, depending upon how the website or information system classifies TSBs.
But, when looking at “case studies,” I found Ford Motor Company TSB 05-22-12 titled, “Buck, jerk and miss during drive cycle above 3,000 rpm — no DTC — 3.0L 2v gas and FFV engine.”
I won’t go into the particulars, but the recommended service procedure required the now-obsolete Ford NGS scan tool. Nevertheless, the TSB described a speed-related issue with the camshaft synchronizer assembly.
Most important, the TSB explained that, since the camshaft position (CMP) sensor monitor is not active above 2,500 rpm engine speed, no DTC P0340 cam sensor code will be set during the failure, which makes it difficult to diagnose the CMP as the “root cause component.”
During the late 1990 and early 2000 model years, Ford equipped early-design engines that originally used distributor ignitions with waste-spark ignitions. On this engine, Ford engineers replaced the conventional distributor with the camshaft synchronizer and CMP sensor assembly.
In that era, cam synchronizer failures were common and, quite frankly, I had forgotten that there still are a number of these engines on the road. In most cases, the shaft bushing would wear out, which allowed the sensor shutter to wobble in the housing and create a faulty signal that stored a P0340 DTC.
Needless to say, the main engine wiring harness and a host of vacuum hoses fully concealed the synchronizer assembly. The key point here is to hold the harness and hoses up to allow the synchronizer to be lifted straight up without dislodging the hexagonal oil pump drive rod from the oil pump.
The next step is to select the correct alignment tool to verify the synchronizer shutter alignment at top dead center (TDC) compression stroke before removal and to mark the position of the synchronizer housing in relation to the engine.
The Ford timing set is available through a major aftermarket supplier. Aftermarket replacement synchronizers may also include an alignment tool with the part.
After removing the CMP sensor from the synchronizer assembly, I felt some wear in the shaft bushing. Moreover, I noticed that the synchronizer shutter was slightly polished in one area, which indicated that the shutter had been touching the CMP sensor. With these observations in mind, my initial diagnostic scenario was that, when the engine was cold, the synchronizer maintained an adequate air gap.
After 20 minutes of operation, the air gap increased beyond specifications. Due to normal wear in the timing chain and the pulsations of the oil pump at the end of the synchronizer shaft, the signal probably began to deteriorate above 3,000 rpm.
As a point of clarification on CMP function, the Ford’s profile indicator pickup signal indicates TDC on each cylinder. In contrast, the CMP signal indicates TDC on #1 cylinder at compression stroke. On Ford products, the CMP signal occurs at 26° after top dead center (ATDC). The PCM uses the CMP’s 26° ATDC signal as a reference point from which to time the fuel injection.
Always remember that CMP operating strategies are application specific, which means that there are no “generic” failure patterns for CMP sensors. Consequently, my diagnostic scenario was that the CMP sensor was skipping a signal as the engine reached peak speed in passing gear and the PCM was reacting by momentarily failing to actuate the fuel injectors.
Without the Ford NGS scan tool, I couldn’t verify the condition described in the TSB. Because I could observe wear in the shaft bushing and verify that the shutter had been touching the sensor magnet, the synchronizer assembly needed to be replaced, regardless. A subsequent 20-minute test drive verified that the TSB was correct because the Taurus performed flawlessly in all transmission ranges.
At least half of my calls are for no-code diagnostics. While some are well-known pattern failures, others often require a fair amount of research time before a diagnostic direction can be established.
Sometimes my research reveals a TSB, and sometimes it’s anecdotal information.
Whatever the situation, it’s vital to verify or duplicate the owner’s complaint and reconcile those symptoms with the information at hand. In the above case study, a TSB was available that perfectly described the symptoms. If the TSB had not been available, I would have had nothing to work with other than the +20% fuel trim number.
I might also say that, while symptom-based diagnostics can be taken into consideration while solving a no-code performance complaint, pursuing a symptom-based strategy can lead the diagnostic process wildly astray.
he positive fuel trim, for example, could have been caused by a component like the MAF sensor, which operates outside the fuel delivery system. More often than not, finding the solution to a no-code problem depends on having good knowledge of all operating systems and not jumping to conclusions.