Think Like Sherlock Holmes To Avoid The Wrong Diagnostic Conclusions – UnderhoodService
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Electrical Vehicle Diagnostics


Think Like Sherlock Holmes To Avoid The Wrong Diagnostic Conclusions

As a boy, I read most of the Sherlock Holmes mysteries. Sherlock Holmes was a genius-level detective who could break a case by analyzing seemingly insignificant clues like the ash from a cigar. In contrast, Inspector Lestrade, who was Holmes‘ competitor from Scotland Yard, would inevitably arrest the wrong person by jumping to a premature conclusion.
In the real world of driveability diagnostics, I‘ve adopted many of Sherlock Holmes‘ tactics by looking more at technical details than at the apparent symptoms. Thinking like Sherlock Holmes, a diagnostic tech should conclude nothing until he collects data that explains the known symptoms.
While the following three case studies in this month‘s Diagnostic Dilemma aren‘t technically complex, they do illustrate why it‘s important to never jump to a diagnostic conclusion.
Although this Subaru’s engine architecture is very basic, it still provided an ample opportunity to jump to the wrong diagnostic conclusion.

Although this Subaru’s engine architecture is very basic, it still provided an ample opportunity to jump to the wrong diagnostic conclusion.

The Case Of The Missing Spark

This minor Diagnostic Dilemma began when a mechanic friend called about a no-start problem on a ‘93 Subaru Legacy. With the battery charger attached, the 2.2-liter Subaru engine seemed to crank evenly and at an acceptable speed, which indicated that the timing belts were intact and had equal compression in all cylinders.
With that done, I checked for spark by clamping the scope‘s secondary ignition probe to one of the four spark plug wires. The waveform remained flat as the engine was cranked, indicating a no-spark condition. Surprised, I verified the condition by mechanically testing spark availability at the spark plug. Next, I tested the crankshaft and camshaft sensor waveforms, both of which are two-wire magnetic reluctor sensors that produce a classic sine-wave signal. Both tested perfectly, which led me to examine the ignition system.
As with many vehicles of that era, the ‘93 Subaru legacy uses a remote ignition module to trigger its two waste-spark ignition coils. A quick test with a scope revealed that although battery voltage was present at the coils and the module, the primary coil circuits weren‘t being triggered by the ignition control module (ICM). I‘ve never seen a Subaru ICM of that vintage fail, so I checked the battery charger‘s ammeter to determine how much amperage was actually flowing into the battery. Because the battery was nearly dead, I would have expected at least 10-20 amps. Instead, the battery charger‘s ammeter indicated zero amperage flow — and that clue solved the cranking, no-spark problem.
Lesson Learned: So much for symptom diagnosis. Due to the presence of a crank sensor signal and the lack of spark, it was easy to jump to a diagnostic conclusion of replacing the ICM. The battery charger was the real clue when it showed zero charging amps with the ignition off, which indicated that the battery itself was incapable of absorbing a charge. Since most vehicles need at least 9.5 to 10 volts to activate the ECM and ICM, the battery wasn‘t supplying enough voltage to power both while the engine was cranking. In this case, the permanent-magnet starter could live comfortably with only maybe 8-9 volts at the battery, but the ECM and ICM could not.
Never attempt to diagnose a vehicle with a discharged or outright bad battery. If I had been working in my own shop, I would have substituted a known-good, compact universal battery before wasting valuable diagnostic time chasing the wrong suspect.
Although this Dodge Cummins is a later version of the engine mentioned in the text, it serves to illustrate the complexities of diesel engine diagnostics.

Although this Dodge Cummins is a later version of the engine mentioned in the text, it serves to illustrate the complexities of diesel engine diagnostics.

The Case Of The Missing Throttle Plate

This Diagnostic Dilemma was a customer complaint of a no-code “sticking throttle” on a 1996 Dodge Ram 1500 equipped with the Cummins diesel and with only 40,000 miles on the odometer. The owner‘s specific complaint was that the engine would temporarily continue to accelerate after passing another vehicle.
Although the complaint sounded like a sticking throttle, let‘s not jump to any diagnostic conclusions. Like all diesels, the Cummins engine doesn‘t control engine speed with a throttle plate mounted in the air intake, but instead controls engine speed by limiting fuel delivery through the fuel injectors. And, like most modern diesels, the Cummins uses an electronic throttle. Just a small variation in voltage output from the throttle sensor potentiometers mounted on the throttle pedal or from the throttle position sensors mounted on the throttle body will immediately store a trouble code, illuminate the check engine light, and possibly force the throttle management system into a default operating mode.
During shop testing, the overacceleration problem became apparent when I let off the accelerator pedal after snap-throttling to about 2,500 rpm. Instead of decelerating to idle speed, the engine continued accelerating to about 3,500 rpm. The diagnosis was complicated by an aftermarket programmer or power chip that the owner installed to improve performance. While the chip was removed at my request for testing, I still had problems with maintaining scanner communications during the testing session.
Despite those problems, I noticed some anomalies in the Cummins‘ fuel delivery system that confirmed my theory that the fuel injectors weren‘t shutting off fuel delivery when the throttle was released. At this critical point of the diagnostic process, my scan tool began intermittently losing communications with the ECM, which prevented me from graphing fuel delivery information for this column.
But with the symptoms verified, I checked my database for any case studies or any TSBs relating to the problem. I found none, so I posted the problem on the International Automotive Technician‘s Network (iATN) Heavy-Duty Forum and got a few responses pointing to leaking fuel injectors or excess oil leakage from the turbocharger seals that can cause a “sticking throttle” complaint. My client technician had already inspected the air inlet to the engine for traces of oil contamination from the turbocharger, so we ruled that out.
Later in the day, a diesel tech friend responded via email describing his experiences with overacceleration on Cummins engines. He said using the Cummins fuel bypass test would determine if the fuel injectors were leaking diesel fuel into the cylinders at higher fuel pressures and engine speeds. The test produced a set of data indicating that we had at least four marginal or out-right defective fuel injectors, which was surprising given the low mileage of the vehicle.
Two injectors had previously been installed to cure a cylinder misfire, so the owner decided to replace the remaining four fuel injectors and install an optional fuel filter to remove any traces of silicon-based residue from the incoming fuel. A long test drive later indicated that the overacceleration problem was solved, and the problem with intermittent scan tool communications was not of concern to the owner.
Lesson Learned: The clue that led us to a successful diagnosis was the overacceleration in the shop. Although the injector bypass testing took time, it justified the need for replacing the remaining four fuel injectors. And, last but not least, this case study illustrates why it pays to network with others when you can‘t find solutions elsewhere.
The Case Of The Missing Cylinder
A client shop called recently to help diagnose a 1998 Chevrolet Cavalier equipped with the 2.2-L, four-cylinder engine with a complaint of severe loss of power. This vehicle was nearing 300,000 miles on the odometer and, while the owner changed the oil regularly, he was reluctant to follow the shop‘s other repair and maintenance recommendations.
The shop found that the No. 4 cylinder was misfiring so badly that it was hard to keep up with the scan tool‘s misfire counter. Meanwhile, the actual misfire trouble code was stored for the No. 2 cylinder rather than No. 4. The shop had already replaced the electronic EGR valve to remedy an EGR-related trouble code. Also, the engine‘s electric cooling fan remained on, even when the engine was cold.
Due to the Cavalier‘s advanced age and mileage, the Diagnostic Dilemma was to quickly estimate the cost of repairing the lost power complaint. Since the spark plugs and wires are very accessible on the 2.2-L, I tested the secondary ignition waveforms. While the No. 1 cylinder was normal in all respects, the waveform on No. 2 displayed so much “hash” that it was indecipherable. The No. 3 wire was open-circuit, pegging the 30KV scale on my scope at idle. The horizontal spark line on the No. 4 cylinder floated very high, which possibly indicated a no-fuel condition on that cylinder.
Fortunately, the exhaust manifold on this 1998 Chevrolet Cavalier was ­completely exposed, which made a quick diagnosis much easier.

Fortunately, the exhaust manifold on this 1998 Chevrolet Cavalier was ­completely exposed, which made a quick diagnosis much easier.

Next, I evaluated the mechanical condition of the engine by using a First Look Sensor (FLS) to do a cold-cranking pressure wave test at the exhaust, which indicated no substantial loss of compression through the exhaust valves. Then, we did a cold cranking intake test, which indicated that the intake valves weren‘t causing the rough-idle complaint. After replacing the wires, the engine still idled rough and the cooling fan remained activated. Last, I concentrated on the short-term fuel trim (SFT) numbers, which were hovering at -6%.

Despite the negative short-term fuel trim numbers, the engine was idling in an extremely lean condition. The exhaust manifold is mounted at the front of the engine, so my next step was to use an infrared pyrometer (heat gun) to measure the exhaust port temperatures. Approximate readings included the No. 1 cylinder at 430º, No. 2 at 325º, No. 3 at 325º, and No. 4 at 200º. Clearly, No. 1 cylinder was essentially “carrying” the other three cylinders, and the No. 4 exhaust port was operating at coolant temperature, which indicated no power contribution at all. To add to the confusion, a scope test indicated that the No. 3 and No. 4 injectors were functioning electrically.
Fortunately, I brought my 12-pound propane bottle equipped with pressure regulator and metering valve to help diagnose a vacuum leak problem in another bay. Immediately after adding propane, the engine smoothed out and the cooling fan shut off.  More important, each of the engine‘s three remaining exhaust port temperatures gradually increased to over 400º after adding propane. This indicated that fuel injector No. 4  was clogged and injectors 2 and 3 might be partially clogged. After checking the vehicle‘s service records, we found that the No. 2 and No. 4 injectors had been replaced less than a year earlier.
Lessons Learned: Because of its advanced age and mileage, an Inspector Lestrade might have concluded that the engine had a compression or timing problem, which isn‘t cost-effective to repair on a vehicle with advanced mileage. The FLS test disproved this theory in less than five minutes without so much as removing a spark plug. And, although this case might have been solved much faster by using a five-gas exhaust analyzer, the propane test proved beyond a doubt that the No. 4 cylinder had a clogged injector. The injector was replaced under the manufacturer‘s warranty, leaving the customer with only the cost of new spark plug wires and the diagnostic process, something of which Sherlock Holmes might be proud.