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February 19, 2021 09:09 AM

Marinucci: Effective road test may eliminate issues

Dan Marinucci
[email protected]
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    Here’s the capture command button on the Snap-On scan tool. A proper road-test format, which takes about 15 minutes with a scan tool, aids a technician in plotting a troubleshooting path.

    A brief but effective road test may save unimaginable time on seemingly difficult drivability diagnoses.

    This column concerns that road-test format and reasons why some sharp technicians rely on it to plot their troubleshooting path.

    Typically, a technician with a scan tool can perform this test — start to finish — in less than 15 minutes.

    To begin, many tire dealers gradually have expanded by adding automotive maintenance and repairs to their menus.

    These expanded repairs often include drivability-related work. Over the years, the word drivability has come to encompass things such as idle quality, acceleration smoothness and briskness, fuel consumption and tailpipe emissions output.

    Drivability diagnosis may challenge — perhaps frustrate – some technicians because a vehicle's problem may not trigger a trouble code. Note that many service personnel refer to a trouble code as a Diagnostic Trouble Code or DTC.

    You see, a failure within the massive realm of the engine control system may not spur a DTC until the problem's consequences exceed a particular threshold or level.

    However, informed techs know that certain patterns or trends in the control system's data may flag approaching failures — before the engine control computer sets a DTC and turns on the Check Engine light.

    Many techs routinely connect a scan tool to a problem vehicle. But in reality, they only use the instrument to extract DTCs. They actually treat this invaluable tool like a low-buck trouble code "reader."

    However, savvy techs use this tool to monitor vital engine control data during road tests. These techs repeatedly emphasize that the more often they see normal data on healthy vehicles, the more readily they recognize data that is trending in the wrong direction — before a DTC appears.

    Let me put this into perspective. In 1981, I took my first classes on domestic engine systems diagnosis at Ford Motor Co. as well as General Motors.

    "No-code" drivability conditions were topical back then and still occur today. Suffice it to say that the lack of a DTC does not necessarily doom a diagnosis.

    In my last column, I stressed that skipping a road test ultimately may waste time and foster costly mistakes. Here, I'll discuss a scenario that tire dealers and service shop operators dread — a drivability condition that hasn't spawned a trouble code yet.

    Complicating this task may be a motorist who is unwilling or unable to communicate the vehicle's symptoms to you — let alone do it clearly.

    Plus, you may hit an unwelcome trifecta when a careless service writer does not interrogate the car owner. Therefore, a tech must tackle the task without an accurate vehicle history.

    Dan Marinucci

    Scan tool line graph

    OK, a tech connects his scan tool to this vehicle and adjusts the tool's display so it only shows two data values; the first is long-term fuel trim (LTFT).

    The other data item is the mass air flow sensor (MAF) signal, which appears in grams per second of air flow.

    Furthermore, the tech sets up the scan tool to show the data in easy-to-read, easy-to-analyze line graphs.

    The accompanying data captures came from an acceleration test I performed on a popular Toyota Camry with a 2.4-liter engine. The scan tool was Snap-on's Solus Ultra.

    I keep my eyes on the road while driving. But I also have my right hand on the scan tool control pad. Here, touching one button makes the scan tool capture a movie of the data. Then I review these test results safely on the roadside or back in a bay.

    Meanwhile, an engine management computer controls fuel injectors that squirt gasoline into the engine. The computer rapidly pulses the injectors, tailoring the amount of sprayed fuel to the engine's operating conditions.

    Engine control computers have been providing fuel trim data for years. To grossly simplify, fuel trim reflects how the computer is managing the ratio of air to fuel being blended together.

    For instance, positive fuel trim means that the computer is forcing the air/fuel mixture richer — adding extra fuel to the mixture.

    But negative fuel trim means the computer is forcing the air/fuel mixture leaner — reducing the fuel within the mixture.

    As its name suggests, LTFT data reflects the engine control system's overall trend adjusting the air/fuel mixture.

    Ideally, LTFT should measure 0%. In other words, zero would indicate that the engine computer does not deem it necessary to tweak the mixture toward the richer or leaner state.

    A common guideline is that LTFT should remain within a range of 10% positive or negative.

    To give you some real-world perspective, I have tested many well-maintained, higher-mileage engines with a positive LTFT within the range of 5 to 7%.

    However, the data graph from this Camry tells a very different story. Its engine computer has pushed LTFT far into the positive range — a whopping 19.49%.

    The first unavoidable conclusion is that this engine control computer has not set a DTC yet. Nonetheless, it has been making a major correction to the air/fuel mixture.

    The second unavoidable conclusion is that one or more problems have tricked the engine control computer into richening the air/fuel mixture.

    The third conclusion is that this job demands additional diagnosis. Here, a tech should begin mentally cataloging common conditions that would prompt an engine computer to richen the air/fuel mixture.

    Acceleration test

    A brief acceleration test is the last step in our quick-check format. The objective is to floor the gas pedal in drive or first gear (manual), accelerating from a standstill up to approximately 5000 RPM.

    As soon as the engine reaches 5000 RPM, quickly release the gas pedal and touch the button on the scan tool. Then pull off the highway or return to the bay.

    OK, suppose the car owner provided little or no useful description of the vehicle's ailment(s). First of all, a sudden "accel" test provides is a useful, seat-of-the-pants impression of the vehicle's overall performance.

    For example, a car that accelerates smoothly, consistently and briskly suggests that its basic mechanical condition is OK. Here, my mystery Camry performed fine.

    But a car that bucks, hesitates and/or misfires reinforces the need for additional diagnosis. It also may point to mechanical trouble inside the engine.

    Second, this road test reveals the MAF sensor's reaction to sudden acceleration. (The vast majority of vehicles rolling into your service bays utilize a MAF sensor.)

    For instance, repeated acceleration tests on this genre of Camry have yielded MAF sensor responses within the range of 85 to 90 grams per second.

    Please check the accompanying line graph for MAF sensor grams per second: It shows an anemic 68.75 grams per second. Air flow is abnormally low for some reason.

    On the one hand, abnormally low air flow could suggest a major restriction somewhere. Likely, a severe restriction would impair acceleration.

    On the other hand, experience has shown that feeble air flow values — coupled with a severe positive LTFT correction – strongly suggest a dirty MAF sensor.

    Contaminated MAF sensors are relatively common; you often can rejuvenate the sensor by cleaning it properly.

    I cleaned this Camry's MAF sensor with one of the popular, professional cleaners and returned the car to its owner. Due to time constraints, I could not retest the car until the following week.

    For one thing, the car's LTFT was a meager correction of only 2.31% positive. For another, the MAF sensor signal measured 86 grams per second during the sudden acceleration test.

    Last but not least, the owner finally admitted that the original concern — never clarified — had been poor fuel economy.

    To wrap up, there surely is more than one way to diagnose a no-code drivability condition. Critics may nitpick the methods I have described here.

    But the key takeaway is that a brief but highly focused road test may help a tech eliminate some potential problems outright.

    What's more, it may prompt the tech to begin categorizing more-likely causes versus less-likely ones before he even returns to the shop.

    Ultimately, this kind of thought process fosters more-accurate diagnoses more often. That means dollars and sense to any business performing auto repairs today.

    Related Article
    Marinucci: Road test may help fix problem the first time
    Marinucci: Basic steps to email efficiency
    Marinucci: Embrace visual inspections of problem vehicles
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