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.
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.