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How to efficiently perform diagnostic test drives

Monday, April 1, 2019 - 06:00
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Understanding the data

Say we have naturally aspirated vehicle that has a hesitation under load/low power complaint. The condition can be duplicated and during the test drive the oxygen sensors go low instead of high during WOT, the VE calculates at 77 percent, the MAP reaches very close to Baro, and the fuel trims were overly positive at cruise, going into Open Loop under heavy acceleration. These things should lead you to the following conclusions. First, the engine can breathe properly — the VE reflects this — so no clogged cat. Second, the intake is not restricted — the MAP reflects this — so a dirty air filter is out of the question. Third, the fuel trims point to a lean condition. And finally, the oxygen sensors not going full rich point to a fuel delivery issue. This is where I want to concentrate my testing efforts. Perhaps a fuel pressure and volume test as well as current ramping the fuel pump would be the next diagnostic steps to take. 

Good MAP making Baro - poor fueling

Here's our second scenario. A similar vehicle to the one above with a similar complaint of a hesitation. The test drive reveals the oxygen sensors are falling short of full rich, the VE is calculated at 58 percent, the MAP makes Baro and the fuel trims under cruise are erratic and appear to” follow the throttle blade” with their positive trim trends. While one might find the vehicle may exhibit some of the same drivability symptoms, what is it that differentiates this failure from the previous example? The answer is VE. Does a failed fuel pump affect the engine’s ability to breathe? It does not. So, what could cause a hesitation compliant that mimicked a fuel delivery issue and reflected itself as low VE. 

Good MAP making Baro

The answer is an air measurement error. What measures the air entering our engine and its PID is mission critical to the VE Calculation? The MAF sensor. It has been my experience that most MAF sensor failures tend to overestimate airflow at idle and underestimate airflow under load.  While a failed MAF doesn’t affect the engine's ability to breathe, its does affect the VE calculation. It’s the old Garbage In = Garbage Out (GIGO) formula.

Bad MAF fuel trims

Now let’s switch things up a bit and look into some issues that may very well exhibit some of the same drivability symptoms and are revealed in the flatrater test drive by low VE. They are issues that deal with engine breathing restrictions. The first is relatively common to low power/hesitation complaints — the exhaust restriction.

After duplicating the complaint and analyzing my snapshot, I calculate my VE and it's low at 65 percent. Looking at my other three key data points, I want to be mindful of the oxygen sensors going full rich and making almost 900mv. Second, my MAP sensor at WOT reads very close to Baro at 97kPa. Finally, my fuel trims appear to be irregular but less so than the bad MAF example mentioned earlier, trending towards the negative rather than the positive. These all point me in the direction of a restricted exhaust/clogged cat, which I now can focus my testing efforts on, using either a backpressure gauge or an in-cylinder transducer.

Restricted intake

Again, VE is calculated and appears to be low, say 58 percent. The review of the other three things show that the O2 went full rich over 850mv revealing no issues with the fueling of the engine. The fuel trims are very close to normal, in this case total trim being +3 percent. But the what was the third thing we wanted to analyze? If you are thinking the MAP sensor’s behavior under WOT, you are absolutely correct! This is the telltale; the MAP usually falls short of making Baro or may make it for a brief moment but then falls away from it. I like the description the MAP will “ratchet away” from Baro. These things and low VE all point to a breathing issue with the engine but on the intake side. You may be asking how can the intake get restricted? The obvious cause is a SEVERELY neglected /clogged solid air filter. I have also seen those supermarket plastic bags get sucked up in the air cleaner box and cover the air filter, thus restricting the intake. 

Good Snap-on enrichment capture

I tell techs repeatedly in my classes that the most powerful tool in their diagnostic arsenal doesn’t rest on the tool box — it rests on their shoulders. Analyzing scan data is doing just that, using the most powerful diagnostic tool you have. It illustrates the old adage that states “Work smarter, not harder!” I think that we as techs have been test driving cars ever since we have been working on vehicles, the crux of my argument is if you are going to test drive a vehicle do so with a scan tool hooked up to it. Moreover, if you are test driving a vehicle with a scanner hooked up RECORD A SNAPSHOT! Our scan tools have gotten so much more powerful than years passed, and few techs tend to take advantage of all their features. The scan tool allows me the garner as much as info as I can with the least amount of effort. As with anything in automotive there are exceptions, you can’t say always and works on everything. However, speaking from my experience, the flatrater or diagnostic test and recording a snapshot and calculating VE and be mindful of how my O2s, my MAP (if equipped) and how my fuel trims behave has served me well over the years.  I hope you are using these techniques or are open trying them and it helps you streamline your diagnostic process and point you in the direction you need to focus your testing!

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