Scope & Scan - Service Repair

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Vehicles from the TST bays

War stories and lessons learned
Monday, November 2, 2015 - 09:00
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Our first vehicle is a 1999 Toyota Rav 4 2.0L with an automatic transmission that came in with the customer concerns of running rough, stalling and an illuminated Malfunction Indicator Lamp (MIL). When we connected our scan tool to the vehicle, we found two Diagnostic Trouble Codes (DTCs); a P0171 (System Lean – Bank 1) with Freeze Frame data and a P1130 (Air Fuel Ratio sensor circuit performance malfunction) (Figure 1). Our visual inspection under the hood revealed that the AF sensor had already been replaced. With the scan data readings we saw for the P0171 and the AF sensor voltage, one could question if the AF sensor was defective or the wrong one for this Rav 4. We always want to test before we ever condemn any component and in this case, I wanted to see if the AF sensor could distinguish between rich and lean.

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But before we move on to testing components, we should first take a hard look at the Freeze Frame data to better understand the conditions that were in play when the code was set. My tech (Bill) had already started to perform a bunch of tests that should have been performed only after reviewing the Freeze Frame data. So Bill and I reviewed the Freeze Frame data together and discovered an important clue as to what may be causing the lean condition. I asked Bill to accelerate and decelerate the engine as we monitored the Bank 1 downstream oxygen sensor (B1S2) voltage.

That split-second shift from fully closed throttle to wide open throttle should cause a momentarily lean mixture to pass through the engine, and when the throttle is abruptly closed again, a rich mixture should be present. The rich and lean mixtures going through the catalytic converter should have caused a shift in the downstream sensor voltage, but we did not see any change. Bill was surprised to learn that the rear oxygen sensor could have any control on fuel delivery. He thought that the rear sensor’s job was only to check the operation of the catalyst. I proceeded to explain that the rear oxygen sensor on many vehicles plays a role in fuel delivery.

Had we found the problem?
The first test that Bill had performed (before our little Freeze Frame session) was a smoke test on the engine to see if there were any vacuum leaks — a not uncommon problem on Toyotas — but none were found. However, Bill had tested the engine “hot” and would need to wait for the engine to cool off to smoke the engine to make sure that there was not an intake gasket contraction problem. The next step was to check the operation of the AF sensor by introducing propane to see if the sensor would respond with a rich shift. The results of the test confirmed that the sensor was able to read a rich condition. Our next step was to introduce a vacuum leak by pulling the power booster hose to see if the AF sensor would respond in the opposite direction. The AF voltage went high and confirmed that the sensor was able to distinguish a lean condition.

Obviously the next step would be to check fuel pressure and more importantly, fuel volume. Both were in specification. With no vacuum leaks found and fuel delivery verified as good, we next checked the load sensor (in this case, an MAP sensor) and the PCV system, as both can cause a lean condition, but both were good. Take a look at the Freeze Frame data (Figure 2) that shows the Long Term Fuel Trim (LTFT) at 44.48 percent while the Short Term Fuel Trim (STFT) was at 24.17 percent. The question to ask is what can cause such a condition that would command such a high fuel delivery?

Figure 1 Figure 2
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