Vehicles from the TST bays

Nov. 2, 2015
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). 

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.

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

We decided to look at the current engine data that still displayed the LTFT at 44.48 percent, along with the AF sensor at 3.15 volts. The AF sensor reading from a normal running engine should be at 3.3 volts at idle and not what was currently being displayed. An important fact to remember is voltage under 3.3 volts is a reading that the engine is running rich, while a reading over 3.3 volts indicates the engine is running lean. Our problem Rav 4 AF sensor confirmed the P0171 DTC that it set in the Engine Control Module (ECM), now all we had to do is find out the root cause.

Remember the Freeze Frame review?

Looking carefully at the current engine data revealed that HO2S B1S2 (the oxygen sensor after the converter) was reading 0.0 volts. As we raised and lowered the idle, we monitored the rear sensor expecting to see a change in the voltage, but it did not. Could a rear sensor cause our problem of high LTFT command? The answer is yes and here’s why: on many engines, the B1S2 sensor has authority to adjust fuel trim. On this vehicle it would have been easy to blame or replace a load sensor or other sensor without finding the cause of the problem. We ordered a new Toyota rear oxygen sensor, installed it and reset the fuel adaptation. Once the repair and the procedure were completed, we drove the vehicle until the monitor was “Ready.” As you can see by the after repair engine data (Figure 3), the LTFT was down to 9.34 percent while the AF sensor voltage reading was at 3.275 volts, just about the perfect 3.3 volts that Toyota recommends at idle.

Next on the list

A 2015 Acura MDX came in from a body shop that I do work for with a problem with a DTC B1E77 Blind Spot (open in the right side BSI radar unit indicator circuit) (Figure 4). The possible causes associated with this DTC are problems with the signal line open, ground harness open, indicator driver failure or right BSI indicator failure. The description tells it all, an open circuit means that something has been disconnected or broken open. The fix for this vehicle was simple thanks to the information provide with the DTC description. The problem was as simple as removing the passenger door (Figure 5) and connecting the open connector then clearing the DTC.  

Figure 3 Figure 4 Figure 5

This type of DTC is becoming more and more popular as many vehicles are being outfitted with lane departure and front and rear radar systems. Take a look at the 2014 Caddy ATS that I had to clear a DTC and reset and relearn the Long Range Radar Module. The process is very involved since it entails more than just reading and clearing the DTC. In fact, the module has to be programmed with the GM GDS and then be relearned while driving the vehicle at different speeds. The vehicle has to be driven for 10 to 30 minutes at different speeds while the radar system makes adjustments as the vehicle approaches other vehicles or objects on the road. Once the system has learned the low speed calibration, the scan tool prompts the operator to accelerate to a higher speed to perform the next calibration. This procedure adds more time to programming many of the vehicles that are on the road using a radar system. Some European and Asian vehicles need special lane departure /radar alignment (Figure 6) tools that have to be used to reset the systems.

Since this is my last regular TST “Scope and Scan” column, I want to thank all of you, especially those of you that have been kind enough to email me your comments. I hope that these articles have been helpful and have shed some light on the problems you find in your bays. Oh, I’ll still be writing feature articles for Motor Age and look forward to some new writers I’ve been hearing about that I am sure will provide all of us with some great information.

Figure 6

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