Vehicle details: Toyota Avensis
Year: 2002
Symptom: MIL on,
P0171,
P0174
Author: Dave Hill | www.londonroadgarage.com

Toyota Avensis | System too lean

Quite a simple one this, but I thought it was interesting.

A 2002 Toyota Avensis came in with the MIL on and two DTCs present: P0171 and P0174 System Too Lean on banks 1 and 2 respectively. On the first visit I was asked to clear the codes (it was a motor trader’s car!) which I did, with the suggestion that it would most likely return.

Sure enough a couple of days later the car is back for a proper look. Fuel trims suggested an issue…

  • Short FT 1 = -18.0%
  • Long FT 1 = 0.7%
  • Short FT 2 = -19.6%
  • Long FT 2 = 1.5%

So out with the PicoScope and time to look at some relationships between both upstream oxygen sensors and the mass airflow meter. These are 1 volt zirconia types and there are 4 in total. Two upstream and two downstream. The MAF is a Denso type.

Here is an overview look at both O2 sensors and MAF. The O2 sensors can be seen to switch nicely at the start of the capture, but as we zoom in, it can be seen that there is a problem when the throttle is “snapped open”.

Next is the more conventional capture that I make regularly. It is pretty clear here that the MAF sensor is performing badly, not just in the fact that the peak voltage is less than 3500 mV but also the “gulp” section of the capture is very weak too. Notice how both O2 sensors remain lean throughout the wide open throttle phase.

Not having seen many of this type of MAF, I am a little cautious about just fitting one without a bit more diagnosis. Other factors can lead to weak MAF sensor output and it is important to rule as many things out as possible. I use a sensor simulator often to help prove a point. This time I am pushing a more expected 4.5 volts down the MAF signal wire at the same time as an assistant performs a “snapped” throttle test.

The boost given to the MAF signal has given the desired response from the oxygen sensors and I feel a little happier about replacing the MAF. I then took an unusual step and decided to have a look at the MAF to see if it was dirty. I was surprised to see the two elements were very dark and dusty. A quick spray of carb cleaner rejuvenated it to a nice shiny state.

Once refitted, the same captures as taken previously were taken again to confirm a fix. Here is the same overview-style capture as before, but now the O2 sensors can be seen to go rich throughout the snap test and they also start to recover quicker afterwards and start to switch again.

Again the classic capture. Showing a good “gulp” and a much healthier peak voltage. The engine revved much more responsively too!

An emissions test proved successful and here are the fuel trim figures now…

  • Short FT 1 = 0.0%
  • Long FT 1 = -3.2%
  • Short FT 2 = 0.0%
  • Long FT 2 = -3.9%

Sorted!

A simple problem by today’s standards, but I feel it is made even clearer with the use of a scope.
Cheers all.

Comments

18 comments | Add comment

Darrel
December 10 2014

thankyou Dave and Carl for a very sound article I really admire the way you kept it cool&factual; so many people start getting abusive when discussing things from slightly varying viewpoints Your O2 captures are identical to the ones I’ve seen,the way the cyclic fuelling changes are made on Toyota tccs is somewhat unusual because I haven’t yet seen one that changes on a one second rich lean cycle,( or maybe all the ones I’ve seen are running on adaptive values)  I too realised a while ago that O2 sensor “switching” should really be called “response”. ie response to oxygen content in exhaust stream. ECU makes a change to injector PWM and looks at the resulting gas through the “eye” or O2 sensor to calculate the next change and also to ensure that there is sufficient oxygen going into the cat to facilitate combustion of any Hydrocarbons that got through the cylinder unburnt

sherif
June 18 2014

I have a question concerning the sensor simulator you use,what simulator do you recommend and where I can buy it.
thanks

B.
November 16 2012

Dave Hill,

I have a question concerning one of Carl’s staements:

“If technicians are to form some conclusion from this throttle opening event, then there should be some set of guidelines to adhere to. One trainer might show a spike and the next may not. It depends on how they do it. I’ve seen it both ways. One trainer might bring it up on the revolution limiter, and another might just snap the throttle as quickly as possible without the RPM raising much at all. These results would be different.”

The data you show here is 3 channels (MAF, O2-B1S1 & B2S1). The scope being used to diagnose and gather this information on this post has 4 channels, I’m assuming. Would utilizing the 4th channel and tapping into Throttle signal help identify the problem, say, if I recorded the data and had you or Carl analyze it? You’re aren’t there with the car, so all 3 of those channels could help you remotely diagnose the vehicle?

Brendan
March 05 2011

i have a avensis 03 model and engine management light is on. i have cleaned MAF and no luck so i have changed Sensor and engine running alot better but engine management light is still. Got it reset and it came on again showing fault PO 171. any others ideas. its been great reading here as im not a mechanic

mick c
November 20 2009

isn’t this the 1zz-fe engine?

Tony s.
September 13 2009

System lean issues on Toyotas with DENSO maf are common. Remove the 2 screws and inspect the heated wires - it will show a badly comtaminated sensor. A quick clean will repair the fault every time.

Carl Grotti
December 02 2008

Hello Michael,

I would also like to add that the way this test is carried out on opening the throttle is of much importance before making a decision. I’ve been involved in hands-on demonstrations of this from many different Educators/Instructors/Technicians and almost always the technique isn’t the same.

If technicians are to form some conclusion from this throttle opening event, then there should be some set of guidelines to adhere to. One trainer might show a spike and the next may not. It depends on how they do it. I’ve seen it both ways. One trainer might bring it up on the revolution limiter, and another might just snap the throttle as quickly as possible without the RPM raising much at all. These results would be different.

I’ve diagnosed many MAF sensors relying on the spike you refer to, but a very quick snap of the throttle to allow this in-rush of air to evaluate it needs to be done. Otherwise, I have no idea of what it should be at the peak.

As a side note, I’ve not used a scope in awhile to condemna MAF sensor. Usually I have enough scan data to form a conclusion. Don’t get me wrong, there have been times when a scope was needed because the scan data was too slow to show a fault.

Regards,
Carl

Mike
November 29 2008

Dave,

This code is fairly common on several Toyota models in the states. I have seen them on Camry, Avalon, Corolla, Echo and Tundra. Toyota used that type of MAF (Denso) on many of their products. Cleaning the wire will fix it. However the cause of the problem is the potting compound that the MAF chip is encased in. Over time the heat causes the compound to melt and it will bleed down the hot wire where it collects the dirt. You can also tell if it is leaking because it will have an oily residue around the outer casing of the chip housing. I have also ran into the situation where cleaning it didn’t fix it and the MAF had to be replaced or that it came on a few months later because the potting compound had leaked again. Anyway, good article. I always enjoy them.. Helps me to think “outside the box”.

Best Regards,
Mike

Michael McDonald
November 29 2008

From what I have seen of Toyota MAF sensors if they do not produce an initial “spike” voltage on the initial throttle opening is is an indication that it is contaminated. This is particularly visible in figure 2. A common result of this contamination is a rich condition at idle and lean conditions at higher speeds.

Technical Trainer - Standard Motor Products, Inc.

Carl Grotti
November 28 2008

Dave,

You are correct. We are taking about the same thing. The Chicken or Egg is a good analogy since we are referring to a closed loop system. My point was that protecting the cat takes high priority by the original equipment manufacturers (OEMs). This is what I meant by “goal.”

Some OEMs have become wise to avoid exceeding the 1.5 times the FTP standard by disabling injectors when a misfire is present. This is an easliy estabished practice through software since CID is already there. In the future you may see more OEMs doing this to protect the cat. The downside to this is that the OEMs do not have to illuminate the MIL because emissions have not exceeded 1.5 times the FTP standard. One problem this presents is the consumer may continue to operate their vehicle in a misfire state with no MIL illumination. The upside to it is the cat will be ok by disabling injection.

Thank you for the exchange, Dave. I’ve enjoyed it. Feel free to post on the forums if you ever feel a need. wink

ROGER JORDAN
November 28 2008

As usual David a high quality informative post, many thanks for sharing it with us, like a lot of people I find Maf meters can be very difficult to pin down, there are so many variables, much careful diagnosing with the scope called for !!
Best regards Roger.

Carl Grotti
November 26 2008

Hi again Dave,

You seem to have a good understanding of fuel control. It is much better than a lot of folks working on these vehicles. wink

I have a little different view of what you mentioned here:

“Short term fuel trims tend to show what the O2 sensor is doing “now” & long term trims showing the “historical” trend that the system has calculated is necessary to keep the O2 sensor switching.”

The goal isn’t actually to keep the O2 sensor switching, that is just a result. The goal is to keep the engine-out gasses as close to stoich as possible (which for gasoline is 14.7:1) for best catalytic converter operation.

Also, I’m a little surprised the LTFT values were normal. Since MAF sensor contamination is usually a progressive process, these numbers are quite often higher. Not always the case, though. Something as simple as an insect can do it in an instant.

Carl  

 

Dave Hill
November 26 2008

Hi Carl

Thanks for the kind comment grin

Interesting to see your interpretation of fuel trim…...

“The goal isn’t actually to keep the O2 sensor switching, that is just a result. The goal is to keep the engine-out gasses as close to stoich as possible (which for gasoline is 14.7:1) for best catalytic converter operation.”

I would have thought that your explanation was the same as mine, just worded differently. Is O2 sensor switch purely a “result” of the stoichiometric exhaust gas state, or the cause of it?

Chicken or Egg???

Great to exchange views.

Thank you!

Dave Hill

Dave Hill
November 22 2008

Hi Carl

Interesting stuff & thanks for sharing it.

As you say “You didn’t mention what the conditions were when observing the fuel trim data. This is very important”. Very true & in this case it is relevant, the DTC’s were still present & the system was running in an open loop state & as such was running under default fueling values I think.

I must admit that I expected (as I suspect that most folks would) to see the long term fuel trims showing that fuel is being added, so as to maintain a stoichiometric state & a thus a switching oxygen sensor. Short term fuel trims tend to show what the O2 sensor is doing “now”  & long term trims showing the “historical” trend that the system has calculated is necessary to keep the O2 sensor switching.

There is another train of thought however & that is that the ECU sees a lesser amount of air inducted than is actually entering. It therefore fuels according to what it is seeing & of course, the O2 sensor reports a lean state. The ecu will then increase injector durations to bring the air/fuel ratio back on track. The increase in fueling will then add fuel based on preset maps. The trouble is that these preset maps are designed around a system that is functioning normally, when in this case, the MAF sensor is reporting less air than is “actual”, so the added fuel is disproportional to what the program has allowed. This could in theory upset the figures seen. There is also the possibility that the diagnostic tool has displayed the parameters incorrectly (not hard to believe these days).

I make an effort whenever possible to take data snapshots of known good cars & save them for the record. As part of our routine service we will perform a “scan” & this gives us a good opportunity to gain some useful data. I will always perform a snapshot at fast idle & then idle (engine at normal operating temperature & purged)

As you have said, the fuel trim figures alone are not enough to draw any conclusions from, but they do indicate a shift from the normal & taken along with all other information gathered by testing, a diagnostic approach can be decided upon.

Thanks again for your input.

Regards

Dave Hill

Dave Hill
November 21 2008

Not a typo Carl honest! Those figures are copied & pasted directly from the data snapshots saved at the time. I do agree however that they look misleading.

Worthy of a debate on the Pico Forum maybe?

Kind Rgards

Dave Hill

Carl Grotti
November 21 2008

Hi Dave,

You didn’t mention what the conditions were when observing the fuel trim data. This is very important. I’m not going to place much faith in idle fuel trims for condemning a MAF sensor. What we need to focus on are idle and cruise (load) trims. If your trims were in fact in the negative range at idle, then there might be an explanation for that. The “freeze frame” data should have shown what condition the vehicle was being operated at near the time of fault code triggering.

Here is more on how you can have trims in the negative range at idle and positive off idle if that is what you seen:

When a MAF hot wire gets contaminated, there is a layer on it which increases its diameter. This larger surface area will radiate more heat at idle. Effectively, current will rise and there you have over-reporting of airflow. If this happens, more IPW is requested by the ECU and now the O2s report this as rich and your trims will be negative.

This heat transfer function is no longer affected at higher airflows by this surface area. Instead, this coating impedes the heat transfer function. Therefore, current decreases and now the MAF sensor under-reports the incoming air and now you have a lean condition as indicated by positive fuel trims.

I hope you find this informative and don’t hesitate to ask if you have any questions.

Regards, 

Carl

Carl Grotti
November 20 2008

Short term fuel trim on bank 1 = -18%
Short term fuel trim on bank 2 = -19.6%

This would indicate a rich condition instead of lean. I’m sure this was just a typo.

Good article other than that.

Gerardo Vazquez F BnB Auto
November 20 2008

Good Results and good resolutions

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Case study: System too lean