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Anthony
Anthony, ASE Certified Technician
Category: Toyota
Satisfied Customers: 11390
Experience:  ASE Master Automobile Technician with over 20 years exp. in this field.
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PO171 System Too Lean (Bank 1)

Resolved Question:

PO171 System Too Lean (Bank 1)
Submitted: 2 years ago.
Category: Toyota
Expert:  Anthony replied 2 years ago.
Hello! Welcome to justanswer!

How well does it run?
Customer: replied 2 years ago.
It always starts, and runs smoothly. In the winter, I noticed that after cold start, it sometimes lacked acceleration (bogged down when I hit the accelerator) for 5-10 minutes. But it does not misfire or drift. The fuel mileage is good (~30mpg), although that might have to do with the elan burning.
Expert:  Anthony replied 2 years ago.
Ok, so you changed O2 sensors and made sure there are no vacuum or exhaust leaks, right?

Also have you ever changed the Airflow Sensor?
Customer: replied 2 years ago.
Ah yes! Sorry. Two years ago, that was the issue. It was dirty, and I cleaned it--worked fine. Then the second year, I cleaned it--didn't help. Then I changed the MAFS--no effect. It's a real puzzler.

I can't guarantee there is no vacuum leak. We sprayed all major connectors with carb clearer and the engine did not idle up noticeably. It's hard to rule out a vacuum leak entirely. Is there another test?

Analyzer data:
load value: 28.20%
coolant temp: 87
short term fuel trim 1: -3.20
long term fuel trim 1: 15.60
engine rpm: 1528
ignition timing advance: 7.50
air intake temp: 59 F
air flow rate: .96
throttle position: 11.00
ox sensor bank 1: .61
ox sensor bank 2: 0.00
no misfires indicated
injector pulse width: 3.90
Expert:  Anthony replied 2 years ago.
When did you change the MAF?
Customer: replied 2 years ago.
Last July, when I was trying to get it to pass emissions. I failed, and after spending $400 on parts and repairs they waivered me. As far as I know, the MAFS works. Cleaning or replacing it made no difference at all. The only other thing I can think of to do now is change the gas cap. Maybe it's leaking...
Expert:  Anthony replied 2 years ago.
Its not that, or it would be rich. See the Long Term Fuel Trim Reading? See how it is 15.6? Its supposed to be plus or minus 10 max. What that equates to is

1. Low fuel pressure
2. A leak in the exhaust manifold before the O2 sensor, front.
3. Bad MAF.

Try this. Unplug the MAF and drive it. IT will run bad. But let it set its code and upload me the same snap shot you just did with the MAF unplugged.
Customer: replied 2 years ago.
Oh, sorry, I don't currently have access to an analyzer.
Sounds like you think it migtht be the MAFS. But what is the change that the remanned MAFS I bought was bad? Slim.
Expert:  Anthony replied 2 years ago.
Yes I am. Reman MAF's have an extremely high failure rate. So try another one.
Customer: replied 2 years ago.
No offense, but "replace the part you already replaced" is a longshot that will cost me a minimum of $90. That means the last one was bad out of the box (very unlikely). Should I go ahead and change both oxygen sensors, just in case they were bad too? Well, thanks--I think.
Expert:  Anthony replied 2 years ago.
No, the O2 sensors are good. If you want to be sure about the maf, have the intake smoke tested and the fuel pressure tested. Then you will know the MAF is bad.
Customer: replied 2 years ago.
OK, thanks for clarifying. I think this needs to be handed over to a tech now--I don't have the tools.
One last thing: Should I bother changing the gas cap? Seemed like a longshot, but a cheap one.
Expert:  Anthony replied 2 years ago.
No problem. It can not be the fuel cap. Only low fuel pressure, vacuum leak or a bad MAF. Has to be one of the three.
Anthony, ASE Certified Technician
Category: Toyota
Satisfied Customers: 11390
Experience: ASE Master Automobile Technician with over 20 years exp. in this field.
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