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Dan, Technician
Category: Toyota
Satisfied Customers: 24584
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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1999 Toyota camry, v6 p0171. No vacuum leaks found. Front

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1999 Toyota camry, v6 p0171. No vacuum leaks found. Front bank running rich, rear bank running lean, swapped af sensors, with known good ones, still there. replaced plugs, how I know whats rich or lean. Erractic misfire, random. compression good, disconnected egr, still the same. erased code, comes back immediately whether driven or not, Block checked system for possible head issue, none found.
JA: Do you have a scan tool to read the codes?
Customer: Just a cheap mac read only scanner, no verus or other handy.
JA: Are you fixing your Camry yourself? What have you tried so far?
Customer: installed 4 new af sensors, but did get them on ebay, Don't want to have to pay 150 to 200 each from denso. Trying to narrow it down, short and long term fuel way out of specs. Oil was changed so that there is no contamination.
JA: Anything else you want the mechanic to know before I connect you?
Customer: Car runs pretty good, 234k miles. Only shows code po171, bank 1 running lean. If it was fuel pressure it would be both banks, also if it was the maf wouldn't it relate to both. New hoses to throttle body intake

Hello, my name is ***** ***** I'm here to help get your problem solved.

The mass airflow sensor usually effects bank 1 at first then eventually bank 2 so that could be the problem. Other possible problems could be brake booster or flex pipe on exhaust leaking. Using hose clamp pliers, pinched off vacuum supply to check valve, and vacuum supply to booster (aft check valve) and monitored fuel trim. Readings returned to normal on short term side and long term slowly returned. Replaced brake booster and check valve. If that doesn't help and you are sure there are no vacuum/exhaust leaks then I would replace the mass airflow sensor. Other possible option is injector but that would most likely cause a misfire. Let me know if you have questions, thanks.

Customer: replied 2 months ago.
No exhaust or flex pipe leak, checked for that.Question if the brake power booster is leaking wouldn't that be noticeable on brake application, such as no assist. How does the maf sensor effect one bank and not the other.Is there two separate electrical lines, 1 for each at the map. I didn't look at the schematic, so unsure how that works.

The brakes wouldn't necessarily be effected depending on size of leak and usually aren't. The mass airflow sensor doens't have a wire for each bank but computer still causes bank 1 to go lean first. Let me know if you have questions, thanks.

Customer: replied 2 months ago.
No I don't need to call you. The steps you have given me are pretty basic. Thanks, ***** ***** just trying to get a head on how the maf effects one cyclinder bank and then the other.

The website offers call, they don't even tell me they are trying to do it. Let me know how it goes or if you run into questions, thanks.

Customer: replied 2 months ago.
Thanks for the info, will pinch off brake booster line and watch the fuel trim, If nothing then will substitute known good maf sensor.

Sounds good, keep me updated. Thanks.

Customer: replied 2 months ago.
Dan, I figured it out. I pinched off vacuum supply at booster and monitored fuel trim, no difference. I tried a maf sensor, still no difference. Then I thought suppose bank 1 is running lean only because it sees bank 2 is running rich. I took out the af new sensor from bank 2 , swapped another one in its place and monitored fuel trim, wala- fuel trim evened out, vehicle runs a lot better, drove it 60 miles, no check engine lite. Problem found.

Having new parts be bad doesn't make diagnosing the easiest. Thanks for update.

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