Another baffling P0171 code - Toyota Nation Forum : Toyota Car and Truck Forums
Corolla 8th Generation (1998-2002) Specific discussion of the 8th generation

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post #1 of 17 Old 04-10-2019, 02:43 PM Thread Starter
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Another baffling P0171 code

I have a 2002 Corolla. *About three months ago it was giving three codes. *One for a bad catalytic converter, the current P0171 lean code and the other for a misfire on cylinder #1. * Fixed the misfire code and the cat code. *

I’ve done all the inexpensive fixes myself….PCV, *Valve cover gasket, new spark plugs, fuel injector cleaner, new air filter, new intake manifold gasket etc. *No real improvement though maybe a couple of percent.

My long term fuel trim at idle during the beginning was around 40. * Took the injectors out about two weeks ago and manually cleaned them and replaced them with new rubber seals. *Long term trim went down to around 33 at idle. *Took the car to what I was told was an excellent diagnostic mechanic. *They did a smoke test and some other stuff and told me to replace the MAF and the upstream O2 sensor. *With high hopes, I replaced those with Denso parts but no change to the long term trim…still hangs out around 32 at idle. *

Interestingly, when I rev the engine to 2500 rpms, the long term trim will go down and hang out around 17-20.*If I push the rpms beyond to 3000+, the LT fuel trim goes back up to around 35…sometimes even 40. * So fuel trim improves around 2500 rpms then gets worse again around 3000 rpms. *All I can think is possibly bad fuel injectors or a week fuel pump. *Or both?*What are your thoughts? Could a melted cat also be the problem?

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post #2 of 17 Old 04-10-2019, 02:52 PM
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Fuel trim that gets better with higher RPMs points to a vacuum leak. Clamp off the vacuum hose going to your brake booster and observe your fuel trims. It is often overlooked.
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post #3 of 17 Old 04-10-2019, 02:58 PM Thread Starter
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I did try clamping off the brake booster but to no avail. One interesting thing I found was when I open the air filter housing while the car is running, I can hear something that sound like a hum with a tonal quality like a horn. Not really loud but its obvious. When I shut the air filter compartment back, I can still hear it but only faintly. Is this normal?
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post #4 of 17 Old 04-10-2019, 03:07 PM
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Humming is harmonic beat from two different air-flow streams making two different frequencies of sound.

Smoke test is bust because intake is always under vacuum (unless you have turbo). Any smoke you put into engine will never leave out vacuum leaks because engine is sucking air IN through leak.

Proper way to find intake vacuum leak is to plug exhaust with potato, banana or cork. Then pressurise intake to 2-3 psi so there is positive presssure in intake. Then squirt soapy water around every joint, junction, pipe, hose-clamp after MAF. Don't forget every injector O-ring and intake-manifold gasket. Especially on back side underneath intake-manifold. Need to crawl under car with strong flashlight with mirror-on-stick to squirt and inspect that area.

Without measurement numbers, MAF cannot confirmed as bad. Vacuum-leak will throw off MAF readings because a lot of air the engine's sucking in isn't being measured by MAF. Replacing perfectly working MAF (or any other part) with brand-new perfectly working part will change nothing.
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post #5 of 17 Old 04-10-2019, 03:17 PM
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* Fixed the misfire code and the cat code. *
Do you recall the cat code and how did you fix it.


Can you post the MAF in g/s along with 02v/ engine load/fuel trims at idle/ 2500/3000?


Did mechanic provide any diagnostic info that led them to believe changing MAF and upstream O2 would fix the lean condition?
Personally i believe he owes you, at least for the misdiagnoses.
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post #6 of 17 Old 04-10-2019, 03:24 PM
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Quote:
*If I push the rpms beyond to 3000+, the LT fuel trim goes back up to around 35…sometimes even 40. * So fuel trim improves around 2500 rpms then gets worse again around 3000 rpms. *
Sorry I missed this on the first read. Looks like you have restricted injectors. Fuel pumps aren't typically an issue on these Corollas.

However, I would check two things before replacing injectors:

1. MAF: Check your MAF values at 2500 and idle. They should be:

Condition . Air flow rate (gm/s)
Ignition switch ON (do not start engine) . 0 gm/s

Idling : 4 to 6 gm/s
Running without load (2,500 rpm) : 13 to 20 gm/s
From idling to quickly accelerating : Air flow rate fluctuates

2. o2 Sensor: Make sure your upstream o2 sensor flips between rich and lean at .8 and .1 volts on live data
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post #7 of 17 Old 04-10-2019, 03:31 PM Thread Starter
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Fixed the misfire code with new sparkplugs. They were all pretty bad. Also, the sparkplug wells were full of old black oil which I think was also causing areas for the spark to find a different path so I replaced the valve cover gasket. Not sure if it is what fixed the catalytic converter code but I had read to run a couple tanks of high octane fuel to clear up a cat code. That cat code has gone away but I have no way to prove the high octane fuel is what did it.

Edited for clarity

Last edited by davidography; 04-10-2019 at 03:38 PM.
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post #8 of 17 Old 04-10-2019, 03:37 PM Thread Starter
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Thanks Doctorbee, I will make a note of the MAF readings tonight. Could I test using a single new injector before replacing all four? Would a single injector provide any improvement that could determine whether or not to purchase the rest?
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post #9 of 17 Old 04-10-2019, 03:52 PM
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Quote:
Originally Posted by davidography View Post
Thanks Doctorbee, I will make a note of the MAF readings tonight. Could I test using a single new injector before replacing all four? Would a single injector provide any improvement that could determine whether or not to purchase the rest?
That is possible. I'm not sure how much of a difference you would notice. If you're tight on cash I would try used DENSO injectors. This seller seems highly rated:

https://www.ebay.com/itm/Toyota-Coro...frcectupt=true

Last edited by doctorbee; 04-10-2019 at 03:54 PM.
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post #10 of 17 Old 04-11-2019, 04:32 PM
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I'd disagree about the fuel pump not being an issue, most of the 8th gens with high mileage that I've come across have had a failing OE Denso fuel pump, and replacement with a new Denso pump was an easy fix.

I haven't seen a fuel pump be the sole reason for running lean or a P0171 code, but it is possible for a weak pump and dirty/clogged injector to combine and be a limiting factor to getting enough fuel to one cylinder, at least enough to throw a code, even if it's no happening consistently or just under heavy acceleration.

Dependiing on mileage, a new fuel pump can be worth it anyway, just for the restoration in performance/acceleration, plus you can keep the old one as a spare.


Usually the P0171 is an air/vaccum leak, so you really need to check and double check and triple check ALL the hoses going to the throttle body, air cleaner, etc. Sometimes there's a cut in the rubber or even just cracks in the end of a hose that allow air in. Just like with the intake manifold gasket, sometimes the paper thin factory gasket for the throttle body needs to be replaced with a newer one to give an airtight seal.

You might also make sure you're running OE parts as necessary. As in, only Denso or NGK Iridium spark plugs. OE Toyota PCV valve. Maybe even Toyota or Denso air filter in case a cheapo brand one is overly restrictive. Sometimes aftermarket parts for fuel/ignition/etc. give weird problems and codes that go away when the OE stuff is used instead.
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post #11 of 17 Old 04-12-2019, 08:49 PM Thread Starter
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Thank you everyone for your input. I've not yet been able to do more testing but hopefully I will get to this weekend.

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post #12 of 17 Old 04-14-2019, 07:53 PM Thread Starter
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Ok. I was able to do some testing.

At idle around 750rpm it was averaging 2.3 g/sec

At 2500rpm it was averaging 7.3 g/sec

At 3000rpm it averaged 9.2 g/sec

These figures are in grams per second based on the car scanner app I am using.

I couldn't seem to get a reading with just the key turned on without the car running.

The O2 sensor voltage seemed to be switching high and low at the rate it should.

What are your thoughts on these figures?

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post #13 of 17 Old 04-14-2019, 11:04 PM
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MAF reading at 3000 should be at least 20 g/s.


One interesting thing I found was when I open the air filter housing while the car is running, I can hear something that sound like a hum with a tonal quality like a horn. Not really loud but its obvious. When I shut the air filter compartment back, I can still hear it but only faintly. Is this normal?"



Low MAF reading can be from restricted intake.
Intake to/from filter clear?


Restricted exhaust can also cause low MAF.
Back pressure test?


Finally MAF itself can be bad.
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post #14 of 17 Old 04-14-2019, 11:17 PM
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There's an error in the FSM - MAF should be 13-20 g/se at 2,500 RPM.

with fuel trims so high it is unlikely to be a restricted exhaust.

Make sure your air filter is clean.

if it is then you probably need to replace your MAF.
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post #15 of 17 Old 04-14-2019, 11:40 PM
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- clean the MAF
- check the air filter
- spray water all over the engine to check for vac leaks

if you still have the code, move on to getting some FP readings





1995 2.2L Camry 5sfe engine
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