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Old 10-06-2019, 05:43 PM   #1
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LTFT (Long Term Fuel Trim) too high - What to check?

If someone can help me solving a lean condition issue, I'd appreciated!

I bought a 2002 Ford Focus ZTW (Zetec, at 100k miles) a month ago, and I noticed OBD2 always reads LTFT for both bank 1 and 2 reading pretty high at 10-25%. 10-15% at idle, and goes up at higher RPMs. I haven't seen any trouble codes yet, but gas mileage hasn't been very good. I've ruled out vacuum leak since LTFTs don't improve at high RPMs. The both banks read similar values, so I've concluded the cause is either a dirty/faulty MAF sensor or fuel delivery issue.

I cleaned the MAF sensor with the CRC MAF sensor spray a few days ago with no noticeable improvement.

Today I unplugged the MAF sensor, disconnected the negative battery pole for 10 minutes and started the car. While on idle, OBD2 continuously read STFT (short term fuel trim) high at 10 to 30%, again with similar numbers for both banks. After 5 minutes of idling, the engine stopped by itself.

I'm cleaning the MAF sensor for the second time, this time more thoroughly with isopropyl alchol as well as spray, just in case.

On the fuel-delivery side, the fuel pump was just replaced a month ago (thus ruling out), and the fuel pressure reading on OBD2 is pretty steady around 40psi. I read that a clogged fuel filter affects fuel volume but not always pressure, and with my car having reached 100K miles, I might as well change the filter now.

I've also noticed a small crack on the PCV hose (longer piece), and since this seems a common issue for the model, replacing the PCV valve and hose is next on the list.

Am I on the right track on diagnosing? Anything else I should look into? I would certainly appreciate sharing your knowledge and experience!


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Old 10-06-2019, 07:27 PM   #2
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Bank 2 numbers means nothing as you have no bank 2. Bank 2 is the other bank of the engine on a V-6 or V-8.

Fix the PCV hose, the most common vacuum leak on those engines.
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Old 10-06-2019, 09:08 PM   #3
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Originally Posted by amc49 View Post
Bank 2 numbers means nothing as you have no bank 2. Bank 2 is the other bank of the engine on a V-6 or V-8.

Fix the PCV hose, the most common vacuum leak on those engines.
Thank you for your reply! Ah, please excuse my ignorance. I see a Dorman hose (part# 46030) for only $16 on Amazon with good reviews, so it's well worth a shot, plus the valve replacement is part of required maintenance anyway.

What I've learned is that vacuum leak makes LTFT go down as RPM increases...? My LTFT goes up as I rev up. In any case, I'll update after replacing PCV hose/valve!

Last edited by smokinhiro; 10-06-2019 at 09:11 PM.
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Old 10-07-2019, 02:53 AM   #4
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Depends on the size of the vacuum leak, they can vary wildly.
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Old 10-07-2019, 02:05 PM   #5
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Depends on the size of the vacuum leak, they can vary wildly.
I see. I did find so many posts here discussing failed PCV hose causing lean condition, so it must be very common. Thank you!
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Old 10-07-2019, 03:18 PM   #6
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Originally Posted by smokinhiro View Post

I've ruled out vacuum leak since LTFTs don't improve at high RPMs.

While on idle, OBD2 continuously read STFT (short term fuel trim) high at 10 to 30%, again with similar numbers for both banks. After 5 minutes of idling, the engine stopped by itself.
FTs below +/- 10 are normal. LTFTs are just that - long term, over many drive cycles - and should not change much just reving the engine. In your case with positive FTs the engine is lean so fuel is being added. I would NOT rule out anything - especially a vacuum leak, either at the PCV hose or anywhere else. I also would not rule out bad (old) O2 sensors. Are your O2s switching normally and have you ever replaced them?

Inline engines only have one bank. Are you sure you have a bank 1 and bank 2 sensor and not a bank 1 sensor 1 and bank 1 sensor 2 (sometimes abbreviated HO2S11, HO2S12 on data PIDs)?

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Last edited by Paul1958; 10-07-2019 at 03:52 PM.
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Old 10-07-2019, 03:38 PM   #7
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You should take the Batt loose about 5 min and back on and be watching the stft not the ltft

The stft is as its happening

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Old 10-07-2019, 03:38 PM   #8
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You also havent listed any mods ??

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Old 10-07-2019, 04:39 PM   #9
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Quote:
Originally Posted by Paul1958 View Post
FTs below +/- 10 are normal. LTFTs are just that - long term, over many drive cycles - and should not change much just reving the engine. In your case with positive FTs the engine is lean so fuel is being added. I would NOT rule out anything - especially a vacuum leak, either at the PCV hose or anywhere else. I also would not rule out bad (old) O2 sensors. Are your O2s switching normally and have you ever replaced them?

Inline engines only have one bank. Are you sure you have a bank 1 and bank 2 sensor and not a bank 1 sensor 1 and bank 1 sensor 2 (sometimes abbreviated HO2S11, HO2S12 on data PIDs)?

Paul
Thanks for your input! Right, I did read that for some people LTFTs didn't change until they drove 100s of miles. For the O2 sensors I get readings from bank1 sensor 1 and bank 1 sensor 2 and no data for any sensors from bank 2. I think they were switching correctly between 0.1 to 0.9v, at least for sensor 1 from what I remember, but I'll double-check and try recording a video of live graphs for both sensors - I'll post it here.
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Old 10-07-2019, 04:50 PM   #10
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You should take the Batt loose about 5 min and back on and be watching the stft not the ltft

The stft is as its happening

Tom
Thanks for your tip! When I took the battery pole off and started afterwards, LTFT was flat at 0% for about 10 minutes and STFT generally kept increasing to as high as 35%, never going into negative.

As I learned in YouTube videos, maybe I could do a carb cleaner test for hunting vacuum leaks - if I hit a leak, that should affect STFT and O2 sensor reading, right?
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