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Old 02-10-2021, 09:16 PM   #1
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8.1L fuel trim out of spec?

I have a 2004 P32 Holiday Rambler with the 8.1L V8, 33000 miles. It has just failed the California Smog test for high hydrocarbons at idle- hi spec is 150, mine tested 160. Ran well within spec at high rpm. I had just adjusted my scan gauge 2 to read LF1, LF2, SF1, SF2. SF1/2 is running about 3+-3. LF1/2 is running around 7 most of the time, once in a while down to 5 or 6, and running up to 9-10 fairly regularly. Both banks run the same within 2 points most of the time. Would these fuel trim readings indicate the engine is running slightly rich, or lean? Seems to me a + reading should indicate the injectors are running rich most of the time. Or do I understand the readings backwards? Positive readings meaning it’s running lean. I don’t have a good RV/Workhorse service center near me (Santa Rosa, CA- 60 mi N of San Francisco).

The engine has had a new MAF about 18 months ago, Brazel’s UltraPower Magnum wires/plugs at the same time. New fuel pump 14 months ago after the infamous “P32 engine quits during a hard climb” problem.

So I have two issues: 1) Pass the Smog test at idle, 2) Make sure the engine is not running lean.

TIA,
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Old 02-10-2021, 10:14 PM   #2
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Did you use a genuine Delco brand MAF? Also the first thing I would try since you are so close to passing is cleaning the MAF with CRC brand MAF cleaner.
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Old 02-10-2021, 10:17 PM   #3
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Did you use a genuine Delco brand MAF? Also the first thing I would try since you are so close to passing is cleaning the MAF with CRC brand MAF cleaner.
X2....
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Old 02-11-2021, 08:41 AM   #4
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High hydrocarbon emissions indicate a misfire. You do not have a misfire indication on your Scan Gauge suggesting that the misfire is random cylinders. I that case I would suspect a small vacuum leak.

A small leak might be enough to cause random misfire that would dump unburned fuel into the exhaust. I would go look for a vacuum leak.
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Old 02-19-2021, 09:34 AM   #5
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i just drove close to 200 miles in mine yesterday. First long drive since I got the SCII. My LTFT reads zero or close to it at idle but around +4 to +7 most times at light throttle. It reaches +11 or 12 climbing grades, but if I let it downshift it drops back down to just under +10 at higher revs.

i just cleaned my MAF last week. These numbers seem high to me, but are not much different from what you're seeing. My coach has 17k miles on it.
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Old 02-21-2021, 06:26 PM   #6
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i just drove close to 200 miles in mine yesterday. First long drive since I got the SCII. My LTFT reads zero or close to it at idle but around +4 to +7 most times at light throttle. It reaches +11 or 12 climbing grades, but if I let it downshift it drops back down to just under +10 at higher revs.

i just cleaned my MAF last week. These numbers seem high to me, but are not much different from what you're seeing. My coach has 17k miles on it.
I would not be happy with fuel trims that high while the engine is under the load required to pull grades. Your a/f mixture is going lean and the computer is adding fuel to keep it at the correct mixture. Normally you don't want to see any fuel trim numbers greater than 10, especially pulling grades
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Old 02-22-2021, 11:25 AM   #7
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I would not be happy with fuel trims that high while the engine is under the load required to pull grades. Your a/f mixture is going lean and the computer is adding fuel to keep it at the correct mixture. Normally you don't want to see any fuel trim numbers greater than 10, especially pulling grades
Tom
Should I try replacing the MAF? Seems to be high on both banks.
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Old 02-22-2021, 06:58 PM   #8
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It is the TOTAL Fuel Trim that you need to consider.

You need to look at STFT as well as the LTFT, you will add both of those numbers to get a the fuel trim reading

Its not just about LTFT.

If LTFT is 7 and STFT is -2 then your fuel trim is 5

Here is a short article about fuel trims.

https://www.autoserviceprofessional....for-you?Page=1

https://www.mechanic.com.au/news/und...rm-fuel-trims1
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Old 02-22-2021, 10:27 PM   #9
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It is the TOTAL Fuel Trim that you need to consider.

You need to look at STFT as well as the LTFT, you will add both of those numbers to get a the fuel trim reading

Its not just about LTFT.

If LTFT is 7 and STFT is -2 then your fuel trim is 5

Here is a short article about fuel trims.

https://www.autoserviceprofessional....for-you?Page=1

https://www.mechanic.com.au/news/und...rm-fuel-trims1
Thanks for that. I had thought of STFT as being instantaneous and LTFT as being an average, but I see that's not quite right. I'll program STFT into my ScanGauge and see what we get. In the meantime, changing the fuel filter can't hurt in any case.
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Old 02-23-2021, 06:26 PM   #10
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Thanks for that. I had thought of STFT as being instantaneous and LTFT as being an average, but I see that's not quite right. I'll program STFT into my ScanGauge and see what we get. In the meantime, changing the fuel filter can't hurt in any case.
Here are a couple of pictures of my STFT and LTFT, these are with the RV not moving but increasing the RPMs. I had replaced the MAF about a year before the pictures, before that, I had cleaned the MAF several times but my numbers where still a little high, I just opted to change the MAF out.
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