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 A question of fuel trims

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Karma
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Name : Andrew
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PostSubject: A question of fuel trims   Mon Feb 16, 2009 10:08 pm

Got a question here relating to long term and short term fuel trims....

Lately I've been getting some slight engine knock at WOT where once there was none. The KR started slight at only a quick jump of 2-3* but as time goes by has been getting worse and worse. After looking around on a bunch of forums, I'm pretty sure my cause is a weak/failing fuel pump. The pump I intend to replace shortly just to be sure, but the nature of my question is a bit different..

Whilst looking around at fuel related problems I noticed that the LTFT is supposed to be as close to zero as possible and that it *might* range as high as 25... and that an abnormally high number would indicate running very lean with the cause being in the area of fuel pressure, FP regulator, etc...

The thing is, my LTFT at idle is 128... A number i've never heard of being that high...

so besides the normal "drive the car carefully" and "check fuel pressure" (which are all happening whithin the week anyway) anyone have any insight to this? does the fuel trims get handled differently on the obd1 as opposed to the obd2? Has anyone heard of a LTFT number as high as 128-130?

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PostSubject: Re: A question of fuel trims   Mon Feb 16, 2009 11:20 pm

It must read differently with OBDI. OBDII varies from +16.4 (very lean) to -23 (very rich). I have seen both extremes, only when I had a severe problem, and I made sure it got fixed right away.

I wonder though, could it be that your 128 actually means +12.8? Might want to check with the manufacturer of the scanner. Maybe someone else with Series I engine has run into this?

What is the LTFT when you are gently accelerating? Does it get closer to zero? Does it ever see negative?

Good idea to check fuel pressure before replacing the pump. Could be a clogged injector causing the lean condition - if you even have one.

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PostSubject: Re: A question of fuel trims   Tue Feb 17, 2009 10:35 am

First of all, remember that you can't read fuel trims at WOT. Fuel trims are not in effect when you are at heavy throttle. You need a wideband O2 sensor to read WOT fueling.

Try reading fuel trims at idle. LTFT should settle on a number and STFT should get very close to 0.
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PostSubject: Re: A question of fuel trims   Tue Feb 17, 2009 12:35 pm

thats why I was wondering, cause the 128-130 WAS at idle...

looks like with gentle driving they go as high as 130, and as low upon letting up on the petal as 111, but no lower .. and never a negative... the STFT are also roughly the same but change faster... I suspect the 128 IS 12.8+, but regardless it a wee bit high

as i mentioned , the fuel pump has been showing all the classic symptoms of failing... sometimes I have to hold a bit of gas petal to get it started (starter seems good as is batt), just needs some juice...

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PostSubject: Re: A question of fuel trims   Tue Feb 17, 2009 1:40 pm

If that's true, and the closest to zero you see is "+11.1", that's not good. That means your PCM is adding 11.1% more fuel than normal - pretty lean. I would guess your engine would not run well in that condition. For example, mine usually stays between +4% and -2% during most normal driving (decel and idle excluded). On cruises, I usually see it between +3% and 0%.

But I don't think it's right that the number represents actual LTFT %. I suspect you're reading some other value, like DC voltage.

Assume that the PCM calculates the input from the O2 sensor as a % different from a known voltage, say 12vDC. So, 12.0 would be 0% - perfect A/F mixture. 12.8 could mean +7%, while 11.1 could mean -8%. That makes more sense to me, but I'm still just assuming.

_________________
'98 SC Riviera 281k miles 298 HP/370 TQ 0-60: 5.79s ET: 13.97 @ 99.28 4087 lb 20.1 avg MPG Nelson Ledges Lap: 1:30
3.4" pulley AL104 plugs 180 t-stat FWI w/K&N 1.9:1 rockers OR pushrods LS6 valve springs SLP headers ZZP fuel rails
KYB GR2 struts MaxAir shocks Addco sway bars UMI bushings GM STB Enkei 18" EV5s w/ Dunlop DZ101s F-body calipers
EBC bluestuff/Hawk HP plus SS lines Brembo slotted discs DHP tuned Aeroforce Hidden Hitch

^^^ SOLD ^^^ frown

'05 GTO 85k miles 0-60: 4.8s 16.9 avg MPG Nelson Ledges Lap: 1:26
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PostSubject: Re: A question of fuel trims   Tue Feb 17, 2009 2:06 pm

hrm, you might have something there... the manual for the CP9145 only has text about "Long Term Fuel Trim" or "LT" listed in its glossary, where it says that LT=Long Term Fuel Trim... so no clue to the format its giving it to me in. I guess I could fire the tool onto another car and see if the results are similar or not... I should really do it with another "obd1.5" car though... as the info is greatly different if I hook this thing up to an ACTUAL obd2 car

another option I might try is disconnecting the batt for a few hours which in theory should set all the fuel trims back to 0, then do a scan at first start and see what the numbers start at, it would give me an idea of what format things are being displayed in anyway...


edit:
after much searching, it looks like LTFTs might be displayed differently on the series one..

http://pontiacbonnevilleclub.com/forum/viewtopic.php?f=19&t=3185&hilit=ltft

"Paul, 128 is essentially the zero point for fuel trims. How about finding out how the car was driven before the overheat?

You may want to put something in there to flush out the cooling system, run it for awhile, then change it out.
"

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PostSubject: Re: A question of fuel trims   Thu Apr 02, 2009 6:58 pm

Mine is about the same LTFT is 121 and STFT is 130, I have the same scanner CP9145

I have a code P0300, can anyone help me with this one?
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PostSubject: Re: A question of fuel trims   Thu Apr 02, 2009 7:57 pm

I believe p0300 is a random misfire code, have a look at your wires and the other usual suspects for misfires....

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PostSubject: Re: A question of fuel trims   Thu Apr 02, 2009 9:50 pm

Karma wrote:
I believe p0300 is a random misfire code

It is.
p0301 is cyl 1miss
p0302 is cyl 2 miss and so on.

Start at the plugs and go back. Plugs have too big a gap or corrosion? Is there any corrosion where the plug wires connect to the plugs or coil? Are there any cracks in the plug wires?

Do you feel this misfire? Does it happen under load as in accelerating up a hill?
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PostSubject: Re: A question of fuel trims   Thu Apr 02, 2009 11:02 pm

Sorry, I'm not sure how the fuel trims read out on OBD-I systems. frown I can tell you all about OBD-II, but your numbers are way way different than what I know.
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PostSubject: Re: A question of fuel trims   Thu Apr 02, 2009 11:45 pm

Thanks for your help
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