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Fuel trim ?

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

V70xc1999
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V70xc1999

Fuel trim ?

Post by V70xc1999 » Thu May 16, 2019 11:49 am

My fuel long term will be at 12.5 and when I press the peddle quite hard and acelorate the long term goes down to 0 and it all goes to the short term( brings short term from normal to 12.5 on live data.) Would a week fuel pump couse this? I am shure there are no vac leaks...but could be wrong



jimmy57
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Re: Fuel trim ?

Post by jimmy57 » Thu May 16, 2019 7:12 pm

When you get hard enough on pedal (something like 2/3 pedal) you go out out of the fuel trim controlled range and get into acceleration and high throttle enrichment. The mixture is richer for high load on purpose as catalyst can't stay in safe temp range and perfromance is compromised at stoichiometric mixtures at high load. The enrichment is why a car or truck in front of you that accelrates hard away from you will often have stinky exhaust smell you will notice. It is coincidence that your long term is 12.5% and the enrichment shown by S/T is 12.5% too.
You should have three fuel trims: idle, lower part load and upper part load plus the short term.
The two part loads are for more or less steady load driving. Short term is always the transitional fuel trim control number.
12.5 is high and would usually mean it is a little lean. If it were -12.5 then it would be rich for some reason and ECM would be leaning to correct.
Is there a problem you are trying to correct or just noted the fuel trim numbers?



V70xc1999
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V70xc1999

Re: Fuel trim ?

Post by V70xc1999 » Thu May 16, 2019 7:33 pm

Trying to correct....I did the head gasket on it and all is well. I am just trying to get it running tip top.

What should fuel presure be at...at the rail?

I installed an adjustible fuel presure regulator and have it set to around 53(its a 1999 v70xc add)the FPR is on fuel suply line. But it was doing the same thing befor I switched FPR. ( high LTFT)

it has new turbo vac lines and I have smoke tested. (Dont think vac leak)
I hooked up a presure gage to the fuel rail conected by a fuel line ran to the inside of car to watch. It seamed to keep steady presurewhile driveing.

Just to be shur what tcv ports go where (vac lines) i have it yellow=Westgate. Blue= fresh air inlet tube. Red= bottom of turbo. Blow off= intake manifold.

It got up to 15.3 LTFT today while I was on a test drive after work done on motor.
Has
=new fuel filter
=New fuel lines
=I have cleaned injectors (whats the best way to test them) I have tested them and they seamed to be good but there may be a better way to test.
Thanks



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DeadEric
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Re: Fuel trim ?

Post by DeadEric » Thu May 16, 2019 8:55 pm

12% lean is a a bit odd at WOT, I would expect the opposite as enrichment for power is ideal as Jimmy said.

As far as your vacuum leak concern, wide open throttle negates any vacuum leaks on fuel trims.

Think of it this way...

At idel everything is as closed and sealed as possible. Only enough air is let in for the engine to keep running. At that point you have negative pressure in the engine or a vacuum. Any unmetered (not flowing passed the MAF) air getting in is a vacuum leak. At wide open throttle, the amount of air moving reduces the vaccum to almost nothing and air won't be sucked in thru the leaks.

Lastly, while fuel trims are important to look at when you have an issue and can tell you a lot... you are looking at processed data not actual sensor data in real time. Unless you are 100% on the fuel trim strategy, logic, theory and operation... leave it to the computer to let you know when there is an issue that needs your attention. Trust me, it has a light it turns on to let you know of a problem.



V70xc1999
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Re: Fuel trim ?

Post by V70xc1999 » Thu May 16, 2019 9:02 pm

Will keep that in mind.....what should the air flow rate be for the maf at idle?



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DeadEric
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Re: Fuel trim ?

Post by DeadEric » Thu May 16, 2019 10:17 pm

Rough rule of thumb is... at 500rmp, grams per second should be the same as displacement.

Ie. At 500 rpm a 2.3 liter engine will read 2.3 grams per second.

Standard idel being higher that 500 RPM some algebra is involved and deviations occur. Other factors such as air temp, engine temp, elevation, fuel quality and other things the computer takes into account can affect this reading also... as I said it's a rough rule of thumb.

If you're not spot on don't be concerned.



V70xc1999
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Re: Fuel trim ?

Post by V70xc1999 » Fri May 17, 2019 5:53 am

Thanks. Its idleing at around 4.2 4.5 so its good to go thanks again!



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