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Fuel trim after rebuild

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Old 10-31-2018, 11:27 PM
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Default Fuel trim after rebuild

Recently finished my 4.6 rebuild(crower cam, new cam bearings, head gaskets, compete new cooling system, new front cover and oil pump assembly, new injectors, new every seal I could possibly get my hands on) and my fuel trims are wacked the heck out.

I initially had 2 vacuum leaks on reassembly(cracked vacuum fitting for the booster connection, leaking clamps on the IAC valve) which I rectified and reset my adaptations afterwards. Within 5-10 miles, my LTFT will be about 20% on both banks and my short term fluctuates from -3 to +9 or so across different throttle and load conditions. upstream sensors appear to be cycling correctly, and rears appear to do their job as well when watching the voltages with my scan tool. MAF is reading about 10grams at idle and maxes out at 100 under WOT(don't know if those 2 figures are right)

I set a p0174 twice in quick succession this past weekend when cruising on the highway, but it cleared and 60+ miles since, and no returning faults.

Any ideas? I have smoked the system and I see no vacuum leaks anywhere
 
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Old 10-31-2018, 11:39 PM
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What should the values be? I am interested in looking into this after getting my new engine.
 
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Old 10-31-2018, 11:55 PM
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LTFT is the car adjusting its base fuel settings to account for issues in the running condition. Ideally? your LTFT should be 0, that would indicate that everything is running perfect and the car is not compensating for anything. A high positive fuel trim means the car is adding fuel to combat an issue, and the reverse is witholding fuel to combat an issue. STFT should just fluctuate no more than a handful of % points

I don't know what our gram flow rate for the MAF is supposed to be, to be honest
 
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Old 11-01-2018, 12:08 AM
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I'd think that you have a leak or a blockage in either the vacuum, intake, or exhaust. Easy to check the exhaust by duct taping a shop vac to a cold exhaust tailpipe with motor off and listening.


Also possible to have a cracked/blocked/faulty O2 sensor?
 
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Old 11-01-2018, 12:38 AM
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Check fuel pressure when engine has warmed up. I'm assuming its +20 so its running lean. I went through the 171/174 code thing. Redid head gaskets because they started to leak...did injectors ,etc while in there. Put all back together and LTFT was badly leaning out on both banks...all vac hoses had already ben replaced. Checked fuel pressure and it was 20 psi....spec is 50.. I couldn't believe that engine drove and ran with only 20 psi...lol.. Replaced fuel pump and now fuel trims good.
 
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Old 11-01-2018, 12:56 AM
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Can fuel pressure be read with a scan tool? My gut was pointing to that next but i do not want to dig around at the back of the intake for that Damn schrader valve.
 
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Old 11-01-2018, 07:22 AM
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Originally Posted by Jason_B View Post
What should the values be? I am interested in looking into this after getting my new engine.
LTFT outside of +/- 10% will throw a too lean/rich code I believe.
 
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Old 11-01-2018, 07:43 AM
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Originally Posted by Llamasayswhat View Post
LTFT is the car adjusting its base fuel settings to account for issues in the running condition. Ideally? your LTFT should be 0, that would indicate that everything is running perfect and the car is not compensating for anything. A high positive fuel trim means the car is adding fuel to combat an issue, and the reverse is witholding fuel to combat an issue. STFT should just fluctuate no more than a handful of % points.t
While it makes sense that LTFT should ideally be zero that is rare. In my truck it is typically somewhere in the -5%/+5% range.

Ideally STFT should fluctuate between -5% and +5%. That's normal. Readings from 5-10% either way are not uncommon however, but they should not persist and if they occur STFT should return reasonably quickly to the 05%/+5% range.
 

Last edited by mln01; 11-01-2018 at 11:41 AM.
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Old 11-01-2018, 10:07 AM
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Only way to read fuel pressure is through the shraeder valve

Same thing happened to me with my rebuild, I was around 7%-12% LTFT and intermittent/pending P0101, P0441, P0505, 121. I can't be certain what actually fixed it, but now I sit exactly at 1.56% LTFT with no fluctuation.

I suggest picking up a cheap intake pressure pressure ($15 bucks) and testing the vacuum in the intake. You can also use it test if the exhaust is clogged.

Some things I did
- redid intake manifold gasket
- replaced brand new genuine purge valve with a Hyundai unit (go figure)
- replaced MAF
- new air filter, o-ring
- new fuel pump (bosch)
- different brand spark plugs (in the OE spec heat range)
 
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Old 11-01-2018, 08:49 PM
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Checked the fuel pressure today after work, steady 50, and when revved it blips down to about 45 and then returns to 50. Car off, pressure falls to about 45 and holds there indefinitely, looks like the pump is OK

I did smoke it again, and I do have a small vacuum leak... coming out of the throttle shaft of the throttle body looks like I will need a new throttle body unless there is an available seal option for the throttle shaft
 

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