Teach me about fuel trims and such...


G8orFord

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I noticed I had a code for lean condition on Bank 2. I think it was P0174, but I cleared it before I wrote it down. The truck is running fine and I figured it might have just been a glitch since I didn't know exactly when the code was set and there was no CEL on. Anyway, I still decided to look at the long term trims. I probably should have looked at some other items too, but that's the first thing I thought of and I had to leave for work, so I didn't spend lot of time deciding on which PIDs to show. I did a screen capture video of the Forscan screens and have linked it. This is after letting the engine warm up for about 10 minutes. The video starts as I'm leaving then my driveway and for about the first 2.5 miles. The LongFT1 is running anywhere from 4-6 percent lower than the LongFT2. I assume, in a perfect world, they should be pretty close to the same. I'm fairly ignorant on how these trims and such work since I've never really had to deal with them much before. After the 60 mile trip to work, the code had not re-appeared, so I'm not even sure if there is an issue here or not. I figured I'd post it up and let you guys educate me a little on what I should be seeing or what other PIDs I may want to monitor to try and narrow it down, or if I'm just paranoid and there is really nothing major to worry about.

Screen Capture Video
 


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Dirtman

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+20% is close to the limit of setting a DTC for system too lean (I think the code sets at 25%). Since it's both banks it's most likely a vacuum leak. The difference between the 2 banks is irrelevant.

Run the live data for the fuel trims, watch the numbers at idle, then accelerate hard. If the numbers drop way down you have a vacuum leak.
 
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RonD

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Fuel injection computers have air/fuel mix tables in memory for the engine they are programmed for
These air/fuel mix tables are made for engine size, fuel injector size and expected fuel pressure at the injectors
It uses these tables in Open Loop, cold start, before O2 sensors are warmed up above 650degF

Closed Loop, O2s are warmed up
Computer uses RPM, throttle position, air temp, coolant temp, and MAF sensor to calculate air/fuel mix second to second
Thats STFT of 0
It then looks at O2 sensor feed back for that STFT
If O2 voltage is low, .1-.3, it was lean burn
So STFT goes to +2, thats 2%, so computer is opening injectors 2% longer than it calculated
If still lean it goes to +4, then +6, ect.......until O2 shows .4/.5volts
This all takes less that 2 seconds

If O2 voltage was high, .6-.9, its a Rich burn
So STFT goes to -2, computer opens injector for 2% less time, -4, -6, ect........

As an engine gets older there will be smaller air leaks and lower fuel pressure, low compression, ect....
Thats what LTFT is for, so computer doesn't have to relearn STFT every day
LTFT is a long term average of STFT

On cold start computer add/subtracts LTFT to/from the Tables in memory

Then in closed loop it does the same
So if your STFT is +5 and LTFT is +5 then fuel trim is actually +10

Generally Codes are not set until STFT or LTFT is above +15 or below -15, for a length of time
i.e. at Wide Open Throttle(WOT) computer ignores O2s, and just pumps in the fuel for best power, maybe +20
And at idle to prevent over heating and to keep Cat Converters hot, computer will run +5 to +8 STFT
O2 sensors are not used at WOT


On a V engine each bank/side has its own calculation, lean/rich on just one bank could be a misfire on that bank or a lower intake vacuum leak

An exhaust manifold leak sucks in air, so O2 on that side would give a false Lean
 
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G8orFord

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Thanks for the responses.

This is a 4.0 SOHC, just so we're clear. I have very recently rebuilt the engine. 0.030 overbore, new pistons, cams, valves, rockers, tappets, timing chains, injectors, and more. The fuel pump was replaced shortly after I acquired the truck since it had been sitting for 3 years. I cleaned the tank and replaced the fuel filter. It idles great, so if there is a vacuum leak, it must be small. The most recent work I've done was to pull the drivers' side valve cover to repair a crack. I did have the fuel line apart at that time, though I can't see where that would have caused the code. BTW, the code has not yet returned, but it's only been about 70 miles.

Here's another video from my laptop showing a bit more information on a short drive. Does this tell you anything?
 

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When you did all this to the engine, did you clear the ECU?

The ecu may just be relearning everything since the motor is essentially new and it's still has fuel trim data stored from before.
 

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I have cleared the codes a few times, but I have not done a total reset on anything. I suppose I could do that with Forscan. It's probably got around 6,000 miles on it since the rebuild.
 

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6k miles is more than enough time for the ecu to learn new fuel trims... They are too high in my opinion which means computer thinks its running leaner than it should be, but I'm not an expert on reading data numbers. Seems at idle you got high long term but under acceleration the long term drops but short terms come way up. Which technically means its lean throughout the rpm/load range. (Not a vacuum issue).
 

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I did find that the CCV breather line to the intake pipe was disconnected today. I assume I failed to get it back in the last time I messed with it. So it was getting some air after the MAP sensor, but I don't think that would cause the code.
 

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That would absolutely cause a lean code. That's a huge leak...
 

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Well, technically, there's a certain amount of air coming from the crankcase too. Obviously, not as much as when the tube was disconnected, or at least it shouldn't be. That would be a lot of blow-by. That leak is pre-throttle body too, so it's odd to me that only one bank would show a code if it were a real issue. IDK, I haven't really driven it since I plugged the tube back in, but I'll see how the numbers look when I do.
 

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Both your fuel trims were high, so it was an issue on both banks. One bank just crossed the threshold before the other for the code to set. If the code sets at 20% you had one bank at say 19.9% and the other at 20% your only going to get a code for one side, but they both suck.

Your pcv valve is connected directly to engine vacuum pulling air OUT of the crank case to be burnt by the engine. The other line on the intake side allows fresh air INTO the crankcase. This air coming into the crank case has to be metered. When the fresh air side is disconnected the pcv system is now pulling unmetered air through it. (I.e a major vacuum leak).
 

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The fresh air side of the PCV system is not a vacuum leak. It is before the throttle plate. It is always allowing air to enter the crankcase... just in this case... it's filtered vs. non filtered air entering the crankcase.

It would allow unmetered air into the air tube before the throttle body... not sure that would be causing your trim numbers at the threshold points though.
 

Dirtman

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The pcv side IS a vaccum... and its pulling from the crankcase. It's a sealed system. If the fresh air side is not hooked up where do you think the vacuum is pulling air from? That's why the fresh air side is on the intake tubing so that the air its sucking in is being metered by the MAF sensor. If the air isn't metered it is a vacuum leak...

You will technically get a vacuum leak just by taking the oil filler cap off...

PCVcircuit.png
 
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That has to be the worst picture of a PCV system ever drawn... but totally beside the point.

Vacuum is created below the throttle plate... up to the PCV valve. Then it is a metered vacuum leak.

If the fresh air pipe is off the intake tube... it doesn't change the PCV system a bit... it's just taking in an unfiltered air source. It's still drawing in the same metered vacuum leak.

If you want to call it a fresh air leak I'm ok with that... but it not a major manifold vacuum leak.
 

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Dirtman… I get what you're saying... totally.

The air entering the crankcase for the PCV system is from a metered source (inlet tube through MAF). So the unmetered air could cause a miscalculation and give you wacked fuel trim numbers... enough to exceed the threshold and set a DTC... ?

This situation with the disconnected fresh air tube could have a two fold effect. Not only issues with the PCV... but air speed through the tube could draw additional unmetered air to enter mix through the open breather port.

Curious if just connecting it back up settles the numbers down and fixes the DTC.

Keep us posted G8tor...
 


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