George B
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Your looking for a vacuum leak to find a negative trim? Wouldn’t a negative trim mean the pcm is pulling fuel due to an excess? Wouldn’t a positive trim mean a vacuum leak?
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swathdiver
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I don't think so, not if I am reading people's posts correctly. Everything I read points to a vacuum leak. Maybe I need to run the smoke machine again with a little more pressure?
The unit has another gauge to tell you how much pressure is inside the unit you are testing so I don't blow anything up.
George B
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I always understood it this way:
“How does a vacuum leak affect a non-turbo engine’s fuel trim values?
During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This forces the PCM to react to the lean condition with positive valued fuel trim readings. The fuel trims will return to almost normal during cruise and wide-open throttle when the throttle plate is open and there is little vacuum in the intake but lots of air flow into the engine. The small amount of un-metered air creating an idle vacuum leak is hidden or masked when the engine’s airflow increases, so the effect on fuel trim is less evident.”
REF: https://www.autoserviceprofessional...k-and-performance-issues-using-fuel-trim-data
So a vacuum leak of unmetered air causes a lean condition and the pcm adds fuel.
Am I wrong?
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swathdiver
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I just read the exact opposite based on our earlier conversation today. So now I'm confused and cannot say for sure what is right!
George B
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Could you share a link to where you read that?
George B
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You could always create a vacuum leak as a test to see which way the STFT goes. Pull the PCV and see. If I get a chance I will do it to mine tonight and share what I find. I suspect unmetered air will cause a lean condition that the PCM will correct with more fuel.
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swathdiver
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I think this was it. Frustrating myself because I could not find the exact articles using the history feature of the browser.
Negative fuel trims at idle | EricTheCarGuy
Eric, I’ve got an 2003 Acura TL-S which exhibits the following: At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected.
www.ericthecarguy.com
I think this was it. Frustrating myself because I could not find the exact articles using the history feature of the browser.
Negative fuel trims at idle | EricTheCarGuy
Eric, I’ve got an 2003 Acura TL-S which exhibits the following: At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected.
Funny, I saw this, too, in a search, except what's being discussed there is a speed density system vs our MAF systems. So it doesn't apply.
EDIT: negative fuel trims (rich condition) could be caused by too much fuel pressure, a leaky injector, or a bad O2 sensor. Maybe some other things I can't think of right now.
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swathdiver
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This condition usually became less of an issue every time we torqued the intake manifold bolts down. Then we went and replaced the intake gaskets and along the way have replaced the oxygen sensors, all 4. Fuel pressure is within spec and the pump's fuel trim is 1.54 last time I checked. Spark plugs all look the same, Magnaflow cats have 53K on them now. No codes of any kind and no leaks observed with the fancy smanchy smoke machine. Maybe the motor is just getting tired?
I would like to replace the fuel pump assembly and the injectors at some point. Actually, maybe I ought to pull the injectors and clean them in my sonic machine with simple green? We use the truck too much to send them out to the injector guy for proper cleaning.
I saw another article today where the problem turned out to be two vacuum hoses connected on the wrong parts, in that person's case, different wastegate solenoids. Swapped the hoses and the trims went back to normal at idle.
George B
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Even the linked article in that article says that negative fuel trims mean too much fuel and positive fuel trims mean a vacuum leak.I think this was it. Frustrating myself because I could not find the exact articles using the history feature of the browser.
Negative fuel trims at idle | EricTheCarGuy
Eric, I’ve got an 2003 Acura TL-S which exhibits the following: At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected.
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swathdiver
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Well, I guess I'm having reading comprehension issues again! LOL I about drove an ebay seller crazy the other day.
I had a lean idle issue on my C5 vette for years and it drove me nuts, and yes it was adding fuel trim up to +25 then CEL if idling a lot. Turns out one day I had my ex gf watching the O2's graph going up and down and I busted out the propane torch (NOT lit) and sprayed. Second I got near the injectors (ALL of them !!) they went rich. Turns out all of my injector orings were hard as glass. $9 later and car was purring like a kitten again.
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swathdiver
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You know that has been in the back of my mind. I did buy new o-rings for the injectors when I replaced the intake gaskets but was rushed and didn't install them. Thanks for bringing it up, I'll change them out when it starts to cool down.
George B
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How about the MAF sensor. If it is not reading the correct flow at idle it will trow off trims. I tried a search but want to confirm its your LTFT thats climbing right?
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swathdiver
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Yes, it is LTFTs at idle. Reckon I could find the acceptable values for the MAF and watch it while idling after it is warmed up. I run the thicker, HD ACDelco air filter A3085C and this one has been in since Christmas Eve, about 10K miles. Shouldn't clog up the little 5.3.
Is there a way to compare values between the MAF and something else to see if the air intake or MAF is a restriction or reading correctly? I seem to remember LT Holman mentioning something like that when he was testing his orange pickup out.
With all the mistakes I've been making lately, I'm a little reluctant to even pop the hood!
How much do your LTFTs change at idle? Seems like you'd have to be idling a long time to affect them much. Normally they're a mix of all your driving, over time -- hence "Long Term" -- so probably a high percentage of above-idle rpms.
Now your STFTs will change instantly, so if they're good at idle, time to look elsewhere for your issue.
Now your STFTs will change instantly, so if they're good at idle, time to look elsewhere for your issue.
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swathdiver
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Before we changed the O2 sensors, the LTFTs while cruising down the highway for example would rarely get to zero. They were usually -3% or so on both sides. Coming to a stop they would begin to creep up into the negative upper single digits.
Now the LTFTs on the highway usually hit zero and creep into the upper negative teens coming to a stop or pulling into the driveway and sitting there for a few minutes.
George B
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Found one similar issue online
https://www.silveradosierra.com/thr...uel-trim-at-idle-only-flat-43psi-fuel.722357/
https://www.silveradosierra.com/thr...uel-trim-at-idle-only-flat-43psi-fuel.722357/
As your LTFTs are dropping into the negative bracket, what are your STFTs doing? It seems like they'd also be very negative.
What's the name of this truck, Christine?
What's the name of this truck, Christine?
George B
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Probably would be good to graph that.As your LTFTs are dropping into the negative bracket, what are your STFTs doing? It seems like they'd also be very negative.
What's the name of this truck, Christine?
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