Fuel trims

Tiny
SUPERSAAB
  • MEMBER
  • 2005 CHEVROLET COBALT
  • 2.0L
  • 4 CYL
  • TURBO
  • 2WD
  • MANUAL
  • 120,000 MILES
I've been watching my fuel trims and I'm confused on what it's doing. My ltft is around -12 at idle and -9 or so when driving. But my stft is between -5 and +5. Never gets even close to -12. So why is my stft relatively normal but my ltft is so high? It does have a more aggressive exhaust cam then stock, cold air intake, and header. Stock sc pulley. Any help would be great. Thank you
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Wednesday, September 30th, 2020 AT 2:30 PM

10 Replies

Tiny
KASEKENNY1
  • EXPERT
So the exhaust cam shaft is most likely what is causing this to be this way. Basically when you increase the duration of the exhaust cam alone, you are opening the valve early and longer to expel the combustion. This causes more of the unburned fuel to go through the exhaust. Hence the long term negative fuel trim.

Just because you are watching the short term and it seems to be switching this is only a segment of data points that goes into the long term calculation. What this is saying is that the fuel trim is rich (less oxygen) and it is commanding the injectors to shorten their pulse widths.

Remember that the o2 sensor can only measure the amount of oxygen in the exhaust. So when there is less oxygen (negative number) it interprets it as having more fuel so there isn't enough "room" for oxygen so it commands it lean.

So the short term switching around 0 is a good thing, as it is showing the PCM is able to adjust but in the long term, the exhaust is more rich for more time then lean. That is due to the exhaust cam.

If the engine is running well, I would not worry about it as the changes you made would make sense.

Hope that helps.
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Wednesday, September 30th, 2020 AT 2:49 PM
Tiny
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Okay, thank you. It runs very well. Sometimes gets a little rpm fluctuation on a cold start for a few minutes, then straightens out.
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Wednesday, September 30th, 2020 AT 2:54 PM
Tiny
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I did a engine swap and did the cam before installing. Can I reset the fuel trims and maybe get them down or would it probably be the same because of the cam? Or do you have any pointers on how to lean it out a little? I'd like to see no more the -5.
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Wednesday, September 30th, 2020 AT 2:56 PM
Tiny
KASEKENNY1
  • EXPERT
Sorry for the delay. This question was closed so it didn't show up. Your new post triggered me to go look for this one.

You can reset fuel trims by using a scan tool capable of resetting adaptives or disconnecting the battery and touching the cables together for 10 minutes. This will wipe the memory of the PCM. However that is not going to fix the issue as they will continue to adapt now and adjust themselves.

Is there a reason you want it around -5? If there is no performance issue then the alterations are causing your mixture issue. At this point, you need to get a tuner and adjust injector pulse width to match the amount of air going into the engine. Do you have a performance tuner?
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Saturday, October 3rd, 2020 AT 10:03 AM
Tiny
SUPERSAAB
  • MEMBER
Now it's running lean after I put stock air box back in. Here is a video of what it's doing now with stock airbox. I have checked for vacuum leaks 3 times and can't find one. Yes, I have a tuner.
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Saturday, October 3rd, 2020 AT 10:14 AM
Tiny
SUPERSAAB
  • MEMBER
  • 2005 CHEVROLET COBALT
  • 2.0L
  • 4 CYL
  • TURBO
  • 2WD
  • MANUAL
  • 120,000 MILES
I've checked for vacuum leaks 3 times and can't find it. It was running rich until I took the cai out and out stock airlock back in. Now running lean. Is it a vacuum leak or something else? Under load the ltft will drop to about normal. Has a long tube header. Little more aggressive exhaust cam. Everything seems to be reading correctly. Runs alright but seems to fall on its face off the line until about 2,000 rpm's then picks up.
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Saturday, October 3rd, 2020 AT 3:30 PM (Merged)
Tiny
KASEKENNY1
  • EXPERT
I just responded to your other post. I suspect the alterations that you made are not allowing the PCM to properly adjust the fuel to match the amount of air coming in. The injector pulse width is determined off the MAP sensor which may be the cause of the flat take off. However, the more likely cause is the fact that you have a turbo installed. They have to be wound up in order to produce the desired power so low power at take off is a common issue. This is why most turbo cars have multi step staging for racing so they can build the needed pressure and wind up the turbo for take off.

Also, the fact that you paired long tube headers with a turbo could be contributing to this as well. Long tube headers are great for reducing back pressure but shorty headers are normally what get paired with a turbo so the can fee directly into the turbo for a quicker wind up.

At this point, I think the solution to your issues are you need to get into tuning the engine at this point. You have altered quite a bit and most that do these alterations just enjoy the novelty of the items, but if you are getting this deep into the numbers then we need to tune the controller so that it can optimize the performance upgrades.

Which ties in the other post as well. Let us know if you have a tuner that can alter pulse widths and we can try to get a little deeper into tuning the engine. However, most of them have options where you tell it what you did to the engine and then it sets the programming for you.
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Saturday, October 3rd, 2020 AT 3:30 PM (Merged)
Tiny
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  • MEMBER
It's supercharged. It wasn't an option.
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Saturday, October 3rd, 2020 AT 3:30 PM (Merged)
Tiny
SUPERSAAB
  • MEMBER
I can upload tunes to my tuner and run them if that's what you mean. I tried to change some things on the tuner but didn't help. I'm just confused on why it was running rich then found my cai had got a hole rubbed into it somehow so switched to the stock airbox and now Is running lean. Thought it should be the other way around if anything.
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Saturday, October 3rd, 2020 AT 3:30 PM (Merged)
Tiny
KASEKENNY1
  • EXPERT
Okay. Let me just confirm that if the engine is now running lean that your fuel trims should be positive. I thought you said they were around -12 and you wanted them to -5.

Lean is high oxygen content in the exhaust so the number is positive so it wants to add fuel. If they are positive with the stock intake on, that would indicate that the stock air system flows more air then your aftermarket one. I know this is counter intuitive but it is not uncommon.

If it is supercharged and not a turbo, then clearly you don't have the turbo lag. So if it is lean and is lagging at take off then I would think we need to add fuel at take off. Does your tuner have the ability to pull data recordings from the tool? If so, can you upload one so we can see what is happening at take off? Also, you may want to contact the company of the tuner and ask if they have a program that will help with this. Or they may be able to write one for you.
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Sunday, October 4th, 2020 AT 8:16 AM

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