Y33 - Short and Long term fuel trim value questions

[Ludeaem]

I'm looking at you Vstar!

I did the knock sensors a few years ago and used new gaskets but there was one vacuum line I had to tee because I couldn't find the spot on the intake manifold. Fast forward to today and I used a smoke machine to find it! I did not see smoke coming out anywhere else however I realize gaskets can fail where it might not be obvious.

Anyways, this is for my 2000 Q. I do have an aftermarket cold air (filter sits in the fender well)

Idle after warming up
Short term -20-25%
Long term 6-7%

I've read that fuel trims should be as close to 0 as possible so with that said it seems like I have a vacuum leak still present. Hence the high negative STF values.

As far as the long term, could it be the intake affecting that? That value seems a tad high but still within spec. Fair?

I've also checked (with the same code reader) the voltages on the four 02 sensors and they are good.

[VStar650CL]

If it was a vacuum leak, both STFT and LTFT would be going rich and not lean. In fact, one of the tricks we use to identify small vacuum leaks is to watch the STFT (A/F-alpha on a Nissan scanner) at idle and then 2K RPM, if it goes rich at idle but normalizes when throttled-up then a small vacuum leak is indicated.

Lean trim values are usually from wrong or leaky fuel injectors or high fuel pressure. If your IAT sensor (Intake Air Temperature) is working properly, your CAI shouldn't affect anything other than to drive the long-term slightly rich (which it is).

[Ludeaem]

I did some 2-3k steady throttle positions to see what it did but I don't remember off the top of my head. I am resetting ECU now and will test later.

As far as the IAT, I moved it from the duct to the silicone coupler which is about 10" after the filter i'd say.

I'll report more later. Thanks for the input

[Ludeaem]

Alright, reset ECU and have only driven about 10-15 miles but readings are better.

STFT -8 to -15. Was -20 to -25 (idle)
LTFT 0 to -2.

Readings all go to 0 for all trims when roughly 30% or more throttle is applied. Will continue to monitor

[VStar650CL]

That's still bizarre for it to run so lean. Besides fuel leaks or high fuel pressure, the only other thing that occurs to me which would cause such behavior is an over-reporting MAF. If the ECM thinks more air is entering than there really is, the front O2's or A/F's will tell it otherwise and it will correct the map lean. But the MAF would need to be way off to cause that much compensation. The choices are pretty limited, lots of things can cause over-enrichment but not many can cause adjustment lean.

[Ludeaem]

My guess is with how i reinstalled the fuel rail/injectors when I did the knock sensors a few years back. I know it was a two stage torque but the cheap torque wrench I used did not give me confidence. The other from what I've researched is old O2 sensors not being able to keep up but my scan tool readings were .2 -.9 or something which I read was in spec.

Do you think it is that lean? Even with the higher values before today's reading I never got a lean code.

[VStar650CL]

With negative readings you'd get a rich code and not lean, but with most ECM's that doesn't happen until it pegs the needle on adjustment (usually +/-25%).

[3Q Jay]

der well)

Idle after warming up
Short term -20-25%
Long term 6-7%

I've read that fuel trims should be as close to 0 as possible so with that said it seems like I have a vacuum leak still present. Hence the high negative STF values.

As far as the long term, could it be the intake affecting that? That value seems a tad high but still within spec. Fair?

I've also checked (with the same code reader) the voltages on the four 02 sensors and they are good.

Two questions:
1) is it possible that your scanner protocol is backward? One way to prove correct or not would be to deliberately introduce a vac leak at idle and see which way the STFT goes.
2) just taking the above at face value as written, this says your ST is too rich (ECU trying to lean it), but your LT is too lean (the trim is positive). I know this was prior to your ECU reset, but other than a brief transient scenario, I'm having a hard time coming up with a good reason why the ST would be rich AND the LT would be lean (perhaps VStar's MAF theroy holds if MAF non-linear reporting). Is the LT a typo in the original post?

[VStar650CL]

1) is it possible that your scanner protocol is backward? One way to prove correct or not would be to deliberately introduce a vac leak at idle and see which way the STFT goes.

Good thought! Wouldn't be the first scanner I've seen with weird firmware.

2) just taking the above at face value as written, this says your ST is too rich (ECU trying to lean it), but your LT is too lean (the trim is positive). I know this was prior to your ECU reset, but other than a brief transient scenario, I'm having a hard time coming up with a good reason why the ST would be rich AND the LT would be lean (perhaps VStar's MAF theroy holds if MAF non-linear reporting). Is the LT a typo in the original post?

The ST value is basically "instantaneous" and being measured here with no load, but the LT is an amalgam of different driving conditions over time, mostly driving and not idling. So a MAF with an altered slope could definitely cause the injector map to cross between unloaded and loaded operation. In fact those numbers are usually what we see when somebody tinkers the injectors a bit larger than stock but the ECM still has a stock map. The map is generally set up to run 2% or so rich at idle to keep it smooth, but the A/F's will contradict what the ECM expects and cause it to dial back the mixture. Under load a minor difference in flow doesn't matter so much and the reading will more or less normalize.

[Ludeaem]

I will try a few different scanners to see but small update now that I have a couple hundred miles on it.

Short trims were are still -8-15 as they were after the most recent reset. So still a tad rich. Long Term was -2 to -5%. Which is odd I would think because like you guys have mentioned there isn't much correction from the short terms. I would think there would be a bigger swing. Full throttle shows ST and LT to be 0%

I'm not going to go too deep into this as the car runs well, has new plugs, fluids, filters, etc. I would imagine this car runs better than 99% of the 2nd gens out on the road still.

[3Q Jay]

I will try a few different scanners to see but small update now that I have a couple hundred miles on it.

Short trims were are still -8-15 as they were after the most recent reset. So still a tad rich. Long Term was -2 to -5%. Which is odd I would think because like you guys have mentioned there isn't much correction from the short terms. I would think there would be a bigger swing. Full throttle shows ST and LT to be 0%

I'm not going to go too deep into this as the car runs well, has new plugs, fluids, filters, etc. I would imagine this car runs better than 99% of the 2nd gens out on the road still.

Ludeaem-
Would you be in a position to give an update on this?

[Ludeaem]

I really don't have an update. I know there is a good vacuum leak or even injector o ring issue as the colder it is upon start up the car can either stall or buck (albeit somewhat rare) when putting into drive. Once its had a few minutes to warm up its ok so that tells me its 'sealing' better as it expands. I also have a warm start issue once its warmed up. It will take a few seconds to fire but on cold start it fires up fine. My remedy has been to turn on to 'acc' for a second then fire it up. Works every time. With that said I feel my issues are related.

I sort of dread going back in and taking the manifold off but I know it needs to be done. I have other projects on it i'll address first but if I do, i'll try and remember to update this thread.

[3Q Jay]

Yeah, wasn't 'bugging' you, --just wondering what the LTFT in particular had done after a few months of driving.
I recently fixed some issues on my daughter's B15 involving the infamous snorkel cracking, and also a weak plug coil. So her LTFTs are (still) calling for more fuel (>0 value) at idle, but quickly drive toward zero at cruise. Will check again in a few hundred miles.
As for turning the key to ACC, just for clarity, might you actually be turning the key to RUN (second click)? If so, then it makes perfect sense, as you are allowing the electric fuel pump to pressurise all the way up to the injectors, prior to engaging the starter.

[fontana dan]

Extended cranking on a warm start and negative fuel trims could both point towards a fuel system issue. I think you would want check fuel pressure and do a leak-down test by plumbing a gauge into the fuel lines. Or maybe you would want to investigate the electrical components while they are warm? not sure on this
I have identical cranking issue with my car but instead have +15% LTFT idling last time i checked...and im willing to challenge you for best running 2nd gen.