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Q's: STFT & LTFT - Vaccum leak, high/low trims... more =
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CaStylin
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PostPosted: Mon Aug 10, 2009 12:09 pm    Post subject:  Q's: STFT & LTFT - Vaccum leak, high/low trims... more = Reply with quote

Well I’m pretty sure i have a vacuum leak. As mentioned in the L26 swap the L26 upper intake manifold has an evep sensor that in on the TB on the L36. 04DLS suggested that I make a “block off plate” which I did but it wasn’t the best and I can hear the high pitch whistle and the idle is rough. Rather then attempt it again I’m going to get it professionally fixed so I can use it a the Evap for when I get/use ZZP Billet TB cuz it doesn’t have one

Anyway to the point… what kind of effects does a vacuum leak have on the fuel trims… I went scanning last night and my LTFT trims constantly hit the -24 when driving (super rich) and the -16 (leaner then the Olson twins) but my STFT where between +3 and -3 which is acceptable

I’ve decided to ground the impala until the leak is fixed but I wanted to know if this was the cause for the issue of is the leak something completely different

P.S. I thought the LTFT where to be a learned average of the STFT? Also is there some where I can host my scans so yall can look at them?

Thanks for the help

CaStylin

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Last edited by CaStylin on Tue Sep 22, 2009 2:12 pm; edited 2 times in total
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Sabrewings
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PostPosted: Mon Aug 10, 2009 1:35 pm    Post subject:  Re: Vaccum leak effects on STFT & LTFT Reply with quote

CaStylin wrote:
I went scanning last night and my LTFT trims constantly hit the -24 when driving (super rich) and the -16 (leaner then the Olson twins) but my STFT where between +3 and -3 which is acceptable


Which one was positive, the 24 or the 16?

And the STFTs will drag the LTFTs to wherever they want to be. If the LTFT is 0 and the STFT is -7, the STFT will drag the LTFT to -7 and then the STFT will read 0. But the STFTs are always jumping around so the LTFT will just lazily follow them.

Unplug your evap solenoid so you know it isn't doing an evap purge and causing really negative trims. Also pull up your O2 MVs. If you're just driving around and the LTFT is extremely positive and your O2s are staying above 900 your car is getting stuck in the cat diagnostic test.

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CaStylin
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PostPosted: Mon Aug 10, 2009 1:56 pm    Post subject:  Re: Vaccum leak effects on STFT & LTFT Reply with quote

Sabrewings wrote:
CaStylin wrote:
I went scanning last night and my LTFT trims constantly hit the -24 when driving (super rich) and the -16 (leaner then the Olson twins) but my STFT where between +3 and -3 which is acceptable

Which one was positive, the 24 or the 16?

And the STFTs will drag the LTFTs to wherever they want to be. If the LTFT is 0 and the STFT is -7, the STFT will drag the LTFT to -7 and then the STFT will read 0. But the STFTs are always jumping around so the LTFT will just lazily follow them.

Unplug your evap solenoid so you know it isn't doing an evap purge and causing really negative trims. Also pull up your O2 MVs. If you're just driving around and the LTFT is extremely positive and your O2s are staying above 900 your car is getting stuck in the cat diagnostic test.

Sorry +16 (max of positive)

So if it is purging then its dumping fuel while its doing so which causes the negative (-) trims? I will log the 02 MV’s on my next run… being that its cazy hot I don’t know if it will be tonight. last time I had to wait till 2am to get clear roads and a decent temp

As for the cat diagnostic test... I reinstalled the rear 02 (was buged) and put the code back and flashed the PCM with a full write so all of its previous memory would clear as well. Is it likely that it went back to the car self test?

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PostPosted: Mon Aug 10, 2009 2:33 pm    Post subject:   Reply with quote

If you have a functioning rear O2 it shouldn't do the self test.

The evap purge pulls fuel fumes in and as a result the PCM needs to use less fuel to keep everything where it wants. So during a purge the fuel trims will go negative by quite a bit.

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PostPosted: Mon Aug 10, 2009 4:20 pm    Post subject:   Reply with quote

FWIW, I didn't suggest the block off plate. Or at least, I didn't suggest that you make one out of sheet metal. That was 04DLS.

And like it has been said, vacuum leaks will usually run you lean because unmetered air is entering the intake manifold.

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PostPosted: Mon Aug 10, 2009 5:05 pm    Post subject:   Reply with quote

whitelightnin04MC wrote:
FWIW, I didn't suggest the block off plate. Or at least, I didn't suggest that you make one out of sheet metal. That was 04DLS.

And like it has been said, vacuum leaks will usually run you lean because unmetered air is entering the intake manifold.

Correction it was 04DLS ... I will change it in the first post Embarassed

If a vac leak will cause leanness then that would explain the idle issues which is what i thought in the first place, hopefully sabre answer will fix the high LTFT's

new question (not to overshadow the first), vac leak effect on boost? pretty sure that it is a decent size one but i would guess that if the leak wasn't too large you could still see boost

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PostPosted: Mon Aug 10, 2009 5:09 pm    Post subject:   Reply with quote

You won't see as much boost with a vac leak.

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PostPosted: Tue Sep 01, 2009 10:50 pm    Post subject:  Re: Vaccum leak effects on STFT & LTFT Reply with quote

Update -
I went and had the L26 evap whole pluged L26 UIM, the wisle the was there is now gone (i think)...

Sabrewings wrote:
Unplug your evap solenoid so you know it isn't doing an evap purge and causing really negative trims.

I also unpluged the evap and got the evap code (p0443) a couple seconds after it was running but the trims at idle where STFT 0, LTFT +1.5. it ran like this for 5 or so minute, i shut it off, pluged the Evap back in started it again and the STFT & LTFT stayed the same

Hopefully this fixed the crazy trims i was getting but i didnt have time to go out for a good scan... maybe saturday night but who knows

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PostPosted: Tue Sep 22, 2009 2:10 pm    Post subject:   Reply with quote

Update

Crazy trims are back... to be really they only left for a couple of drives but since i don't drive it much it took awhile to go back to crazy mode. So again I'm looking for other reasons that this is happening

Of course i am again looking into sab's "cat test cycle" theory but i have flashed the PCM twice now and it should have fixed the issue. does the memory of this self test hold in the PCM or could it hold in the BCM/other place... If it is the PCM and it hasnt be over written in the last two flashes then i may just swap to a new PCM work from there

As i think about it more i also think that these 3 objects could be causeing the issue

- 1 -Could my injectors be spiting out more fuel because of bad connections ( the tables are right so connections would have to be messed to cause issues)
- 2 - Could a bad FPR cause this type of behavior?
- 3 - Could a bad evap cause this type of behavior?

As always... thanks again for the help

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PostPosted: Tue Sep 22, 2009 9:02 pm    Post subject:   Reply with quote

There's no way to stop the cat test. The only way to turn it off is with a program like EFI-live or tunercat. With the PT, the only thing you can do is leave the rear 02 codes enabled and make sure you keep a sensor installed. Without a sensor, the pcm can't see that the test is or isn't working. With the codes deleted, the test will continue to run until you go into PE mode.

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PostPosted: Tue Sep 22, 2009 9:15 pm    Post subject:   Reply with quote

my block off workes fine Wink I used 1/4" sheetmetal and RTV to seal it ....

but i had been chasing down other vac leaks for a while, not fun but a can of starting fluid helped me Wink

If your Injs are spitting out MORE fuel its prob a bad inj, not connection (unless you soldered them in)

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CaStylin
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PostPosted: Wed Sep 23, 2009 8:53 am    Post subject:   Reply with quote

whitelightnin04MC wrote:
There's no way to stop the cat test. The only way to turn it off is with a program like EFI-live or tunercat. With the PT, the only thing you can do is leave the rear 02 codes enabled and make sure you keep a sensor installed. Without a sensor, the pcm can't see that the test is or isn't working. With the codes deleted, the test will continue to run until you go into PE mode.

I dont want to stop or delete the test (well actually i do but as you said i don't have the software) but if it completes the issue would go away. I have re-enabled the rear O2 codes and put the O2 back in but the test hasn't stopped (in belief that is the test causing this)... but on the same note the check engine hasn't come on either and i have not thrown the rear O2 code. Tonight / this weekend i going to look back through the files and make sure every code is set as if it was stock

By the way that sab describe the test i wonder if the rear 02 will even read the difference that it wants to see to complete Confused

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PostPosted: Wed Sep 23, 2009 3:40 pm    Post subject:   Reply with quote

Not sure if you've read this or not, but it explains the cat test a little better.

http://www.grandprixforums.net.....-7092.html

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PostPosted: Wed Sep 23, 2009 4:11 pm    Post subject:   Reply with quote

I've read it a couple of times... the cat self test as covered would explain the +16 LTFT's but they never mention the -25 LTFT's that i also see so its got me wondering what is doing on... If the self test is strictly adding fuel to get a response from the rear O2 then i should never really have an issue with negative LTFT's Confused

I wish it was still summer so i had much more time to devote but as it isn't its hard to track down issues as they arise

I think I'm gonna make a video of the aeroforce gauge and post a scan tonight if i get home before 10pm

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PostPosted: Wed Sep 23, 2009 4:22 pm    Post subject:   Reply with quote

CaStylin wrote:
I've read it a couple of times... the cat self test as covered would explain the +16 LTFT's but they never mention the -25 LTFT's that i also see so its got me wondering what is doing on... If the self test is strictly adding fuel to get a response from the rear O2 then i should never really have an issue with negative LTFT's Confused

I wish it was still summer so i had much more time to devote but as it isn't its hard to track down issues as they arise

I think I'm gonna make a video of the aeroforce gauge and post a scan tonight if i get home before 10pm


Stop looking at the aeroforce gauge for your trims. Just don't do it. I stopped doing it because I'd look at them and think I had a problem and the minute I hooked up the laptop and PT, the LTFTs read completely different. Strictly use the PT for your trims. I don't care what anyone else says about it. It doesn't read mine accurately. After 5 or so MAF tunes, I was still getting the same LTFTs on the aeroforce.

Another thing worth noting is how you are tuning. Are the -25 trims usually coming up when the throttle is closed? If so you need to do a scan and use the table modifier and limit the adjustments to 0% throttle. Doing this will only make changes to deceleration cells.

I'm all tapped out of ideas aside from having an actual fuel leak. If this is the case, you should be able to tell just by looking at the plugs.

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