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Discussion Starter #1
I am currently running a stock 02Z with only a cold air induction set up by MTI. I have read from several folks that the pcu will adjust for the increase in air, but at WOT the car may run lean due to an "open loop" (whatever that is). My question is, although I do not hear any detonation at WOT under load, if I were running lean would a code be thrown? I am not throwing any codes but am paranoid as hell I may damage something.

Thanks,
Nervous Willy
 

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Don't get hung up on open loop vs closed loop as far as how the car determine appropriate A/F. Regardless of what you do the PCM will run the car at 14.7:1 A/F at partial throttle. WOT (open loop) it goes to a different set of tables (PE).

Don't worry about running lean at WOT, the LTFT's will dump more fuel if needed, but if you want to get maximum performance from your car get a tune and get the timing set (around 26), A/F (around 12.8) and LTFT's (at zero or slightly negative) by someone with experience.

Just my .02 - other opinions will vary.

Les



John Youngblood said:
I am currently running a stock 02Z with only a cold air induction set up by MTI. I have read from several folks that the pcu will adjust for the increase in air, but at WOT the car may run lean due to an "open loop" (whatever that is). My question is, although I do not hear any detonation at WOT under load, if I were running lean would a code be thrown? I am not throwing any codes but am paranoid as hell I may damage something.

Thanks,
Nervous Willy
 

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Discussion Starter #3
Thanks Les, Anybody else???
 

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The problem is usually the opposite. The PCM adjusts for the added air in closed loop operation, typically the LTFTs go up. At WOT, the car doesn't run closed loop, it uses the LTFTs, and this usually produces a rich condition.

If you're lean, it'll ping and you'll hear it.
 

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Agrees with RocketSled.

I have a few cells near the WOT entry point that are perfect in cool air but go red (lean) in hot air. Thus when I go WOT, the whole run gets too much fuel (AFR commanded = 12.5, AFR actual = 11.8 or less) and that kills my WOT power.

Last 2 weeks, 3rd gear on the strip was particularly pathetic and I'm down to running 12.8's or worse.

John.
 

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The VCM is set in closed loop to run at the given stoich value, which is 14.62857 in our case. This is controled in part by the o2 sensors. The VCM will never pull fuel to acheive this but will always add. Now when you change the amount of air to the intake the VCM will adjust the amount of fuel needed to reach the stoich value. On the other hand blocking some of that air the VCM makes no adjustments at all and you get a rich condition. (The VCM will not pull fuel)
Now while your car is running in closed loop with all that new found air it is recording the extra fuel being added to keep the stoich value. This is your Long Term Fuel Trims (LTFT).
Now when you go into open loop all bets are off. It no longer reads the o2s., MAF , or stoich value. The VCM now gets it's AFR from the PE Table and the recorded LTFTs.
The stock PE table is rich to begin with running in the low 12s and as rich as 11.81 between 4400-5200 rpms.
While in open loop as the VCM is reading this table for the target AFR it also reads the LTFTs and if they were running on the positive side in closed loop (adding fuel) they will add fuel to the PE table.
In this case then if the PE table at 3000 rpms is set to reach an afr of 12.1 and the LTFTs say add fuel your afr could be as rich as 10.5.

When you change the air coming into the engine the MAF has to be scaled to reflect that change. The MAF has been scaled stock for a restrictive filter, change the air change the MAF settings.

The '01 Z06s have the same MAF as later models except for the screen and the hole in the top of the filter on the later cars. GM adjusted for that with a different scaling of the MAF.
Yes '01s have a different scaling then '02s - '04s.
Same engine and MAF is in the CTS-V with a different scaling yet. Why, air intake is different.
 

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Tom,

Are you saying that the VCM will not pull fuel in closed loop (i.e. when the cells read green)? Or just in open loop?

[sorry if this is a dumb question]

John.
 

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Another quick [probably dumb] question:

Do LTFT's reset to zero when the car is turned off?

John.
 

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DELLZ06 said:
Tom,

Are you saying that the VCM will not pull fuel in closed loop (i.e. when the cells read green)? Or just in open loop?

[sorry if this is a dumb question]

John.
The VCM will never pull fuel.
If your LTFTs are on the negative side you are running rich.
I had a member come up to me from Miami 2 weeks ago running in the 10s and 11s in closed loop. He has a LC-1 wide band.
Scanning his car his LTFTs were deep into the negative side.
First thing I looked at was his MAF scaling, which turned out to be stock. Next thing was the injector scaling, bingo.
Someone scaled his injectors from 30.94 down to 25.
What happens in this case is the VCM thinks it has a smaller injector and forces more fuel to them.

Lets say the injector has a 1/2 inch hole to disperse fuel and you then tell it it only has a 1/4 inch hole. The VCM thinks the hole has gotten smaller and adds more pressure to the injector to make up for it. So the pressure to acheive stoich is greater as far as the VCM is concerned but the injector is now shooting way too much fuel.
 

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DELLZ06 said:
Another quick [probably dumb] question:

Do LTFT's reset to zero when the car is turned off?

John.
The LTFTs are always learning. They do not reset automaticly.
My HP Tuners scan tool has a reset.
They can be reset by pulling a fuse, can't remember which one.
 

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Hey Tom,

Thanks tons for the comments!

The LTFT's don't learn in open loop right? Since the STFT's lock at zero, they would have nothing to learn from. I have +2's across the top of my LTFT in HPTuner's scan -- in the 95 or 100 row. Is this because the cell just before I entered WOT was +2? Since I would never be in these cells at non-PE, how would they learn? (assuming my first statement above is true)

Somewhat confused...

John.
 

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Not sure what you mean by the 95-100 row.
The LTFTs only record in closed loop. The avearage is added to the PE table across the entire rpm range in open loop.
The higher the LTFT average the more fuel added to the PE table.
Even though you may be recording positive LTFTs at 3000 rpms fuel will still be added through the entire rpm range in open loop.
 

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Discussion Starter #13
WOW!!!!! Now I'm confused. Tom, does that excellent explanation you gave mean I don't have to worry about a lean condition? Sorry for my stupidity. :crazy:

John
 

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John Youngblood said:
WOW!!!!! Now I'm confused. Tom, does that excellent explanation you gave mean I don't have to worry about a lean condition? Sorry for my stupidity. :crazy:

John
That's correct John. If anything you will be rich. :)
 

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Discussion Starter #15
Thanks Tom, :thumb:

John
 

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That is what I was trying to say .. Tom said it better.

Les


John Youngblood said:
Thanks Tom, :thumb:

John
 

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The PCM has a set of values called the Short Term Fuel Trims. These adjust continually, whenever the PCM is in closed loop operation. But they don't adjust instantaneously, changes "average in" over time. On a periodic basis (some number of minutes, don't remember what, exactly), the PCM takes the STFT values and it adds them to the LTFT values while immediately setting the STFT values to zero and the process starts again. When you go WOT, the PCM stops all this and switches to the PE and LTFTs as Tom describes. At WOT, nothing is "recorded". Changes only occur during closed loop operation. The LTFTs "remember" their last value through a start/stop cycle. The STFTs always start at zero on each new restart. You can rezero the LTFTs by pulling the two PCM fuses for a minute and then reinstalling. Afterwards, you need to do the idle relearn thing.
 

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The thing I'm confused about here is why the VCM would attempt to learn negative LTFT's ever if they are not used for anything - even in closed loop...

This guy seems to think fuel is removed during closed loop:

http://ackthud.com/shawnfogg/mixture.htm

This article seems to state the same:

http://www.motor.com/MAGAZINE/Pdf/032005_07.pdf

Negative fuel trim
percentages indicate that the PCM is
attempting to enlean the fuel mixture
to compensate for a perceived rich
condition

http://www.ls1edit.com/pipermail/ls1edit/20030723/008491.html

says:

In closed loop mode, the PCM adjusts with a negative LTFT to keep AFR at
14.7, and we know that negative values lock at zero at WOT.


Tom, this is probably a misunderstanding on my part with what you're saying, please forgive me if this sounds noobish. When you say "The VCM will never pull fuel" -- you are talking about open loop only right?

Thanks,

John.
 

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That's correct, in open loop the car runs off the PE table and will not run any leaner then the set mutipiers. Fuel will be added to that via the LTFTs but not taken away.
In closed loop the VCM tries to keep the AFR at stoich value and will try to adjust the fuel flow to reach that end.
I have found in the couple of cars I was able to scan on the street that have a wideband (including mine) that when the LTFTs are -15 or greater that the AFR is in the 12s even though the VCM is trying to acheive stoich.
 

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The reason the neg LTFTs are recorded is for a balanced average.
+1 and a -1 cancels each other out.
 
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