After every fill up

[killer01ws6]

My LTFTs go heavy - like say -10 or so, but then about 20 to 40 miles they start to go pos again. ( I am fighting a pos condition)

They ease back to the +3 to +5 I am chasing.

I recently took a trip to fla and filled up 4 times and was checking the MPH and of course logging and tring to figure how to get my pos LTFTs Neg.

And I saw this happen each time......

But what I am curious about is why it does this,

I have both my Low and High Octane tables set to the same settings so I can try to dial in my tune, and I will get 0 kr during this process......


It there a section I am missing which is why I can't get my ltfts right, or
some other reason.

I know if you have KR on the high table it will put you on the low and after you add 5 gal of gas it will put you back......


but I have the tables the same and other than cold startup surging on idle (warm is solid) and a little surging at 35mph my tune is great ( with pos ltfts that is)


Can someone give me some insight please?

Thanks,

Pete

[jimmyblue]

Normally the tank is full of nothing but gasoline vapor
and the EVAP system purges this into the engine (when
active). But when you fuel, you introduce some air into
the system. So the contribution of the EVAP is pushed
around some. It's surprising that it would be to that
extent though.

I wonder if there are any funny "fill-up routines" that
check fuel type / quality and maybe challenge mixture
on purpose. Though what I've seen discussed, is just
the EVAP having to get "back on plane".

[killer01ws6]

Well if I had Neg LTFTs and it made them Pos that would make sense....

more air leans you out....

but I am going the other way........


First time it happend I was happy as could be till I started the car the next time..........

then good ole Pos ones again.

[jimmyblue]

If your +LTFT problem is only at low airflows, you may be
having O2 sensor problems with the long tubes. Might
want to observe the O2 voltages in the scanner, looking
at the switching character of the voltage - particularly for
long idle periods, do they start switching slower, then take
a little nap, then wake up vs a normal medium-cruise fast
switching, continuous behavior?

If so then you may want to swap in the 'Vette rear O2s
for your fronts and see if that makes the trims come around.

Though this has roughly nothing to do with fill-ups as far
as I can see.

[killer01ws6]

If so then you may want to swap in the 'Vette rear O2s
for your fronts and see if that makes the trims come around.

.

LOL, now you tell me Jimmy..

I just put new 02s in it last month, end of Aug at the latest.....

[killer01ws6]

LOL, now you tell me Jimmy..

I just put new 02s in it last month, end of Aug at the latest.....

You have the Part # for those, by any chance.

[jimmyblue]

Corvette Rear O2 Sensors:

Bosch 13111
Denso DEN 234-4087


I wouldn't go changing them just to change them; check
out whether you are seeing the stock stye ones "drop
out" in the logger first, if you are seeing continuous fast
switching in a positive-trims cell then you can probably
discount the cold-O2s theory. No point in spending that
kind of money if it isn't the problem; it's just a possibility.

[killer01ws6]

I am sure I can get some of my money back from the sale forum on the boards....... :-/

[John_F]

Heh... A few friends and myself were trying to figure this one out too ???
The only thing we could come up with that made any sense was that the gas is COLD when it comes up out of the underground tanks. Cold gas = Dense gas. We have no clue if its right or not, but it made sense to us after a few beers ;D

[rattle]

Have you tryed dumping injector cleaner into tank?

I bet you dont see the trims start drifting positive until your near or LESS than 1/2 tank of fuel. The crappy fuel is beginning to plug up your in tank fuel strainer which results in some slight fuel starvation and vapor entry into the fuel pressure line, if left unchecked you will be buying a fuel pump soon.

The fuel pump sits inside a plastic reservoir, fuel enters through the pickup strainer. I have seen strainers that looked worse than a used coffee filter.

[softy91]

Guys,
Leaning the engine down over a time/distance senario is my guess.

I would think that there may be a problem of building up a vacum in the gas tank as the car is driven. This would cause the fuel pump to have to work harder and maybe not be able to keep the corect fuel pressure to the injectors.

You would have to rig a direct read fuel pressure gage off of your fuel rails to check this out. If your readings start dropping fuel pressure, try loosening the gas cap and driving to see if the condition goes away.

I guess it would be more expedient to just loosen the gas cap and logging to see if the LIFTS go Pos. from a Neg. starting point.

Good luck!

[killer01ws6]

Nope,
It happend again last night, I filled up boom my LTFTS went Green, stayed there till I got to my sons soccer practiace, when I restarted afterwards....


Right back to +5 LTFTs....... >

[SideStep]

My LTFTs go heavy - like say -10 or so....

I am seeing the same thing but only +/-6

[guardsman01]

I have been told that when you fill the tank up, the PCM resets the trims....could that be the cause?

[Screamn03]

Command the purge and vent solenoids off (scanner-vcm controls) the next time you fill up. I was playing with them yesterday in my driveway and I noticed when I turned them on the engine ran different.

Just a thought and something to try.