EVAP system - how far can this throw fueling off?

[cc-rider]

So I've spent a few weeks running in open loop mode with my MAF failed and tuning in my VE and I've got it to the point where I'm pretty happy with it. I did all this VE tuning with the EVAP hose unplugged from my intake (with intake nipple plugged). So I finally decided to re-enable closed loop and put things back to "normal" and see what it would do.

Here's what I did:
- re enabled closed loop and STFT's (but kept LTFT disabled)
- reconnected my EVAP host back to the intake
- re-enabled COT
- re-enabled DFCO
- kept the MAF failed

After going for a drive, I was a bit surprised to see my STFT running so negative. I had my VE table dialed in...my MAF was still failed so the base fueling should have been coming from my VE table. The only think I can think of that might be doing this is that EVAP being reconnected. Perhaps the COT?

Can reconnecting the EVAP throw off fuel trims by 10% - 20%? Is that normal?

Attached are some log and tune files. V24 is my VE OL file with no EVAP influence. V25 is where I put it back into CL and reconnected the EVAP. Please note that I drove around for 15 min first to get rid of the rich-after-flash issue before I did these datalogs.

v25 CL enable with VE OL - maf failed.hptv25 drive 2.hplv24 VE tuning set up.hptv24 drive.hpl

[schpenxel]

FT's on mine would always go nuts when re-enabled, especially when first re-enabled. I don't know if it's a bunch of fumes built up or what, but this is why I started tuning in closed loop using fuel trims instead of in open loop for part throttle areas. It is royally obnoxious to have spent time getting everything right then it would be wrong again

So.. I can't say that those exact numbers are normal, but I know I've seen numbers that bad before under the same circumstances.

[jtrosky]

So is this similiar to how the fuel trims go negitive after filling the car with gas? I've noticed that re-fueling makes the fuel trims go crazy for a bit. Never really understood why, but it sounds like this may be a similar type of situation?

[schpenxel]

I've noticed the same, though I'm not really sure why it happens

[cc-rider]

Probably is something similar. Maybe filling up the gas tank leads to more vapor in the evap system.

Who knows. It's starting to drive me insane that I can't get these different fueling modes to jive with each other. It seems that the only fueling method that I have control over is OL speed density. My MAF is nuts and fueling is all over the board. CL fueling is all over as described above. I can't imagine what a beauty it would be if I turned on my MAF, dynamic airflow, and CL all at the same time (as they are intended to be run by the factory). I thought all these damn widgets and gadgets and settings were supposed to help better control the fueling. F%#?!!!!!

It seems the most old and crude way of speed density tuning gives me the best result. I'm sure I just need to play with things a bit more with my MAF and CL and any and all combinations in between.

I guess the good and bad thing about having a wideband is that you know what's actually happening with actual vs commanded at all times. Sometimes that can make a guy want to pull his hair out.

[schpenxel]

Yeah, there's also a LTFT evap purge reduction table that is somehow added into the mix based on what the evap system is doing? I've never figured out the math behind whatever changes it makes though.

I can only imagine this further complicates things on our lovely gen 4's.

I've had more consistent results just unplugged the evap solenoid and zeroing out that table personally. The problem is it can lead to fuel smells during hotter months.

The good news is @ WOT I don't seem to have these types of variances, and fuel trims make corrections in other areas, so it works out fine.

[LT1Z]

When EVAP purges it can cause large negative swings. It is easiest to just unplug the evap purge valve when tuning and then let the trims take care of it when back in closed loop if you plan to keep Evap.

[Tick]

I'm tuning a 2010 Camaro right now with a Whipple that is doing this same exact thing. I've never seen one swing so badly. If you log fuel trim cell number its clear as a bell that the evap opens and the trims then need to pull out about 20% of fuel at idle.... Fuel trim cell 6 and its rich as can be. Fuel trim cell 14 is back to normal. I'm tuning with the long term trims off as I normally do so as not to get any wierd fuel trims bleeding over into the wot fueling.

[cc-rider]

I'm tuning a 2010 Camaro right now with a Whipple that is doing this same exact thing. I've never seen one swing so badly. If you log fuel trim cell number its clear as a bell that the evap opens and the trims then need to pull out about 20% of fuel at idle.... Fuel trim cell 6 and its rich as can be. Fuel trim cell 14 is back to normal. I'm tuning with the long term trims off as I normally do so as not to get any wierd fuel trims bleeding over into the wot fueling.

Good to see a similar set up experiencing the same thing. But I am getting rich afr in more areas than idle. Not at wot, but just about everywhere else in the normal driving range.