donbrew

The fuel tank is sealed. Gasoline produces vapor. As fuel is used from the tank a vacuum is produced. As the tank is filled vapor is displaced by liquid.

While the engine is running it draws fuel out of the tank, the volume of which needs to be replaced by air. The EVAP vent valve opens to let air in. In the mean time Gasoline vapor is being forced into the charcoal canister. When the positive pressure in the tank needs to be relieved the EVAP purge valve opens letting the fuel vapor from the charcoal canister flow into the combustion chamber to be burned and cleaned by the catalytic converter.

If the vapor line has a hole in it air is drawn into the fuel vapor going to combustion. The O2 sensors sense more oxygen in the exhaust, there are 2 reasons for oxygen in the exhaust, alcohol burning or the A/F mixture is off. The computer gets confused and adjusts for higher alcohol content by adding fuel (rich).

If the purge vale is stuck open the engine will be hard to start because it is the opposite of "choking". After it does start the combustion chamber is drawing more vapor from the tank making the O2 sensor sense less O2 in the exhaust so the computer adjusts less fuel in the mixture (lean).

A hole in the vapor line and a stuck Purge valve makes for unpredictable behavior.

The EVAP vent valve has morte to do with filling the tank and maintaining the proper vapor pressure in the tank. If it sticks closed The pump can't draw fuel out of the tank as fast. If it sticks open vapor escapes from the tank causing pollution and not letting the vapor getting to the combustion chamber causing more computer confusion.

This has been a condensed version other stuff is going on.

PulpFriction

Excellent and helpful explanation. Some of that I knew. Some I didn’t. Thank you.

cturboaddict

Pulp, I highly suggest before you start throwing any parts at it..... Find someone with a scan tool that can reset the fuel trims. Give it a SHOT, it's alot cheaper than buying all the parts that I ended up buying and all of the work that I did to get there.
I'm not sure if your app will show you the data you're looking for or not, as I'm not familiar with it.

PulpFriction

Sounds like the way to go. Did the battery disconnect lobotomy almost 2 weeks ago, made no immediate difference. The car hasn’t been driven much since the holidays, cleared the P0172 a couple times seems to take longer each time for the CEL to return. Today the CEL went out on its own during a 20 mile run, got home and found the code still there but short-term fuel trim now down to mostly -2 to -5 range. Also found I could read alcohol level and it is now calculating only 50%. Slow learner. Only had one fill up, tho. Ready for another soon. Want to get the alcohol level reset but also curious if the next fill up will have any effect.

Oldblue

Let’s wait and see, ohh!! The suspense is electrifying!!

PulpFriction

Stop the presses, finally got the ethanol % reset as Cturbo recommended and now it starts fine, thankyouverymuch.

My normal shop couldn’t/wouldn’t do it so, though about acquiring what was needed but wanted to see if an ECU update was available so, OK after much vacillating, off to the dealer a few days ago. I asked them to run an ECU update if available and if not just reset the alcohol %. They did the latter. Less than $90 out the door, coulda been worse. They didn’t wash it tho.

The car came home with 0% ethanol value stored. Will be watching for changes. Cold starting instantly. Wife is happy to have her DD back, likes it better than my LaCrosse with heated seats and steering wheel, lol.


To recap, back in 12/2020, 2011 LS 70,000+ miles cranks no start the morning after a fillup. Plenty of battery power for protracted attempts. Flood-clearing pedal to the floor starts it with difficulty, chugging, smoke and fuel smell. Never a prior problem. Runs fine once warmed up, restarts normal line while hot or warm. Problem persists, battering replaced because what the heck, it’s the original, ten years old. Doh, didn’t check codes before battery R&R, but problem continues and P0172 sets, no other codes. Fuel line leak found and repaired, ECU has stored 50% alcohol even though E85 was never pumped and fuel was sampled at 11%. Fuel line pressure checked. Purge valve replaced for no good reason. Evap line never checked for leaks, vent valve never checked. Problem persisted even after 12+ gallon fillup.

Will never know for sure what set this off but this car, like Cturbo’s, gets many very short trips and had a fuel like leak. Others have reported similar problems with frequent short hopes.

The the problem only manifested when the coolant was cold and the ECU was running open loop, too rich because it thought there was 50% ethanol in the fuel,and there wasn’t. Once it warmed up it was fine. I would suggest if anyone sees a P0172, check the alcohol % stored against an actual sample from the tank.

Oldblue

Glad it’s running well now! Keep it clean and drive it often!

cturboaddict

This is what I should of done from the start.
Glad this post helped get you there bud!!!! Congrats

donbrew

Alcohol content is calculated with data from the O2 sensors. Excess O = must be alcohol since the air is metered by the TB and there are certainly no leaks (ha-ha). Add gas to A/F since computer has no way to control the Air.
Excess air causes hard cold starts by acting anti-choke. Open loop means the computer does not collect any data from the sensors, it uses tables of assumed conditions.

BTW, it is very important to have the engine OFF during refill operations, The computer needs it off to reset calculations. That is the only logical way I can think of for the stored alc to get that far off.

PulpFriction

That’s a useful tidbit. I know someone (or two) who sometimes cheats and leaves the engine running while fueling. Or used to.

I was taking pains to make sure my fillup was at least 11 gallons. Eleven liters would have been easier, lol.

Car was filled yesterday. Alcohol still set at 0% this morning. Huh. Will be watching.

Thanks all for the help.