One of my kids has a health issue and needs surgery, so I don't have time to troubleshoot now. I took the car to a local mechanic that I have trusted in the past. I rarely take it to a mechanic.

Thanks Don for the note on how the alcohol % is calculated, especially the part about the algorithm closing the canister purge valve temporarily while it determines the %.

I also have a P0455 code, which is a large evap leak. This could be the reason for the bad alcohol %, based on the above information. I explained this to the mechanic and hopefully he takes it to heed. He will do a smoke test on the EVAP system to check it out.

I hate to go to a mechanic, but my hand was forced this time.

Also, I am going to buy the necessary equipment so I can do alcohol % reset and other settings for future problems. It appears I can buy a unit that plugs into the OBD II port, and also get an AC Delco account, which would allow me to do things like VIN programming.

My 2016 Suburban requires ANY switch (power window, etc) to be programmed so it works. I need to get the capability to do that kind of stuff. Since I keep my vehicles a very long time I'm sure that I will use that capability.

Thanks
Steve

 

Our thoughts and prayers for your child and you and your wife and family.

 

Thank You!

They are much appreciated.

Steve

 

The aforementioned TSB says this problem is associated with evap leaks, among other conditions.

One ofcour regulars once suggested a better alternative to a smoke test. Don't remember who. My smoke test was futile. Leak wasn't big enough to detect, or condition that caused the code was never achieved during the test, perhas because the problem proved to be the purge valve.

There is sucha thing as "smoke" with uv (dye? pigment?) in it to aid in leak detection.

The purge valve problem seems to be common enought that with a P0242 or P0255, i would be temped to throw a new one in, cuz it's so cheap and easy. I did this once, using the Autozone garanteed for life item. It didn't work, but I saved the OEM, and many years later I got another P0255, swapped the EOM back in and, voilá! Fixed! I returned the Autozone item for a free replacement so now I have one in stock.

 

But how many ways can a purge valve leak? Now I wonder if it needs to be an external leak or if an internal one could set the P0255 as well.

I thought of the venturi effect. Seems plausible if not likely, with a leak in a perfect constricted high-flow location. What my gut finds less likely is enough flow to cause this, because the internal pressure is mostly higher than atmospheric? In that condition, the venturi effect is VERY inefficient and not terribly probable with a randomly located leak. Again, just a gut feeling.

What my pea brain finds equally (im)probable is that resonance could cause rarefaction nodes with pressure below atmospheric.

 

That valve should be closed, if it allows air to pass through, the code is set. No mystery. No external leak.

 

PWM means PulseWaveModulation, that means there are 10,000 (my math might be off, I took statistics 50 years ago) different ways it could malfunction. It operates on a duty cycle algorithm, from 0 to 100 percent. The computer could demand 90% and the valve might respond at 80%.

The O2 sensors can tell the difference between 1% O2 and 1.5% O2, it is looking for a 17:1 A/F ratio. 18:1 would be lean, 16:1 would be rich.

Your foot controls the amount of METERED air. The computer controls the amount of fuel. If the computer thinks there is more air than it is demanding it responds with adding fuel and vice/versa, that is "fuel trim".

 

Huh. A modulated open-loop solenoid then, not a servo. I kinda expected it to be a simple open/close solenoid valve, not modulated. Now I wonder why it is.

Yes, sort of. Proportioned based on the stored ethanol %, when the O2 sensor is cold and providing no feedback. When (and if) the O2 sensor heats up and begins functioning, the ECU continually adjusts the fuel feed based upon the a/f it thinks is resulting.

An erroneously high stored ethanol % should result in an very rich initial mix, and an excessively large leaning trim, result in a P0172, "[Excessive] Fuel Trim [needed] System [initially excessively] Rich." (Some fuel trim is always needed, of course. I suppose they write the algorithm to warn only of potentially troublesome conditions.)

I would expect unmetered air to, in most cases, have the opposite effect, requiring a trim for more fuel, not less, and if excessive, resulting in P0171.

Obviously, diagnosing a P0172 would be a different kettle of fish in a non-FlexFuel car.

 

That is what I said. I guess I should have used "expecting" in stead of "demanding".

Oh,, and it is PulseWidthModulation.