Can`t disable flex fuel with master switch

[G-Body]

So I believe that I have a bad flex fuel sensor as my commanded afr is around 12.2 when warm and operating in closed loop. My Alcohol percent is stuck at 43% and the sensor was putting out a 92 hz signal (read it with a tech 2). I tried to disable the sensor using HPTuners by setting the master flex fuel to disable. The tech 2 now said the sensor was putting out 0 hz, but hptuners and the tech 2 both still say 43% ethanol content and my commanded afr is still in the 12`s in closed loop. I finally got pissed and reset my stoich AFR to 14.24 across the entire alcohol range, now the commanded afr goes to 14.24 at operating temp. I thought the point of the master enable/disable switch was to be able to use it to turn off flex fuel, so why did I also have to figuratively zero my stoich table also? I attached a starting file and the file from after I disabled the flex fuel sensor AND zeroed the stoich table. The log file is from after I disabled the master flex fuel switch but before I zeroed the stoich table.

[LSxpwrdZ]

Don't quote me on this but this may be an issue with memory locations for the switch... This may also be the reason I am having a hard time enabling flex fuel on non-Flex OS's.

It could also be stuck in the default percentage. If you set the fault percentage which is right under the master enable/disable to 0, when the sensor faults out it will default to 0% ethanol. I believe this is set to around 38% IIRC.

To fully disable you'll also want to kill the sensor codes as well, when those codes are tripped it enables the above default E%.

[G-Body]

Well I finally had a little bit of time to play with the car again. I grabbed the flex fuel sensor off another car and hooked it in. When I hooked it in I was able to get the tech 2 to register between 45 and 153hz but the pcm still reported 43% ethanol. My pcm is set to default at 38% so I don`t think its stuck at the default percentage. I`m wondering if the pcm only updates the ethanol percentage after a certain amount of time? or if theres a change in fuel level? or?

[Eaglegoat]

Are you running fuel thru the sensors when you check them?

[GHuggins]

Fuel composition has to be reset via output controls, tech 2 or with a "like" capable device. Then a relearn has to be performed. I couldn't find the exact proceedure when I looked this time, but basically the ecm will use fuel trims and MAF to calculate the alcohol content in the fuel and must be driven or ran for a specified amount of time for it to make these calculations. It's been a long time since I've had to check into this, so I don't remember the exacts. Sorry about that. BUT, when you changed stoichio or programmed the ecm, you more or less did the reset, which is why it started working correctly.

Hope this helps with some understanding on the problem.

[G-Body]

Yep I`m running fuel through the sensors to test them and isopropyl alcohol to test for alcohol content. The hz output of both sensors varies depending on if the sensor has e10 pumpgas, is empty, or has isopropyl in it so I believe the sensors are working correctly.

I don`t think I can reset/relearn the fuel composition with this system because it has a physical flex fuel sensor. Everything I read seems to say the reset/relearn is for the newer virtual sensor systems. Plus HPTuners has a fuel composition relearn button that is grayed out when I plug the scanner in and my tech 2 does not show an option to do a fuel comp relearn.

I know the newer virtual sensor systems need to see a 3 gallon + increase in fuel level to recalculate the ethanol level. I`m just wondering if the physical sensor systems are the same? I can`t imagine a good reason for GM to set it up that way, but I don`t know why my ethanol % would be stuck otherwise. I would really like to make this car e85 capable again.

[GHuggins]

Just because HP won't let you do it, doesn't mean that it's not possible. From my dealership days I remember having to reset them on a regular basis before they figured out they had a problem with the calibration and came out with bullitens for reprogramming them. If you've updated HP in the last couple years, it should have had that update in it, so don't worry about that being your problem. Just saying that they are resetable/relearnable and it wasn't uncommon for the "physical" setups to be wrong.

[Road]

There is no fuel composition reset on that ecm. The best way to diagnosis it is with a signal generator but being you have tried two different sensors I am leaning towards the ecm has latched the reading for some reason. I would try reflashing it with Tis and/or replacing the Pcm. Have you done a global reset?
Mike

[G-Body]

Well I figured out why the PCM was stuck on 43%. Turns out the PCM needs to see an increase in fuel level before it will check the ethanol sensor even in the systems with the physical ethanol sensor, I assumed that you would only need to see the change in fuel level on the virtual sensor systems. So I put resistors in until the PCM registered 1/4 tank, shut the car off, pulled resistors out so it would see 3/4 tank and turned the key back on. It was still stuck at 43% ethanol, but as soon as I started the car it went down to 9% ethanol which is correct for my fuel. Looks like if you set the master disable switch to disable the PCM will be stuck at whatever ethanol percentage it was last running!

[Road]

Wow that is interesting glad you figured it out and thanks for sharing. I am assuming this is on a conversion that doesn't use fuel level sensor input? I still wonder what it would default to if you flashed it stock W/tech2 then reflash a calibration only.
Mike

[G-Body]

Yep its in a 1978 buick so the fuel sender will not hook up as the ohm range is 0-90 instead of the newer (40-240 iirc?). I didn`t want to try doing a full tech 2 reflash as I have a clone tech 2 so who knows if it would actually work or brick the PCM in the middle. I believe if I had used HPTuners and done a write entire it probably would have gone to 38% as that is the default ethanol %. I ordered a time delay relay and am going to set it up so that when you start the car the PCM will see 3/4 tank of fuel and then a few minutes after its running it will drop to 1/4 tank. That way it should check the ethanol content every time the car is started. I will post back with the results, but the relay is coming from china so it will take a while.

[G-Body]

I was able to trick the pcm into checking the ethanol content every time it starts up. I used a timedelay relay and resistors to simulate an almost full tank at startup and it drops down to below half a tank after 1 minute of run time. So the PCM thinks the tank was refilled every time you turn the car off. It has 2 100ohm resistors in parallel (50 ohms) on the normally closed side so it reads these when the car is started. The normally open side has 4 100 ohm resistors in parallel (25 ohms). I tried using a different relay model FC-34 the first time, and every one that I ordered came in dead on arrival. I had to switch to the TDRM01 board.