how does the computer know how to calc the parameters - How does the computer compensate for different blends of fuel? Specifically how does it read how to increase or ...

How does the computer compensate for different blends of fuel? Specifically how does it read how to increase or decrease volume depending on if you've added E85 to a partial tank of gasoline? Also when it does change, what is used to modify flow? Pulse width or line pressure? Thanks for any help.

If you fill up with e85 or add to a tank of regular you need to drive the truck for at least 11 miles for it to " learn" the new fuel that's in the engine. Search this topic and you will finds tons of info about this.

Thanks for the reply. I read all 19 pages of posts when searching e85. There was mention of a sensor in the fuel line and knock sensors being used to detect ethanol variations. I assume O2's most likely translate the mixture but am curious if there is another sensor of some type. I am also unable to find any information on what the computer then does... does it increase rail pressure via variable pressure fuel pump and injector PW remains constant or does injector pulsewidth broaden and fuel pressure remain constant. I assume the latter but don't know.

There is a fuel composition sensor... beyond that no idea.

Thanks, that is exactly what confuses me though. People are claiming there is no sensor on GenIV's and it's all adjusted in the tune using fuel trims now. However I can't find a single person that did just an injector swap and E85 tune that got the ecm to recognize a change in fuel. I don't want to continually retune for mixture changes... especially since E85 blends are going to 70% for the winter.

Must be pretty serious sourcery if theres no sensor anymore. Ya got me man... gonna hafta look next time I change my oil now.

Hi LC9,

They once used fuel composition sensors... Very expensive alternative...

This is what GM literature now says:

"Hardware changes for flex-fuel operation are limited to the injectors. Because ethanol has fewer BTUs (less energy) than the same volume of gasoline, more fuel is required to produce the same horsepower at wide-open throttle. Flex fuel engines use unique Denso injectors with a greater cone angle and higher maximum fuel-flow rate. The fuel rail matches the injectors, but it’s manufactured of the same stainless steel used for all Vortec V8s.

The flex-fuel Vortec 5.3L doesn’t even require a special fuel sensor. The first flex-fuel engines used a light-reactive sensor to measure fuel composition from 100 percent gasoline to 85 percent ethanol. The Gen IV has a virtual sensor—software programmed in the E38 ECM with no separate physical sensor whatsoever. Based on readings from the oxygen (O²) sensors, fuel level sensor and vehicle speed sensors, the ECM adjusts the length of time the fuel injectors open for the type of fuel used. Within a fuel miles after filling up, the E38 controller determines what fuel is powering the Vortec 5.3L and manages the engine accordingly."

That's why only certain vehicles are E85 approved...

So I guess that's how it's done... Amazing!

Wish you had a "thanks" button. Nailed it.