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Piston Slap: Decomposition of the Fuel Composition Sensor?

Longtime TTAC commentor rrhyne56 writes:

Flex Fuel. I see it more and more. From what I’ve heard, this mainly means the vehicle has a fuel system that alcohol wouldn’t eat up. (Mainly, yes. — SM)

So here’s my question: do the more recent models of these vehicles have the ability to sense what level of alcohol is in the fuel lines and adjust the engine accordingly, to make best advantage of whatever current gasoline/alcohol or alcohol/gasoline mixture is entering the engine? I watched a build on Mighty Car Mods where the Haltech engineer was tweaking just such a system.

I know, I know, I ought to just Google it. But I thought it might make for some lively discussion.

Sajeev answers:

Yes indeed, flex-fuel vehicles have a flex-fuel sensor (b.k.a. fuel composition sensor) to give such information to the engine computer. Considering the following video (wiring diagram and the “blocky” output via the scope), this older Buick’s system meets the most basic need: it triggers a switch — E85 on/E85 off. Then the computer switches between a unique fuel/spark table for whatever’s in the tank.

The only proof I see (i.e. Googled) of a system that eschews the on/off switch for a rheostat is in the aftermarket. That said, who else but the hardcore ethanol racers worry about blowing up a tuned motor from a bad batch of ethanol? Factory sensors and tunes are good enough for most, but the Zeitronix sensor gives a varying signal that interfaces with standalone fuel injection systems.

You can bet it ain’t cheap, and most folks won’t pay for all of this. So let’s get back to the current factory setup.

Replacement sensors (on older models?) are pricey, causing drivability issues and engine codes when they fail. One enterprising company is offering a defeat sensor so that an E85-ready machine returns to being a normal gas guzzler. Considering a large number of Bush-era ethanol vehicles are fully depreciated, on their last legs, etc., this defeat sensor makes financial sense.

[Image: Wikipedia (CC BY-SA 3.0)]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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