Weird reading on ethanol content under boost
 

So I just installed my flex fuel sensor (along with id 1700x injectors and all new lines in the engine bay). I haven't run E85 yet because I haven't had a chance to install my Walbro 450 pump or upgrade my hard lines.

Anyway, I looked at my ethanol content and it's sitting at 8.5, which seems about right. However, I noticed that under boost it drops down to about 2% then goes right back to normal and I very of the throttle.

Any idea why? My guess is that I pretty much maxing out my pump (Walbro 255, running ~18 psi boost). I don't see a dull in fuel pressure, but could it be that I'm right at the limit and not much fuel is going through the sensor?

I'm assuming you put the ethanol content sensor on the return line, right?

Well, is it a possibility there isn't enough fuel in the return line for accurate reading? :)

Yes, it is in the return, and yes it is definitely possible there isn't much in the return at that point. I was just wondering if that would cause it to give false readings like that.

I'd think so. Don't know for sure though. Take the sensor off the car and trickle e85 down it instead of a full flow...see what happens.

I'm a bit late to this thread, but I've heard from a few different people who have experienced this. It's common enough that some ECUs ignore odd ethanol content changes at high RPM / high load, to avoid letting the bad data cause bad behavior.

They probably have a time threshold on it, where fueling changes based on ethanol content have an associated dwell time before being acted on. I don't know honestly.

What this leads me to wonder is, how does the sensor physically work? I'd think if it was truly and "alcohol content sensor" the reading would be the same regardless of the volume flow through the line unless you are running it flat dry...at which point I feel you should re-evaluate your fuel system for the power levels you are at....

Skeese

I think flex fuel sensors measure the capacitance of the fuel, but not 100% sure so don't quote me on that. I suspect the bad readings are usually a sign that there isn't much flow through the return line. It's a good idea to check fuel pressure carefully in boost, if everything is working correctly the fuel pressure should always be a certain amount higher than manifold pressure. For instance, if your base fuel pressure is 40psi with the engine off (0 psi boost) you want to see 45psi fuel pressure at 5psi boost, 50psi fuel pressure at 10psi boost, 55psi fuel pressure at 15psi boost, etc. If your ECU has a fuel pressure sensor connected, check the log data and it should be pretty clear. If you only have a fuel pressure gauge in the engine bay, maybe add a temporary boost gauge nearby and record video showing both pressures during a dyno run. If you don't have any way to measure fuel pressure, watch the shape of your fuel map curve compared to the engine's torque curve. If boost is flat, the fuel pulse will usually go up when engine torque goes up and go down when engine torque goes down. If you're needing to add lots of fuel pulse at high RPM when you know the engine's torque is flat or dropping, this is a sign that you're probably trying to compensate for fuel pressure dropping off.

Even more late to the party! Wouldn't surprise if a lack of return fuel is causing weirdness. Assuming you're running a fpr on the rail...or thereabouts, tuners here have found fuel returning via the flex sensor can be inconsistent in reading even if there is sufficient pump capacity. Thought process is bubbles coming out of solution under reduced pressure, especially with hot recirculated fuel, is causing the problem....so not using the sensor on the return - and readings end up rock solid.

Whether that has implications for sensor life - ?

Sad follow up. This car actually caught fire in July and was a 100% loss. Still not sure exactly what caused it. I was able to get another one with the insurance money; its a far more practical car, but I can no longer keep up with a new Corvette. Its a sacrifice you have to make, I guess.

That said, that's interesting about the sensor on the return line, but makes sense. Assuming I do this again (its a streetported, doweled, motor with a GT35R so its definitely a possibility) I will probably T off a -6 line in parallel to the feed line and put the sensor there.

That's basically what we did, Teeing -6 to the sensor, with -8 main feed line.

Man I really hate to hear that.... glad you were able to recover another car out of it though and not get fully boned. I've only had 1 engine bay fire in 5 years, but it scares me into now lugging a big fire extinguisher around with me full time.

Skeese

Yeah, thankfully I had a VERY good insurance policy on it! I normally always had a fire extinguisher but had taken it out when I cleaned the car a few weeks prior. Needless to say I have a large halatron one mounted in front of the passenger seat of the new one.

It was one of the worst fires I've seen, actually. Plus, beyond the car, it started a grass fire that probably covered a couple hundred acres. I don't want to clog up this thread any more, but I will be making a post about it in the 2nd gen section, probably in the next week or two.