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Old 05-06-2020, 01:04 PM   #1
dreksnot
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Flex Fuel sensor readings drop when in boost

Never noticed this before, but my FF sensor reading drops when I do a run down the strip. I have a Triple Fore pump setup and the FF sensor on the return line after the fuel regulator (no bypass line around the FF sensor). So, my question being, what's going on? Yes, my 2nd/3rd pumps are kicking on. Shortly afterwards, my FF sensor output reading goes from 65% to 41%. IDC happy at 43%, FP hanging fine at 78psi. The issue doesn't appear to show up with low boost, it gradually worsens as my boost goes higher.
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Old 05-06-2020, 06:27 PM   #2
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How does a datalog look ? How long are any dropouts ?


I only just fitted a sensor to my own car a few days ago ( aftermarket setup ) and did notice that when fuel was not flowing the sensor would read zero. But when it was flowing it would read 5% ( pump fuel only )...but again after the pumps had stopped it didnt take long for it to read zero again.

But it did make me wonder what would happen when actually driving. I do have a bypass. Probably be a few days before I drive it and maybe a few more before I add any ethanol to it.
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Old 05-06-2020, 07:54 PM   #3
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Starts at my expected 65%, drops to 52% then returns to 65% after the WOT run down the 1/4-mile. It's the white line.
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Old 05-06-2020, 07:56 PM   #4
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Here's the whole log run (the large picture)
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Old 05-06-2020, 08:27 PM   #5
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Quote:
Originally Posted by dreksnot View Post
Here's the whole log run (the large picture)
Hey, looking at your scanner channels, can you explain what your TORQ(mod) and WHP(mod) are? The (mod) part, that is.

As for your Flex Fuel problem, I wonder if it's a function of flow rate? Maybe the return line is seeing some cavitation, for lack of a better term, when all three pumps are running full tilt?

Any way to toggle the pumps on one at a time, to see if it's a flow rate problem?

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Old 05-06-2020, 09:28 PM   #6
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TORQ(mod) is a multiplied version of the Engine Torque in HPTuners. It was pretty close with the LS3 but I had to do a multiplier with the increase in CID on the LSX. HP(mod) is calculated with the Engine Torque and has the multiplier. I set the multiplier by comparing a dyno session with the HPTuners output, and matched them.

There's a Fuel Composition Sensor (frequency) in the PIDs. I might try that to see if it's dropping. As for stepping the fuel pumps in, that would be easy enough to try. The FF sensor in on the return line (original stock supply line), so it's after the filter, rails, regulator.
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Old 05-06-2020, 10:11 PM   #7
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I worked with Dave Steck quite a bit while developing his triple fuel pump assembly. I also troubleshot his dsx aux pump with him, and own his FF kit. He told me explicitly to NEVER put the FF sensor in the return line. Guys are always reporting odd stuff to DSX when they do so.

That being said, mine is in the return. Lol.
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Old 05-07-2020, 07:35 AM   #8
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I wonder if you're using enough fuel while at wot that you aren't getting much flow of fuel in the return line and its skewing the reading.
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Old 05-07-2020, 09:57 AM   #9
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I wonder if you're using enough fuel while at wot that you aren't getting much flow of fuel in the return line and its skewing the reading.
I can't imagine using too much fuel with my setup. Perhaps fuel moving too fast through the sensor??? But, keep the ideas coming.
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Old 05-07-2020, 01:18 PM   #10
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I can't imagine using too much fuel with my setup. Perhaps fuel moving too fast through the sensor??? But, keep the ideas coming.
But it's in the return line, why would the fuel move faster through the return line while you're using more fuel under WOT. I'm thinking as your engine consumes more fuel the flow in the return is actually diminished which would cause the inaccuracy. Probably not the easiest thing to do but if possible I'd try it before the rail and see if the problem goes away.
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Old 05-07-2020, 02:44 PM   #11
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I would highly doubt speed has anything to do with it.

Although the behaviour sounds odd

Raw frequency signal of the sensor would be interesting though, as they should range from 50-150Hz so if that was still stable...the issue is with how that signal is being interpreted.
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Old 05-07-2020, 03:23 PM   #12
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I start digging and looking and find other weird stuff. This time my E% dropped from my expected 65% to 58%, then 55%, then 50%, returning to the expected 65% after letting off crossing the end of the quarter.

Now, the weird stuff - my FP is dropping when launching. I'm doing my 2-step thing (mine only cuts ignition), building a little boost, easing out the clutch and see my FP tank to 30 (see 1st pic). It recovers. This happened 2 of out 11 runs. The other time was when I activated my anti-lag (timing retard) and while it built up boost, the FP dropped out (see 2nd pic). So, this looks boost/fuel pump related? That 2nd picture, however, did not have the E% drop the whole run.
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Old 05-07-2020, 03:42 PM   #13
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I need these graph set ups!

Flow in the return line under high rpm is going to be my guess. Why is the sensor on the return line? Did I miss something?
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Old 05-07-2020, 03:52 PM   #14
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Quote:
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I need these graph set ups!

Flow in the return line under high rpm is going to be my guess. Why is the sensor on the return line? Did I miss something?
I had it in the supply on my LS3 setup with LPE twin-fuel pump supplying. No issues. When the Triple Fore was installed we used the original supply fuel line as the new return line, keeping the FF sensor in place. Can't really think why that would be a problem.
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