GM gen 4 FPCM dump request OS 23482558

[hjtrbo]

The sensor is down the back, it does drop a few psi but nowhere near what they've modelled once you've got aftermarket pumps in there. Presuming they're modelling the pump as well.

What fuel were you running in it? Next time you're doing one put a mech gauge on it to be sure. There's a test port on the drivers side fuel rail that's easy to get to.

Yellow line is injector delta pressure calculated from the fp sensor volts. fuel_dp_psi = (27.809 x fp_sens_volts - 20.717) + baro - MAP
Orange line is the modelled fuel pressure the ecm uses for injector calcs.
A couple of observations - you can see the step up when the 2nd pump kicks in early in the pull. Over the course of the pull, you can see the natural droop of the system ~3psi.

Screenshot 2026-07-01 185548.png

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Subtracting the droop term from the fp sensor. Good alignment, I'm in the right place.

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[hjtrbo]

Had to get some help figuring out why functions weren't passing in parameters and returning properly in the decompiled c view. Turns out some tweaking is required to the compiler spec. Feed the attached docs into the magic ether and get it to sort it out for you. The smart ones might understand what's going on, I certainly don't. End result is good for the few dozen or so functions I've been looking at so I'm taking it.

[hjtrbo]

Update - hptuners have now added xdf UDP support to the FSCM in the latest beta. Tested with the droop table, all looks good.

[Highlander]

most of these modules you can unlock them via seed and key and then read with mode 23.

[antus]

why the hell would they have that in there if it has an actual sensor to monitor pressure.

Probably designed to function in vehicles without the sensor, and for OEM it was good enough. There is probably some developer somewhere who said the same thing and got told to leave it and get it out the door and move on to the next task.