Fuel pressure sensor doubts

[hjtrbo]

Hey, not sure what to make of this data. Just finished up a boost referenced return fuel system install with 2 x Walbro 460 (staged), base pressure is set at 50psi. Running around 750hp at the engine so should have heaps of fuel.

When I calculate the delta pressure from sensor volts I'm +/-1 psi off base pressure in all driving scenarios so I think yah, the world is good. But when doing the same using the fuel pressure PID, in all scenarios except WOT everything follows nicely, but WOT takes a dive like I'm out of pump. I have tried multiple PID's and they all read the same. I don't know which reading to trust.

FYI
Calculated delta pressure from sensor volts (P/N ACDELCO 13516496, Gain 27.809, offset -20.717)
(Gain x Sensor_Volts - Offset) + (Baro - MAP)
(27.809*[6507.10]-20.717)+([50032.98]-[50030.98])

Just wondering if anyone has encountered this before and can offer some guidance as to how I should interpret.

[blindsquirrel]

Isn't the fuel pressure PID generally considered little better than guesswork, an estimated value or something? My platform predates the FPCM/FSCM stuff but I seem to remember that being mentioned often.

[hjtrbo]

Hey. I'm still new so haven't come across any of those statements yet. I'm going to go for a run with a go pro looking at the mechanical gauge so that will clear it up once and for all.

[hjtrbo]

Well holy fuck.

The fuel pressure PID can't be trusted. Fuck knows what the ECM is doing with it to skew it like that.

Log and video of FPR included so you can see for yourself.

Video Here

[blindsquirrel]

Yup. Some of that can be down to a scanner issue/error too, nothing to do with the data the ECM is seeing/using for calcs. The ECM probably uses the raw voltage directly in its math stuff without needing to do a human-friendly conversion.

See here for a scanner issue: https://forum.hptuners.com/showthread.php?78662
First go-round with the support ticket got it reading pressure, but the conversion was off... needed a second tweak to the scanner to get it to match values shown in the Tech2.

[hjtrbo]

Ta, will log a ticket here locally and see how it goes.

[blindsquirrel]

Hey... is there a GM-specific PID for Fuel Pressure, instead of the SAE one in your log?

[hjtrbo]

No, nothing defined for my OS. Screenshots of whats available to me...

[Sjd6795]

Well holy fuck.

The fuel pressure PID can't be trusted. Fuck knows what the ECM is doing with it to skew it like that.

Log and video of FPR included so you can see for yourself.

Video Here

I'm going to end up running a mechanical gauge off my rail and just have it taped or maybe I'll 3d print a holder just were the cowl raises up at the windshield to have a permanent mount to verify mine. I get slightly hesitation sometimes on WOT but the AFR always seem fine, could just be the remote base tune and never having it on a dyno. I hope my solution is easy as yours haha. I really don't want to put a $1000 aux pump or a $2000 fore setup when I will never make anymore then 650whp for the foreseeable future.

[hjtrbo]

Maybe there is some things that can be done. Here is the response from our local HPT support guru. I'm going to stay with my sensor volts conversion. I'll have to run another mech gauge and take another video of the 2 mech gauges to work out the pressure drop at x flow. I'm all AN8 plus the factory 3/8 supply rail, I guesstimate to be around 2-3psi line pressure drop at 450L/H.

Hello xyz,

Have you changed the Injector flow rates and settings in the FSCM?

There are a couple of issues that may impact this, there are likely parameters in the tune that we do not see. The pressure logged likely contains a friction loss for the line relative to flow, this scale, model or table is probably no well scaled or out of bounds at the fuel flow you are at. The FSCM is limi9ted as you have probably realised. The pressure you are seeing is the ECU estimate at the rail as the sensor is at the rear. It is incorrect due to the high flow.

I can see if there is the possibility of finding the relevant table but there is no guarantee it would be scalable at significantly higher flow.

The voltage conversion math is fine that you have done but does not allow for the pressure drop in the line.

[blindsquirrel]

What thread is that sensor - is it the same as the fuel rail test port or something else? I'm kinda thinking of adding one to a non-FPCM platform if I can figure out a way to log it.

[LSxpwrdZ]

Add the channel: "Inj. Delta Pressure" to your list. That is usually what the injector flow rate, injector voltage offset vs pressure is used with. Also need to make sure the min/max rail pressure isn't set to 400kpa on both as that will force rail pressure to stay at 400kpa and the delta pressure to calculate off that set value rather than the rail pressure sensor.

[hjtrbo]

LSx, I have injector delta pressure. Look at my previous logs. That PID is what started this whole thing. I have now deleted it because it is horse shit. I log sensor volts, apply the transform and i've got the horses mouth so to speak. Minus supply line pressure drop of course.

Geez blindsquirrel, I want to say 1/8NPT but I'm not 100% sure. Mine has an OEM fitted quick connect adaptor which I am still utilising. I never pulled it off to check the thread itself on the sensor.

[4wheelinls1]

There is a table that is missing in the FSCM, the PID shows the estimated pressure at the rail so the ECU can appropriately compensate from the injector pressure differential table. There is a drop in pressure due to the line size and length from where it is measured to the injector, with increased flow comes increased pressure drop, the table is not scaled for the large flow of aftermarket fuel systems hence the error that is seen, ideally if the table were there it could be scaled based on a separate gauge at the rail. The second issue is that the incorrect fuel pressure will cause the injector pressure delta to be incorrect in respect to the actual from the measured which in turn will offset the airflow and torque in tuning.

[blindsquirrel]

FYI the sensor thread is M10x1.0.

[lxlaw]

the delta pressure comes from psi in map vs. fuel pressure requested, not actual. The only way i seem to be able to get this dialed in is to calulate flow rates set the fuel pressure to the boost i want plus requested, i.e. 58psi plus 15 psi is 73. Make high flow 73 and ensure the setting are making it go into high flow at like 80 lbs/hr. I dont see why we cant get it to calculate diff. pressure off actual FP. Any thoughts?