Low Side Pressure Flucuation

[MakesBadDecisions]

Looking for a little push in right direction. I have a 2016 pro-charged Z06 with twin nozzle Alky Control system and DSX AUX low side and E sensor. Car was moved from sea level to 4800ft and customer wanted a checkup to make sure everything was good. Went for cruise and noticed the low side does something I have not seen that wasn't a AUX low side check valve or controller issue. The low side pressure pulses like nothing I have seen. Put mechanical gauge back by the tie in point for AUX low side and pressure pulsed from 60psi to 70psi while the data log showed pulsing from 40psi to over 100psi at times. I know the mechanical gauge isn't necessarily going to show these pulses in good detail but was able to give a picture of what was going on. The mechanical and ECM pressures were within 2-3psi with engine off. Added known good check valve to isolate the AUX low side and the system behaved the same. Worked with DSX to verify low side control and pump are doing what they should, and they are. Dave was also nice enough to verify settings in tune were good and they are. I have to believe it is not a tune problem as the low side setting have only been changed for proper low side control. AUX pump will prime and set system to 95psi, with engine off. The factory fuel system will set system to 65psi. The system does hold pressure for over an hour, slowly dropping and never getting below 45psi. Only when heat soaked will the system pressure bleed off to 0 which then creates a hard hot start with an upset high side and engine for a couple of seconds. The hot start issue will be addressed after the low side gets figured out as one could be leading to the cause of the other. Replaced the low side factory sensor with new GM part and similar results, the high spikes are less pronounced but still there. Meth system activates and goes green like it should, the low side comes on when commanded and the fuel pressure still fluctuates wildly even during a WOT pull. HPFP pressure never drops off, so the fuel has to be there, just cant trust the low side pressure sensor.

Have I gone down a rabbit hole for no reason? Like is this an OS issue or is this signs the low side pump/regulator maybe even the HPFP is dying?

Attached is the log of a idle and cruise.

[GSXRWS6]

Check valve in the stock fuel line that goes to the high side pump is bad

[cadillactech]

Check valve in the stock fuel line that goes to the high side pump is bad

Exactly. Seen this a number of times, sometimes they make quite a bit of noise when they have failed.

[MakesBadDecisions]

I will get one coming to try. I assume those check valves near the HPFP are different enough that a regular check valve would not work. I see LPE has a high flow version. If this does fix it I will see about getting one of those and change the plumbing around to clean things up a bit. Luckily it looks like I may be able to squeeze that little guy out from under the ProCharger intake without pulling intake.

Thanks guys for the push in a direction

Have a good weekend

[HeavyChevy305]

I just delete it. Run a stock hard line (with no check valve) off a lt4 zl1 Camaro. Keeps your high side a bit more consistent when you're pushing the fuel system to the max.

[MakesBadDecisions]

Thanks for more info. This is a log and screenshot of the first WOT pull i made on the car while on the dyno and with the tune that was in the car as it arrived. With the low side moving around so much its hard to see if there is an issue going on. Obviously at the power lever this car should be at I just need to be diligent making sure there are no issues that could lead to catastrophe.

I just have not seen the low side do this on the cars I have done that was not something else introducing it. I understand the reason the check valve is there after reading the TSB and other stuff about this issue. Without a doubt a its a restriction on a system that needs as much volume as possible. I just would like to make sure of things before introducing another factor.


Thanks again


Edit - Add second paragraph

[silvy423]

What did you end up doing or not doing about this. Im experiencing the exact same issue. See my last post, such an odd issue.

[pisces_0]

I always assumed the low side actual pressure fluctuates +/- 15 PSI from commanded, similar to a sine wave. Mine has done this for as long as I can recall. Is this not normal? Should low side actual pressure track commanded pressure much closer?

For reference, I have a Walbro 465 in the stock sending unit bucket and an LT4 feed line (no check valve). So my "normal" could be thrown right out the window...

[silvy423]

Check valve or no check valve my low side pressure readings are moving 40-50 psi fluctuations! However my desired high side and actual are good