The pprv is not a check valve as it's more of a back flow valve it's not designed to hold pressure just volume in the line while relieving pressure. You would not want a true checkvalve anywhere in a returnless pulse modulated setup, will cause pressure issues when loading/unloading the fuel system. You can completely delete the pprv on the hat and not have these symptoms. Slower to prime sure but wont cause stumbling after start ups. It is more likely injector leak by like you mentioned. Engine off pressure should read factory 39psi delta. Wont read less unless there is vacuum on the frps ie around 30psi at idle.
OP you mentioned it started few months ago and its colder time of year. Before your next cold start attempt unplug the maf. If the car starts fine with maf unplugged, no stumbling or hard to start, Then your tune/sensors are adding too much fuel at startup.
Hi JeremyH,
You had me kinda confused w\ the "pprv" abbreviation but a quick search fixed my confusion so I stand corrected as I have always heard this referred to as a check valve (even heard this called a check valve while watching a Go Tech video this morning)....after checking up on what PPRV means now I can relate.....positive pressure relief valve....so I fully know\understand now what you're talking bout & it's relation to a returnless fuel system so you're also correct...a true 1-way check valve would be bad for this fuel system design (or really any fuel system design for that matter), especially if the fuel system is\gets hot....good way to blow a line apart downstream of the check valve or blow an injector out the rails due to potential over pressure from liquid expansion w\o no way to relieve the built up line pressure.
Also read where folks do remove these for performance reasons (this is what you were referring to)....mostly manual trans vehicles to cure potential slow engine RPM\power recovery after throttle cycling during high RPM shifting from fuel delivery cut caused by PPRV's trapping fuel line pressure spikes thus fooling PCM's thru the FRP reading to slow down fuel pump DC output from throttle cycling APP signal 100%-0%-100% thus causing engine output to momentarily drop off until the PCM tells the FPDM to ramp it back up..... Good to know this as I do have a manual equipped car & do plan to run it at the track...…..good reason to install a WOT box to set up no-lift shifting instead of removing the PPRV since these are available now...….
But those that do remove this PPRV are aware of the need to allow the fuel pumps more time to restore adequate fuel pressure to the line\rails prior to starting the car....and the potential issues that derive from it's removal.
1 thing I can think of to do on a car that has this PPRV intentionally removed is to thoroughly
insulate the fuel line\rails from excessive heat sources to prevent fuel vaporization which can cause poor starting among other things...…..just saying.....
This PPRV isn't technically supposed to leak off any fuel pressure
below it's rated relief pressure set generally speaking (to protect from fuel line\fuel rail ruptures from overpressure....the purpose of a relief valve)….it should hold fuel volume in the line as you have stated, but also to prevent
excessive fuel volume displacement from the line past the pump(s) from
vaporization of hot fuel by maintaining some fuel pressure in the line\fuel rails so if the PPRV does leak off while the fuel system is hot, hard starting\stumbling after startup can occur due to vapor trapping (gas bubbles) in the fuel line\rails until the fuel pressure comes up enough to assist the fuel injectors to bleed this trapped gas out. Now if the fuel
doesn't vaporize in the line\rails then all will be as you have said....so the hard starting from a bad PPRV\lack of a PPRV would most likely be noticed on a hot restart as opposed to a cold restart depending on how bad\quick the pressure leakoff occurs to determine how quick\much hot fuel will vaporize (flashpoint of std gasoline is 73*F at atmospheric pressure or 0 psi...ethanol is even lower than this) in the fuel line\rails during cooldown but it
could also happen on a cold start if enough trapped gas accumulated in fuel line\rails from fuel vaporization occurring while the system was hot during shutdown due to fuel line pressure leakoff thru a bad PPRV\no PPRV....the stumbling reaction is somewhat similar to starting a vehicle just after a reconnect of an empty fuel rail & making a single quick key cycle of the fuel pump(s) to load up the fuel rails then immediately starting the car instead of cycling the key several times to fully load up the fuel rails before starting.....
The whole purpose of maintaining fuel pressure in the fuel rails even when shut down is to reduce\eliminate hot fuel vaporization but also eliminate fuel line\rail overpressure....which the PPRV is supposed to accomplish but they do wear out.....I know this 1st hand as I have verified that my PPRV (that I thought was a check valve prior...thanks for the clarification) is leaking off & won't hold any fuel line pressure after sitting overnight so I
do sometimes get some stumbling on some...not all...cold starts but the difference for me is that I
know the issue isn't from other fueling issues (all injectors have been pressure tested & visually verified sound & tune OL cold start fueling has been checked & verified good as well...I had my car intentionally datalogged thru a cold start by my tuner w\ AAT's @ 26*F at the time--I live in the 4 Corners region of the US so it's cold here--due to me suspecting excessive fueling during cold starts thru the tune after checking injectors for leakage...the cause of the excessive fueling found was due to finding COT protection kicking in due to actual plug misfires that I caused unknowingly from using poor dielectric grease causing the cats to overheat from flashing off unburnt fuel\air inside them so the PCM upped fuel delivery to try to cool the cats off but also over fueled the engine during cold starts...yes this can happen) & when the fuel rails have trapped gas bubbles in them it makes cold starting worse when AAT's are colder vs warmer as well due to initially less fuel delivery (or lean, which isn't good during low AAT engine starts) until the injectors "catch up" (fuel rail pressure rises enough to allow the injectors to purge out the trapped gas bubbles & deliver the proper amount of fuel enrichment)….so to say that having no PPRV installed won't cause slow starting or engine stumbling just after a cold start...….
Well let's just say we agree to disagree as my recorded data results says otherwise (also why I made the suggestions to the OP as I did)...………but thanks for helping clarify my confusion of understanding what "PPRV" meant.
