Fuel Pressure Fluctuations....HELP please

[jspaeth]

Hey all,

I am going to be doing a turbo swap this weekend, and then going to Evans Tuning on Monday to have my car tuned.

This will be on a PFC D-Jetro. From what I understand, fuel injection is controlled via the injector solenoid.....hence, the fuel pressure must be consistent during and after the tune, in order for the proper amount of fuel to be injected when the ECU tells the injector to open a certain amount or whatever.

I just recently installed a Nismo Type-A FPR. I have set it at 34-35 psi at idle, per the FSM. However, sometimes, when I start the car, the pressure is as low as 30 psi or so, and sometimes, it doesn't quite get back up to 35 psi. A few times, I tried raising it back to 34 psi at idle, only to find that when I check the car a half hour later, it is up to 40 psi or so.

I am not sure if this is specific to this FPR, or if the stock FPR also behaves this way.

Can anyone chime in on this? What is the best way to tackle this issue?

....My worries here are

A) Inconsistent FP during tuning
B) (Bigger issue) Random FP drop after tune, causing lean conditions

[karmakaze]

Hey all,

I am going to be doing a turbo swap this weekend, and then going to Evans Tuning on Monday to have my car tuned.

This will be on a PFC D-Jetro. From what I understand, fuel injection is controlled via the injector solenoid.....hence, the fuel pressure must be consistent during and after the tune, in order for the proper amount of fuel to be injected when the ECU tells the injector to open a certain amount or whatever.

I just recently installed a Nismo Type-A FPR. I have set it at 34-35 psi at idle, per the FSM. However, sometimes, when I start the car, the pressure is as low as 30 psi or so, and sometimes, it doesn't quite get back up to 35 psi. A few times, I tried raising it back to 34 psi at idle, only to find that when I check the car a half hour later, it is up to 40 psi or so.

I am not sure if this is specific to this FPR, or if the stock FPR also behaves this way.

Can anyone chime in on this? What is the best way to tackle this issue?

....My worries here are

A) Inconsistent FP during tuning
B) (Bigger issue) Random FP drop after tune, causing lean conditions

just to make sure, you set the fuel pressure with the vacuum line removed from the FPR?

also, the pressure of the FPR is not constant. it changes as your vacuum or boost changes. so do not expect a completely constant reading unless you have the vacuum source disconnected from it.

[MELLO*SOS]

I think all nissan require you to remove the vac line to set your FPR at 43.5 psi (aka 3 bar)... Wherever it ends up with the vac line reconnected is fine, as it will change with engine vacuum or elevation...

Hmm what fuel pump do you have? If Walbro, did you re-wire it off its own relay?

[jspaeth]

I set the fuel pressure with vacuum off to 43 psi.

With vacuum on its like 34 psi at idle (around 1000 rpm or so)

I DO understand how the FP goes up as manifold pressure goes up.

What I am saying, is that at idle, with the car parked, I see the FP vary between 30-34 psi sometimes (with vac line on).

This seems to be a big change, considering my RPM at idle is consistently the same.

hmmm

PS I do have a Walbro, but I dunno about the wiring, as it was installed by the person who did my engine swap. I will look into this though. Thanks

[MELLO*SOS]

The suggested thing to do is upgrade your pump wiring when going walbro, supposedly the stock wiring isn't sufficient. It's possible the pump is getting fluctuating current and screwing with the fuel pressure... But regardless of the pressure the pump sends, I think the fpr should bleed off any pressure above (43.5 - eng.vac).

Here's a quick test: unhook the vac line from the FPR and watch your fuel pressure at idle... Does it stay at 43.5 or fluctuate? if it still moves around could be the pump/wiring or FPR itself. If it stays rock solid at 43.5 then it's probably a vacuum issue causing the fluctuation (so watch your boost/vac gauge at idle to see if it's changing). Might want to boost leak check to make sure there are no leaks.

Also there will always be some change as the idle valve opens and closes it will screw with the vac slightly. If you haven't recently, replace your fuel filter maybe it's full of crap...

just my $000.02 GL

[jspaeth]

Thanks^^^^

I should really clarify my statements.

When I say it FLUCTUATES, I don't mean that I stand there and watch the needle move....


It changes over a much longer time.....if I start my car right now, let it warm up, and check the FP with car idling at 1000 rpm, it might read 34 psi.

Drive it around or whatever. Tomorrow, start up the car and let it warm up, let idle get to 1000 rpm, then it might read 30 psi!

I mean it varies over longer time periods, not over seconds or even minutes.

Could it be the weather? Someone else told me I should try raising the pressure because maybe the spring isn't tight enough to hold constant pressure?

I dunno

[MELLO*SOS]

I see. When you fire it up and it's reading 30psi one day, pull the vac hose off and check if the base setting is still at 43.5. Should tell you if it's pump/FPR or a vac issue. I think it will change slightly with the weather.. When your fuel pressure is out of whack what does your boost/vac gauge show? gl with the tuning, I'd reset the FPR when you get there and check the pressure between runs if you can.

[bigOdom1]

yeah it shouldnt be hard to check fuel pressure on the dyno but as stated next time you see it low disconnect the vac. and see what it reads. if it is 43.5 psi eveytime you check it with the vac. off then i would see where you might have a leak somewhere change the fuel filter and make sure there are no leaks

[jspaeth]

Thank you very much....checking the pressure without the vacuum was going to be my next thing to do.

Logically, if there is a boost leak, then the pressure will read HIGHER at idle (bc the manifold will have LESS VACUUM).