Wierd fuel pressure question

[onecrazyfoo4u]

So, I got a fuel gauge put on awhile back, and now I've put a nismo type A fpr so I can turn it down. My question is, at what engine temp do I adjust the fuel pressure? It varies quite a bit, at least on my redtop.

When it's a cold morning start, the pressure runs a little high, about 36psi. When I've run the car and shut down for a couple hours, it'll run right at where I set it, 34psi. And when I've run it hot for awhile, and the engines really hot, it'll be about 31psi. Is this normal, for the fuel pressure to vary with temperature? Just wondering what temp to set it at?

I'm still having cold start problems, where it's flooding every morning I try to start it. Could I turn the pressure lower then the fsm rated 34psi? If so, what's the lowest you can go without running the engine lean? Thanks!

[onecrazyfoo4u]

Anybody know why the fuel pressure varies with temperature? What's the lowest safe psi I could turn it down to?

[CMG]

Anybody know why the fuel pressure varies with temperature? What's the lowest safe psi I could turn it down to?

I believe you should use A/F ratio for tuning, not just fuel pressure.

[onecrazyfoo4u]

Yeah, I'm not really trying to tune the car, just get rid of these nasty cold start flooding problems every morning. I know what the fsm specs are, but it doesn't say anything about turning fuel pressure down. Is it okay to turn the pressure down a bit, or will it harm anything?

Anybody know what to make of the different fuel pressure readings at different temps too?

[CMG]

You have a Walbro right?

[onecrazyfoo4u]

yeah, a 255lph

[inktdevil]

if you are running either a stock computer or a programmed 3 bar computer (which most are) then you should set the fuel pressure to 3 bar with the fpr unplugged with the engine at operating temperature.

3 bar = 44.1 psi

you should NEVER turn down your fuel pressure to try and compensate for a tuning issue... if the car runs too rich, then you need to spend the time to tune it properly with the computer. all of the fuel mapping in the computer is based off of a few constants, one of which is 3 bar of fuel pressure.

[onecrazyfoo4u]

I've never heard anything about that....especially at a pressure of 44 pounds. The fsm says to set it to I think 35 psi, or at 43 psi with the vaccum tube unhooked at operating temp. That's what mine is set up to, right now, or should be. I'm just curious as to why the pressure would be higher at cold temps and alot lower at really hot running temperatures?

[doridori23]

35 and 44 are not relative to a temp setting per say. 35 is pulling between 17-20" Hg Vacuum. When warmed up and idling you should have that vacuum setting on stock components. Anything you do to the intake or exhaust has the potential to lower the vacuum number a bit. The 44 is with the line un-hooked, this will let the FPR see no vacuum and no boost (absolute pressure).

The only reasons they threw the bit in the FSM about being warmed us is due toA. IACV (can raise the idle and effect vacuum)B. Warm motors are sealed up tight due to thermal expansion of the metalsC. Thermal control valve (opens the TB to increase idle RPM until engine is warm)D. Never try to tune or make any adjustment on a motor that is not at normal operating temps and pressures

[onecrazyfoo4u]

Cool, thanks for the good info. When cold my vaccuum is about 17"hg, but when it's warm and hot I'm always right at 15"hg when idling. When I'm driving or boosting, inbetween shifts it hits 20"hg dead on every time. Does it sound like my vaccuum is okay or is it way off?

Thanks again, I'll just make sure to adjust everything on a normal temp engine, not too hot though.