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Discussion Starter · #1 ·
Could you please offer some advice? I bought my '87 GN already set up and I only have a reasonable amount of details re: what all has been done to it. My question is about fuel pressure. I "supposedly" have a hot wired Walbro 340 and a Bosch adjustable fuel regulator and I just recently purchased a fuel pressure gauge. The car is showing a baseline fuel pressure at idle with the vacuum line on of 33 psi. When the vacuum line is removed at idle (1,000 rpm) the pressure jumps to 35 psi. After reconnecting the vacuum line, it is back to 33 psi. On a WOT run, the fuel pressure peaks out in first and second gears at 45 psi. I don't think this is enough fuel pressure for the car. My knock detector goes solid red on every WOT run (which there have only been about 4 or 5 before I got the fuel pressure gauge). The car is relatively stock with the exception of Tomco 32# injectors, a 3 inch downpipe, a BG Custom chip and the aforementioned fuel system mods. I just went out into the garage to try to adjust the fuel pressure regulator and it appears that the Bosch regulator is "maxed out". The nut is bottomed out on the stud, it won't adjust anymore. When I back it off the pressure only drops about 2-4 lbs. So what do you think the problem is? Thanks for the help.-BB

279 Posts
You need the fuel psi to be 43psi with the vacuume line off. Don't worry about what it is with the line connected.
The fuel psi under boost should be:
Fuel psi = base fuel psi + boost
43psi + 16psi of boost = 59psi of fuel psi under max boost.

You need to loosen the jam nut then turn the "stud" into the "cap" (clockwise). This will increase fuel psi. Once you get the psi to 43psi with the vacuume line off, snug up the jam nut, connect vacuume hose and go for a quick drive.




87 GN
12.05 @ 111, 1.60 60', 15psi launch

New mods @ 20 psi: 12.44 @ 107 60' 1.71.... With problems
JJ 62mm
MaxEffort, 22psi
Race fuel only for street
No more 92 octane for this car


[This message has been edited by KEVINS (edited January 22, 2002).]

Premium Member
43,735 Posts
The fp should drop about 5#? when the vacuum line is hooked up...

loosen the nut and raise the pressure to somewhere around 42-43 (line off)and then put the line back on It is does not drop back to around 37#, you have a problem in the regulator or in the vacuum line, I think.

Don't be getting on the car until you have this figured out or you will be learning how to change head gaskets, or worse.
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