random misfire from lean
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random misfire from lean
i have a 96 z28 that has the stalling problem common to fbodys just off idle to 2500 rpms. i finally was able to put a scanner on the car while i drove it. the o2 sensors show lean, .1volts when the car is stalling/bucking. I put a fp guage on this weekend and on cold start about 50 psi, dropping to 41 after a sec of run time. when driving the car its steady about 41, unless you get on the throttle and it raises to 50 while its stumbling. What does that mean, the pressure rises when i show i am running lean?
thanks
matt
thanks
matt
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From: Hell was full so they sent me to NJ
If you are referring to the flow vs pressure curve for the pump, typically as pressure goes up, flow through the pump goes down. But that shouldn't be a factor unless you have an undersized pump.
With respect to the injectors - No. When fuel pressure goes above 43.5psi, the flow through the injectors increases, proportional to the square root of (new pressre/43.5).
Your fuel pressure should not be 41 psi at idle or while cruising with a steady throttle. Test the pressure at idle, with the vacuum compensation line off the regulator. That's when it should be in the range of 41-47psi. When you reattach the vacuum line, the pressure should drop by about 6 or 8psi with a stock cam, less drop with a more aggessive cam. I guess if you were running at the very top end of the acceptable "no vacuum" pressure, depending on the cam, it might explain yoour pressure readings. But you have no "signature", and have told us nothing other than the year of the car, so we have no idea if its stock, modified, whatever????
When you drive, fuel pressure should vary with fuel pressure. Close the throttle, and pressure should drop, because intake manifold pressure is high. When you open the throttle wide, the pressure should rise, closer to the "no vacuum" pressure you saw when you tested it at idle.
Before we spend any more time speculating, give us the info on your engine - is it modified or stock? If modified, what are the exact engine mods? Does it have a stock fuel pressure regulator, or an aftermarket adjustable FPR?
With respect to the injectors - No. When fuel pressure goes above 43.5psi, the flow through the injectors increases, proportional to the square root of (new pressre/43.5).
Your fuel pressure should not be 41 psi at idle or while cruising with a steady throttle. Test the pressure at idle, with the vacuum compensation line off the regulator. That's when it should be in the range of 41-47psi. When you reattach the vacuum line, the pressure should drop by about 6 or 8psi with a stock cam, less drop with a more aggessive cam. I guess if you were running at the very top end of the acceptable "no vacuum" pressure, depending on the cam, it might explain yoour pressure readings. But you have no "signature", and have told us nothing other than the year of the car, so we have no idea if its stock, modified, whatever????
When you drive, fuel pressure should vary with fuel pressure. Close the throttle, and pressure should drop, because intake manifold pressure is high. When you open the throttle wide, the pressure should rise, closer to the "no vacuum" pressure you saw when you tested it at idle.
Before we spend any more time speculating, give us the info on your engine - is it modified or stock? If modified, what are the exact engine mods? Does it have a stock fuel pressure regulator, or an aftermarket adjustable FPR?
Last edited by Injuneer; Jun 5, 2010 at 10:55 PM.
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Sometimes.....we worry about you Fred... Lol, just kidding man! 
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