fuel pressure...which behavior is correct?

[btoscano]

I did a simple test... . sitting in the driveway stationary, looking at my fuel pressure and boost gauge. It idles at 6-7 psi. If I rev the engine, boost goes up to 4 and fuel pressure drops to 4. Does this seem correct? Or does it suggest a collapsing fuel line? If I connect my original hard plastic line the idle is about the same but when I rev the engine fuel pressure goes up to 9 psi. Is the higher pressure air?



Brian

 

[dpuckett]

Air compresses MUCH more than a liquid, so I'd guess if it goes up to 9spi when revving, you have a restriction of some sort in your stock/ unclear line. I'd doubt it would be compressing the air to make 9psi vs 4psi. Have you replaced your rubber fuel lines? Wha does it do under load? I know you said you had some leakage, but even a short drive up to 40mph and back down will give an indication of how it is doing.



The diaphragm pump is marginal at best for stock fuelling, much less with the hotter pumps some (many) of us run. I can suck mine down to nothing with little effort in higher gears, with moderate throttle (20-24psi boost).



Daniel

 

[btoscano]

With the replacement rubber lines, under load, I'm much more likely to see 2 psi on the fuel pressure gauge. With the hard plastic lines I can get it down to 2 psi but not nearly as often. With the replacement rubber lines, when I shut off the truck, the fuel pressure gauge took seconds to drop from 7 to 5 and upwards of 20 seconds to zero out the gauge. With the stock hard plastic lines just go to zero. However, I did not repeat the shut off test 10 times in a row to verify it wasn't something like the cam position on the lift pump causing the difference.



Brian