Cranking Fuel Pressure

[Wing0tj]

This is sort of a dumb question but I cant find the answer Im loooking for. What is the fuel pressure supposed to rise to with key on engine off? also what should pressure be at while cranking, mine is right before 30 psi and Im not sure if that is enough. My car will no start and I cant find out why, I just did timing and installed a aeromotive fpr and fp gauge. I am getting gas and spark and timing is right, so I think there is either not enough gas or too much, or a weak spark (just got new plugs), or idk. if anyone knows the fuel pressures at those time that would be helpful in narrowing this problem down.

 

[steve]

With the engine not running the fuel pressure should be the same as with the reference pressure hose off the FPR or the FPR solenoid activated. On a 1G that's about 36 psi. This is also known as the base fuel pressure.

Steve

 

[Wing0tj]

thanks for the fast response

so if I am around say 26-28 with key on engine off, that could be causing it to not start? I know that it is a fuel issue but I cant figure out what.

 

[steve]

Depends on how and when your measuring. The pump is only run when the engine is cranking or it has started running. The only way to check the pressure with the car not running is to check during cranking, by powering the pump from the check connector, or to turn the pump on with a datalogger.

Aftermarket FPR's often bleed pressure off quickly.

Keep in mind that the actual fuel pressure will be 8 psi or so lower when the engine is cranking and you have manifold vacuum present. If you do in fact have 26-28 psi base pressure that won't be a lot when cranking.

Steve

 

[Wing0tj]

well I have 26-28 when cranking, so I guess that my fuel pressure would be right then considering, the base pressure of ~37 psi drops about 8 psi under vacuum. Let me know if this sounds right.

 

[steve]

Let me know if this sounds right.

It's likely close. You need to check and set using the base (engine running hose disconnected, or engine still) pressure. Manifold vacuum varies while cranking as does system voltage.

Steve

 

[Wing0tj]

ha, that is the problem the car wont start right now so setting the base at idle is gonna be difficult.

 

[steve]

the problem the car wont start right now so setting the base at idle is gonna be difficult.

Please reread post #4.

Steve