Z_boy912

iTrader: (1)

I've just finished installing CJM fuel system with a new walbro pump, 600cc injectors and JWT cams on a stock block APS TT. FP was set to 45psi at idle. when I got the car on the dyno for a tuning session the fuel pressure drops as the car builds boost. at 5500 rpm the fuel pressure drops to 30 psi. that was verified with both a FP gauge and the regulators gauge..

first thing that came to my mind is a bad fuel pump, I've gone through 2 new walbro pumps, same problem. checked for leaks, nothing. battery is fine. what could cause the pressure to drop like this? I'm clueless at this point!!!

GTM

iTrader: (12)

Are you using the Aeromotive FPR? It could be bad.

Z_boy912

iTrader: (1)

yup checked the regulator and its fine. its a new CJM system so i doubt the problem is from the fuel system..

phunk

iTrader: (21)

a couple guys have had issues with the pump housing and had to replace it... some sort of internal crack or leak. but i would pressure test the assembly, make sure the pump o-ring isnt leaking since the o-ring on the pump output nipple meant for the stock pump is a little lose on the walbros

nightcrawler_8

iTrader: (5)

is the problem fixed?

whitt1

Is the fuel regulator boost indexed? If not your fuel pressure will drop by the amount of boost generated.

NETSHARK

+1000 same story happened with a friend when he was ready for the tuning time this exact problem happened to him he fixed it by purchasing a used Pump Housing assembly it was taken form a totaled Z with very low mileages.

when it comes to fuel problems always leave it to phunk

TrackPack

iTrader: (1)

Very good point, is the vacuum line connected to the regulator.

phunk

iTrader: (21)

this is incorrect.

fuel pressure is not effected by boost in the engine... well, not how you are thinking by watching with an analog gauge, perhaps an ultra fast datalog with an o-scope would show the minor fluctuations when an injector opens and how they can vary based on pressure in the intake.

but anyway, pertaining to this thread, pressure is not effected by boost. what IS effect is the pressure differential... so your EFFECTIVE fuel pressure has dropped under boost... which is your RELATIVE pressure... but your absolute pressure shown on the gauge is not effected by boost pressure.

the indexed fuel pressure 1:1 with vacuum line connected will cause pressure to go up or down 1 psi for every psi of vacuum or boost. unless of course something other tahn the regulator is effecting pressure... meaning if your return is too restriction, pressure can creep under low engine load, or if the fuel supply system in inadequate then the pressure will drop below the regulators control