kando83

I have a 96 502 merc in a 26 Scarab and have been fighting a low fuel pressure problem for the last year have had to merc dealer as well as many others . Now here is where I am , 28psi, block off retuen line ,no change New fuel pumps (2) pressure is 14-18 psi back to my old pump back to 28psi clean all of the injection system, all filters ,found no problem in the VST tank and have 13.5 volts at the pump, replace all fuel lines HELP!!

stevesxm

well... thats either not possible or you have a massive internal leak or bypass. if you block off the return then the pressure has to spike hugely and a lot of them will stall. if you are saying you can run your pump into a closed system and it just sits there at 28 psi then either the pump is scrap or the fuel is bleeding off somewhere somehow. if i were standing there , i would have the pump off, in my hand, and hook up a gage right to the end of it and fire it up... i would expect to see 60 or 80 psi
depending on the pump. and from there you just keep addiing components. build yourself a little TEE rig to test the regulator and then each injector one by one. i know you believe everything you wrote is correct but it simply can not be. the system is a cl;osed and sealed system with the return line the only way for fuel to get out. if you fire the system with the return line blocked, the system pressure should spike... it has too unless the injectors are leaking like crazy. all of which is easy to see if you check all the components one by one.

i'm going on the assumption that this has been checked with a proper gage and not some electric gage in the dashboard...

kando83

Thanks for the reply, I know how it sounds, the boat runs good but lean on the top end ,the pump sit inside the VST tank to build pressure the only thing I can come up with is a problem in the tank itself, no fuel leak, just not willing to spend the $ 2000 they want for the system yet

stevesxm

look... there are a limited number of things to check... all easy to do one at a time.

take the pump and dead head it into a gage... see what it says.

take the regulator and dead head the out put of the regulator into a gage with a clear return ... see what it says

take the injectors out and pressureize them ... see if they leak...

like i said... its a closed system. if you close the return, the pressure has to be exactly what the pump would be dead head... which is probably 60 or 80 or so...

i keep reading your last post and i have to wonder... do you know as a matter of absolute truth that you have a low fuel pressure deal at speed by virtue of looking at a realiable mechanical gage connected to the rail when it happens or is " low fuel pressure" a conclusion based on a perceived lean condition at speed ? i mean... are you actually chasing a true symptom of someones conclusion ?

do this diagnosis one componet at a time. when they all pass, the system will work. i know you want the simple answer. the one magic thing that will fix this but if your explanation of the problem and the work you have done is accurate then you have tried all that stuff. all thats left is a component by component examination of the system. and if you hunker down, it won't take that long. the upside is that it eliminates all the guesswork. it the sherlock holmes method. you keep eliminating possibilities until all thats left is the answer.

kando83

Will start again and get back with ya
Thanks