Bonzen

I'm chasing a weird fuel pressure issue at around 3/4 throttle where my pressure goes from 30-ish psi to 20psi instantaneously causing major stumbling/flat spot and then back to 30psi once I push through to WOT. It doesn't matter if I mash the gas or ease into it slow, the same thing happens. I can hold the rpms and the throttle in the flat spot for as long as I want, it never recovers until I let off or push though to WOT. I'm watching this happen on the fuel pressure gauge in real time as the issue occurs. I am now looking at this return/bypass block thing that I have no idea what purpose it serves or what it actually does. It's plumbed to the fuel pump, and the main return line from the throttle body dumps into it. I'm wondering if it has some check valve or bypass inside thats diverting fuel from the pump via the small "USCG" hose which is causing my major drop in fuel pressure in that certain RPM range.

Any ideas?

The plumbing goes like this: Feed line - Line out from tank(new lines) into water fuel separator(brand new) into fuel pump(old but still seems to work and obviously still works at WOT) into fuel filter(brand new) into the throttle body.

Return line - out of the throttle body into the "block thingy" and then straight to the tank.

Small crossover USCG tube from the "in" end of the fuel pump into the top of the "block thingy"

Also it has been verified that the fuel pump is not experiencing any significant voltage drop during the entire RPM range. The fuel pump and fuel system setup/plumbing is identical to my previous engine which was a standard run of the mill 454 330 hp. The new motor is .030over, bigger cam, 10:1 compression. The new motor runs GREAT except in that one rpm area where the flat spot occurs.

How could a fuel system run great, then stumble and drop pressure significantly, and then pick back up and run great again??? It is running fairly rich (another issue to get tuned) and then goes pretty lean on the wide band gauge and then back to fairly rich.

HELP!

buck35

Read artic friends thread in tech section could be a tuning/programming issue. Or demand exceeds supply in this area of powerband. I'd be careful powering thru at 14.5 or you may be starting over.

edit, the thread is modified 502 mpi intake dyno session.

hadleycat

I noticed you didn't mention where your fuel pressure regulator is in your plumbing? I am guessing this is the stock fuel system from the 454/ 330 that used to be in the boat?

Bonzen

FPR is part of the stock TBI setup. same way it came from the factory. I have replaced numerous spring rates/diaphragms with no change in the issue. I have also adjusted the regulator for pressures of 24psi to 32 psi and same issue still occurs.

The fuel pressure gauge is after the pump and filters, about 3 " from the inlet of the TBI. So it's basically reading the fuel right before it goes into the injector.

Yes, same exact fuel system that ran the 330hp 454. I would think that if it wasn't up to the task pressure wise and volume wise it would not run great at WOT. New motor is probably around 375hp. It revs to 4800 which is pretty good for stock iron peanut port heads in a jet boat (diagnostic reading not tach reading in real time)

hadleycat

I would want to to look at the tune in the ECM and see if it could be a programming issue. Hopefully some more experienced guys will chime in here soon. Good luck.

Bonzen

I don't think it's a tuning issue. I have had forum member TRASH out on the lake with me twice pulling real time data with the MEFIBURN software and he has not seen any weird holes in the fuel mapping/curve. We have checked literally everything that the computer can tell us.

Given that the fuel pressure gauge drops 10-12 psi drastically and the wideband goes lean at the same time lead me to believe there must be a fuel volume restriction that somehow occurs only in that RPM range.

buck35

What if your map is actually flooding it out at this rpm range causing a false reading?have you stopped and checked the plugs after lean condition to verify?

Bonzen

No have not done that. Have only checked plugs after a normal shutdown and they are not lean. I don't think it's flooding the cylinder with too much fuel here as the FP gauge drops major pressure and the AFR gauge shows way lean.

buck35

That could explain both, or not just tossing out ideas.

Bonzen

I appreciate all ideas! More heads together the better to figure out a very weird issue.