Things that cause a fuel pressure drop befire TBI

[Bonzen]

Posted a similar thread before but Ive narrowed it down and have more info now...

I'm having a fuel delivery/pressure/volume issue. 1994 454 TBI. This is in a jet boat.

I swapped fuel pumps, hardwired the pump straight to the battery, installed an Aeromotive regulator, had all new fuel lines made, high flow fuel filter...nothing. The motor cuts out like its losing fuel around the mid range of the rpm band and then near WOT and at WOT is picks back up and again and runs perfect with zero miss. The digital AF gauge reads lean at the same exact time the motor cuts out and the fuel pressure gauge right before the TBI on the inlet side drops pressure. I'm pretty convinced that there is a fuel delivery problem (as opposed to an ignition/injector problem) but I don't know how or why it's only in the middle areas of the RPM band and at WOT it screams with no issue. Easing into the throttle doesn't make any difference.

The ID of the fuel lines are 3/8". The Motor is a 454 that has a mild cam/stock iron heads/overbored TBI/.030 over cylinders with 10:1 compression SRP forged pistons. I only run 91 octane. The fuel pump is the $300 962 Carter/Indmar Marine fuel pump. For approx 375HP this should be plenty of fuel pump.

Throw out ideas please!

[Six_Shooter]

What's the pick up like inside the fuel tank?

That seems to be about the only thing you haven't changed. If there's a soft rubber hose it could be collapsing under high suction, which is common near the middle of the RPM band, where the torque peak is for most engines. I have also seen where commanding the injector open for a long PW can cause a similar issue that still indicates a fuel delivery issue, but sometimes commanding a shorter PW (lower VE) can actually richen an engine, especially if it's an MPFI type. I know it seems counter intuitive, but I've seen it a couple times. Again, the delivery issue still needs to be sorted in that case, but it can be a quick diagnostic tool if you know what to look for with that.

[Bonzen]

What's the pick up like inside the fuel tank?

That seems to be about the only thing you haven't changed. If there's a soft rubber hose it could be collapsing under high suction, which is common near the middle of the RPM band, where the torque peak is for most engines. I have also seen where commanding the injector open for a long PW can cause a similar issue that still indicates a fuel delivery issue, but sometimes commanding a shorter PW (lower VE) can actually richen an engine, especially if it's an MPFI type. I know it seems counter intuitive, but I've seen it a couple times. Again, the delivery issue still needs to be sorted in that case, but it can be a quick diagnostic tool if you know what to look for with that.

The pickup tubes are aluminum tube that is welded to the threaded bung on the fuel tank where the pickup fitting screws in. I have removed and replaced those already as well. Being that the tubes are metal, I don't see how they could collapse and given that this issue happens regardless of which fuel tank I pull fuel from, I'm guessing it's something besides the pickups. Ive has the whole MEFI data log and real time fuel maps read and my tuber said nothing looked weird. But I cant elaborate on that too much as i dont know all the lingo.

[Bonzen]

Attachment 10864Attachment 10865Attachment 10866These are screenshots from my current setup. This is all I have.

[Playtoy_18]

Have you checked for voltage drop on the pump during the issue?
Setup a multimeter and go for a run,see if pump voltage drops out.
That would rule out an electrical issue,also might not be a bad idea to pull apart the regulator and make sure there isn't some trash in it. I've seen new ones with production debris like shavings in the ports.
You said you have an aeromotive regulator,did you remove the TBI regulator and spring?

[Bonzen]

Have you checked for voltage drop on the pump during the issue?
Setup a multimeter and go for a run,see if pump voltage drops out.
That would rule out an electrical issue,also might not be a bad idea to pull apart the regulator and make sure there isn't some trash in it. I've seen new ones with production debris like shavings in the ports.
You said you have an aeromotive regulator,did you remove the TBI regulator and spring?

The Aeromotive regulator is brand new and i have already disassembled it to swap in the high pressure spring. So it's clean inside. The factory regulator was replaced with the block off kit(so yes no diaphragm and spring) the problem existed identically with the old factory regulator.

I did run a test while running and there is no voltage drop to the pump.

[lionelhutz]

It sure sounds like something odd is causing your pump to simply not be capable of producing the pressure. For example, cavitation due to an air vortex in the tank. Maybe an engine harmonic is vibrating something in that rpm range, for example that pickup pipe you mentioned starts vibrating which lets air get sucked into the pipe.

[brian617]

The digital AF gauge reads lean at the same exact time the motor cuts out and the fuel pressure gauge right before the TBI on the inlet side drops pressure.

How much fuel pressure are you losing? What are your normal pressures and problem pressures?

[myburb]

Are your fuel tanks full when it does this. If the pickups are not efi friendly and jet boat probably has varied running angles. Also you indicated you had more than one tank so could the switching system cause a problem.

[Kitch]

I Know with a carbureted car you can have a problem if the gas tank isn't vented properly.....I'm not sure if you can have a similar problem with an EFI system and a marine gas tank?

[Bonzen]

It sure sounds like something odd is causing your pump to simply not be capable of producing the pressure. For example, cavitation due to an air vortex in the tank. Maybe an engine harmonic is vibrating something in that rpm range, for example that pickup pipe you mentioned starts vibrating which lets air get sucked into the pipe.

it's possible, but the original engine that pulled fuel with the same fuel pump from the same tanks never had this issue. I installed a new fuel pump that has a higher pressure and the problem remains the same.

How much fuel pressure are you losing? What are your normal pressures and problem pressures?

Im losing between 5-10 psi depending on the throttle range and where im at in the rpm curve.

Are your fuel tanks full when it does this. If the pickups are not efi friendly and jet boat probably has varied running angles. Also you indicated you had more than one tank so could the switching system cause a problem.

full, half, almost empty the issue doesnt change. there is no automated switching system, just the manual brass valves. ive inspected them for blockage.

I Know with a carbureted car you can have a problem if the gas tank isn't vented properly.....I'm not sure if you can have a similar problem with an EFI system and a marine gas tank?

vent lines appear to be good.

[lionelhutz]

If the old setup was a carburetor then you could have easily had the same problem and never knew it.

[Bonzen]

If the old setup was a carburetor then you could have easily had the same problem and never knew it.

the only thing thats different is the new motor has a little hotter cam, and 10:1 comp pistons, and aluminum intake manifold.
Old setup was TBI.

[brian617]

Volume problem rather than pressure? Restrictions in fittings, adapters, elbows, etc?

[Bonzen]

Volume problem rather than pressure? Restrictions in fittings, adapters, elbows, etc?

It could be volume...however that still doesnt explain the pressure drop. Something is causing it to happen only in a certain rpm range...and then it fixes itself at WOT.