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Fuel pressure not rising with boost

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hello all,

i have a return fuel system with aftermarket FPR, vacuumed referenced.

base fuel pressure is 58psi.

in -10psi of vacuum the pressure is 48psi, in 0psi of vacuum it 58psi.

but in boost, it will not go over 62psi

i tested it with a vice grip on the return line from the FPR, the pressure got to 80-90psi at the gauge and also at the fuel pressure sensor.

in my logs i see that in boost / high rpm i have a voltage drop, from 13.5v to about 12.5v-11.8v.

cannot seems to decide where is the problem coming from.. alternator, battery, FPR.

hope to get some insights from you guys [=


attached some screen shots

fuel test


in boost

Attached Files

do you know what voltage yo have at the fuel pump? I used to fix issues like this. The most time Bigger Wire gauge resolves the problem. But if you have voltage drop at the battery at high Rpm, it seems your voltage regulator on the alternator has a problem.

Don't know the pump voltage, but i have a thick wire (8awg i believe) and a 40amp relay to the pump.

I looking to find out if my problem is at my alternator, or what do i need to do to check.

This is the most common issue I see on the dyno. Things to check.

As Josias mentioned, what is the voltage at the fuel pump? I would check this with a multi-meter, ideally you should see 13.5-14.5 volts or there abouts.

If this is ok then it could be as simple as a blocked fuel filter, sometimes I will fit a fuel pressure gauge at the fuel pump and another at the reg, check the pressure difference between the 2 and this will show a restriction. Could be a collapsed fuel line/hose or filter.

If all of these are ok then next I would be testing another fuel pump or making sure it isn't leaking internally, faulty pump will typically show when pressure starts dropping. A faulty reg will typical show the pressure sitting stable and not increasing.

I would also be checking your ecu voltage as that is also a fault (although most likely not related to the pressure). I would match the ecu voltage to the battery/alternator, if the battery/alternator reads the same (low) then you have an alternator/wiring fault. If the battery is reading 13.5-14.5 and ecu says 12 then I would check the power supply at the ecu and see if that matches. If it matches the battery then I would say you have an ECU grounding issue or internal fault like a cracked board. If the voltage matches the ecu readings and is low then I would check wiring/power supply to ecu and repair.

What pump are you using - you may find a flow graph for it? There is a 3D relationship between the voltage ACROSS the pump - you also need to make sure the earth/ground is up to the job - and the potential flow at different pressures.

With the alternator, it may be due to slippage at high rpm, try tightening the belt and/or fit a new one. Depending on the vehicle, you may even be able to fit a slightly shorter one that the tensioner will put more tension on.

Thx for the replays.


In the battery i see 13.5v

In the pump i saw 12.8 replaced the battery to a new one, now it's 13.10 at the pump (at idle)

Nothing is clogged in the fuel lines ( will check pressure difference later on this week)

Fuel pump is not dropping pressure, pressure is stable.

Tested the regulator with a air compressor, taking the fuel pressure to 6-8bar no problem.

No difference in ecu to battery voltage.


Alternator belt is new and checked it today , all good.

Fuel pump is a AEM 50-1000. Good to the pressure.

I'll keep on looking, if you guys have more insight I'll be happy to hear. Thx (;

So it would seem, depending on the fuel you're using, the engine efficiency and the power the engine is supposed to produce -


Note the pressure-volume relationship.

HOWEVER, that is with the pump operating at 13.5 volts - I assume they mean ACROSS the pump? If I read your post correctly, you have 13.1V at idle TO the pump, what is it across it, as that takes the earth/ground out of the equation - can you fabricate a means of monitoring the actual pump voltage, terminal to terminal, under the load conditions - this may be important as the voltage may be expected to drop as current increases and, as the pump's power is roughly proportional to the square of the voltage, a 10% voltage drop from spec' will cost ~20% flow at the same pressure.

Sorry about harping on on it, but seen this cause a lot of problems.

The other thing to try is to monitor the fuel return to the tank under the loaded condition, it'll take a bit of messing around to do so safely, but having the return run to a spare fuel drum would confirm if the pump is able, or not able, to maintain sufficient fuel supply under load.

Here's what the things I would check :

1. Putting a boost gauge tee'd up at the FPR vacuum to confirm same boost is reaching the FPR (unlikely but probably takes 2 min to check)

2. Checking the voltage drop across the Fuel pump during a pull @ full boost (make yourself some kind of adapter if it's an In-Tank)

3. Diagnose that voltage dropping to 11.xx during a pull. Check it at alternator output / battery terminal.

4. I guess that if you don't have sufficient flow, the pressure would drop. Make sure the AEM you have can provide sufficent flow for your power goal.

Pretty much the things Gord said explained differently hahaha!

Hope it helps,


hello all,

managed to do some testing today (covid-19, quarantine.. lol)

i have a difference in my ECU voltage reading and my battery reading, i think i know why, i have the ecu power supply going throw a toggle switch (need say no more). after that been said.

here is what i found

car in not been started over 24hours

multi-meter to battery


multi-meter to alternator - positive to the alternator output, negative to battery



car started - waited until the rad fan kicked in car is at idle

battery 14.13v

alternator 14.34v


hooked the multi-meter inside the car, Alternator positive to alt' positive output, negative to battery

throughout my drive and my 2-3 pulls multi-meter voltage showed me 14.22v

ECU dropped from 13.7v to 12.1-12.3 throughout the pull



13.96v at idle and regular drive

throughout the pull 13.97v

ECU shows 12.2-12.0


Fuel pump - positive is connected to the positive feed that goes to the pump from the relay (have a 40amps relay)

negative is connected to the negative that comes from the relay

(hope that is clear.. )

car at idle 12.8v-12.7v

throughout the pull i see 12.5v

ECU shows 12.2v-12.1v.

also very important to mention,

it's a ek civic STOCK fuel lines, stock fuel filter.

after i saw that my voltage is not dropping at the pump (it's low, but not dropping)

i'm thinking it's the fuel lines + pump maybe..

will replace my fuel lines to -6an soon, thinking abut changing to the walbro 450/252 pump.

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