Fluctuating fuel pressure and fuel pump duty

[1Saucyyy_brz]

To give anyone reading this some background on my car. 63k miles, built motor and with rods and pistons, full blown motorsports turbo kit. 550cc injectors and AEM fuel pump.

So, ever since i got my car tuned a few months ago this issue has been happening. During light acceleration, the car runs really rich (in the mid 9's to 10's) and when i get into boost, the car runs on the more lean side (in the 13's and 14's). During the tuning process the car was only running rich, the lean issue came up later. At the time of tuning, the tuner thought that i just had a faulty AFR gauge and just told me to replace the gauge. After a couple weeks i replaced the gauge and the running rich issue was still present. Shortly after replacing the gauge, i got a SLIP and CEL light. The codes that were shown were P0456 (evaporative emissions system leak detected, very small leak) and P0441 (Evaporative emissions system incorrect purge flow), the SLIP was C1201. I left the lights there to see if they would go away on their own, didnt happen. After clearing the lights, they come back after a week or two.

A bit more information that might help: I have an ecutek blue tooth dongle allowing me to view in real time a lot of parameters for the car. So, while driving, I started watching a lot of the parameters to see if i saw anything out of the ordinary. What I saw was that the fuel pump duty would be at around 60% while the fuel rail pressure is at about 11.00 to 16.00 MPa. At first when i saw this I thought that maybe the fuel pressure regulator is bad and needs to be replaced, but my check engine lights are telling me otherwise. Another thing i noticed when i was watching over the parameters is that while casually cruising (60-75mph) the fuel pump duty would "flicker" between 60% and 75% just randomly and cause the fuel pressure to jump around with it.

So, what im wondering is that where should i start to tackle the problem? After doing some research I saw that a cracked or pinched line going to (or coming from) the charcoal canister could be the issue. I then saw that accessing the charcoal canister is a pain and involves dropping the rear diff. Another possibility that i saw was that maybe it is the fuel cap, which i can confirm is not the problem. The fuel pump is fairly new with less that 10k miles on it so i dont think its that failing on me.

Any and all information is helpful and i am wiling to answer any questions pertaining to the issue. Thanks in advance.

[steve99]

Originally Posted by 1Saucyyy_brz

To give anyone reading this some background on my car. 63k miles, built motor and with rods and pistons, full blown motorsports turbo kit. 550cc injectors and AEM fuel pump.

So, ever since i got my car tuned a few months ago this issue has been happening. During light acceleration, the car runs really rich (in the mid 9's to 10's) and when i get into boost, the car runs on the more lean side (in the 13's and 14's). During the tuning process the car was only running rich, the lean issue came up later. At the time of tuning, the tuner thought that i just had a faulty AFR gauge and just told me to replace the gauge. After a couple weeks i replaced the gauge and the running rich issue was still present. Shortly after replacing the gauge, i got a SLIP and CEL light. The codes that were shown were P0456 (evaporative emissions system leak detected, very small leak) and P0441 (Evaporative emissions system incorrect purge flow), the SLIP was C1201. I left the lights there to see if they would go away on their own, didnt happen. After clearing the lights, they come back after a week or two.

A bit more information that might help: I have an ecutek blue tooth dongle allowing me to view in real time a lot of parameters for the car. So, while driving, I started watching a lot of the parameters to see if i saw anything out of the ordinary. What I saw was that the fuel pump duty would be at around 60% while the fuel rail pressure is at about 11.00 to 16.00 MPa. At first when i saw this I thought that maybe the fuel pressure regulator is bad and needs to be replaced, but my check engine lights are telling me otherwise. Another thing i noticed when i was watching over the parameters is that while casually cruising (60-75mph) the fuel pump duty would "flicker" between 60% and 75% just randomly and cause the fuel pressure to jump around with it.

So, what im wondering is that where should i start to tackle the problem? After doing some research I saw that a cracked or pinched line going to (or coming from) the charcoal canister could be the issue. I then saw that accessing the charcoal canister is a pain and involves dropping the rear diff. Another possibility that i saw was that maybe it is the fuel cap, which i can confirm is not the problem. The fuel pump is fairly new with less that 10k miles on it so i dont think its that failing on me.

Any and all information is helpful and i am wiling to answer any questions pertaining to the issue. Thanks in advance.

Ok I think all is ok


The fuel pressure you measuring is for the direct injection system. ( there is no monitoring or sensor for in in tank low pressure pump that feeds the port intectprs and the di pump only the di pump fuel pressure is monitored.


The di system uses a combination of injectpr pulse width and fuel pressure to regulate the amount of fuel deliveeed by the di system


It perfectly normal to see the di pump pressure vary between about 4 mpa at idle to 20 mpa at full throttle or when there is high demand on di system.


The fuel pump dury cycle is the duty cycle on the intank low pressure pump. Its also normal for this to vary depending on load and rpm. As there is no feedback to the ecu regarding the fuel pressurw on the low pressurecin tank pump the fuel pump duty cycle is just a look up tavle based on engine load and rpm from memory and ahain its normal for this to vary with engine load and rpm and likely also with the settings in the port to direct injection ratio tables.


Fueling is determined by eitger the mass air flow sensor or pressure senspr if your tune used speed density. the ecu then calculated the fuel needed based on air flow rpm and load and tgis delivery will be a combination of the port and direct ibjection systems.


if the fueling is lean in some places and eich in others it can be the mas air flow scaling is incorrect or the speed density ve table is incorrect or the replacement port ibjectprs are incorrectly scaled .

[steve99]

re the evap codes are you runnog boost control ?? if so it likely using the evap purge solenoid output to drive the boost control solenoid so the tuner need to delete all the evap codes in tune

[1Saucyyy_brz]

Originally Posted by steve99

re the evap codes are you runnog boost control ?? if so it likely using the evap purge solenoid output to drive the boost control solenoid so the tuner need to delete all the evap codes in tune

Lots of typos here so its a bit hard to understand, but i am running a mechanical boost controller. The boost controller shouldnt have any interference with the fueling. At one point we tried installing an electronic boost controller and for some reason we couldnt get it to respond, so we ditched it and went to mechanical and it has been controlling the boost just fine.

Your reasoning for the fuel rail pressure fluctuating makes sense, but heres my other question: Why is it that the fuel pump duty (measured in a percentage on my ecutek dongle) would randomly jump from 55% to 78% just while cruising down the highway, with very little throttle change.

Ill have to take a look at the MAS sensor, maybe there is a small leak somewhere around it causing the car to demand more fuel, or maybe even a leak on the couplers that are close to it. Ill take a look, thanks for the input.

[steve99]

Originally Posted by 1Saucyyy_brz

Lots of typos here so its a bit hard to understand, but i am running a mechanical boost controller. The boost controller shouldnt have any interference with the fueling. At one point we tried installing an electronic boost controller and for some reason we couldnt get it to respond, so we ditched it and went to mechanical and it has been controlling the boost just fine.

Your reasoning for the fuel rail pressure fluctuating makes sense, but heres my other question: Why is it that the fuel pump duty (measured in a percentage on my ecutek dongle) would randomly jump from 55% to 78% just while cruising down the highway, with very little throttle change.

Ill have to take a look at the MAS sensor, maybe there is a small leak somewhere around it causing the car to demand more fuel, or maybe even a leak on the couplers that are close to it. Ill take a look, thanks for the input.

yeah the fuel pump duty cycle is just calculated from load rpm and i think injector pulse width, so if the port to direct ratio changes so can fuel pump duty cycle. Load can change a lot with very little throttle movement its not true engine load its calculated load by ecu based on air flow and rpm