** PLEASE READ THIS BEFORE BLINDLY REPLACING YOUR HPFP or LPFP BECAUSE PEOPLE ON FB SAID IT’S THE ISSUE. IT IS EXTREMELY RARE FOR EITHER OF THE PUMPS TO FAIL. **
It is normal for the fuel pressure to occasionally drop down to ~150 psi on throttle lifts. If you have fuel pressure on the AP gauge screen 24/7, then you may occasionally see values below 150 psi.
If the fuel pressure is dropping below 150 psi every single time you let off the throttle and/or constantly dropping down to 30-50 psi, then there is an issue that needs to be addressed.
Low fuel pressure CEL's only happen for 4 main reasons:
1) LPFP O-Ring Leak
- Fuel is getting around the o-rings on the low-pressure fuel pump nipple and leaking back into the fuel basket. This type of leak will present itself in several different ways depending on the severity of the leak:
- Small Leak - random pressure drops down to 100-200 psi. These mainly occur when you let off the throttle after a wide open throttle pull. It is difficult to diagnose a small leak as random pressure drops are not un-common. All you can really do is monitor the fuel pressure and if the pressure drops become more frequent and drop to lower and lower psi, then the data is pointing to this leak. This leak will not likely throw a CEL because the pressure drops do not occur for a long enough period of time.
- Large leak - when the leak has progressed or an o-ring is torn, then the pressure will continually drop to 25-50 psi on throttle lifts. Once the leak has reached a certain point the fuel pressure will just start tanking down to 500 psi between 5,000-6,500 rpm. It will feel like you hit a brick wall and the car will lose all power. The car will take longer and longer to start and will have hesitations at idle and cruise. If the car has been off for 6-8 hours, then you can check the fuel pressure in the morning by just applying power to the car, but not starting it. Power on the AP, put fuel pressure on the gauge screen, then turn the key to power on. In this situation the car has been off long enough and all the fuel has drained back into the tank, so when you power on the car and the LPFP primes to get fuel up to the engine the fuel pressure reading on the AP will be purely from the LPFP. It should go up to 52-55 psi, then slower taper down. If it only peaks at 30-40 psi and falls off quickly, that is concrete proof that the LPFP system is not sealed and therefore not holding pressure.
The LPFP should be installed w/ the white stock spacer and 2 o-rings. Even if it was installed correctly, it can still pop out of place over time and leak, or an o-ring can tear. This is an issue with both the DW300c and AEM340 fuel pumps. Neither fuel pump was specifically designed for the 15+ WRX so the fitment is not very good, which causes all these issues. The AEM pump fits ever so slightly worse because it is 1mm longer than the DW pump. The fitment is so tight on the pumps that it seems impossible for the fuel pump to move around and pop out of place, but it is actually fairly common.
2) Fuel Pressure Regulator O-Ring Leak
– basically the same exact issue described above, but on the fuel pressure regulator. The FPR is the silver thing
3) Your stock HPFP does not like e60-e70
– The stock HPFP’s on some cars simply don’t like the higher ethanol contents in the e60 – e70 range. These issues are due to the internals sticking from lack of lubrication. The pumps are lubricated by the fuel, so as the ethanol content is increased, the lubrication is decreased.
- This can easily be tested by adding pump gas and lowering the ethanol content down to e50-e55 and resetting the ECU to clear the CEL’s. If you need to drive and burn off some ethanol to make room for the pump gas, you can turn off the car and reset the will get rid of the CEL and make the car drive normally.
- If the fuel pressure drops go away when the ethanol content is lower, then it means that your stock HPFP simply can’t handle the higher ethanol. The tune will need to be adjusted to have the car make max power at e50. The car will still make the same exact power, the only difference will be that it occurs at e50 now instead of e60.
- If the fuel pressure continues to drop below 200 psi om e50, then #1 or #2 are the reason for the fuel pressure issues.
4) Fuel Pressure Sensor Connector/Wiring Issue
- The 0191, 0192 and 0193 CELs are for the pressure sensor itself and are usually thrown if one of its wires are cut or if it is simply not plugged in all the way.
Occasionally these fuel pressure CELs may also be thrown if you were simply low on gas and gulped up air bubbles in the gas tank.