Fuel pressure

[mnxtrik8r]

Greeting everyone,
I was wondering what fuel pressures we should be seeing at the rail under boost when using a BAP. I realize the FRPS looks at the pressure drop across the injectors not rail pressure. I am just trying to do some troubleshooting. Thanks in advance.

[FordNut]

The pressure should be whatever the tune tells it to be. The BAP will not change the pressure except under 1 condition...

Before BAP: the fuel pressure is dropping at high rpm because the fuel system is maxed out (if you datalog the FPDC will be 100% and pressure still dropping).
After BAP: The fuel pressure stays constant at high rpm (if you datalog FPDC it will be less than 100%).

By fuel pressure, I am referring to pressure across the injectors. A fuel pressure gauge is referenced to atmosphere while the FRPS is referenced to vacuum. So if you look at a vacuum/boost gauge next to the fuel pressure gauge they both go up and down together.

[FordNut]

Oh, to answer the original question the pressure across the injectors is commonly programmed to be 39 psi, but some tuners set it to 60 psi.

[sailsmen]

Please telll us what your mods are so we can help. The lowest I see is 39psi.

[FordNut]

Note that the fuel pressure gauge will go up & down based on intake manifold vacuum/pressure. When under boost, the offset is simply additive (ex. 10 psi boost + 39? psi programmed deltaP = 49 psi) but when under vacuum the scales are not the same. Since 18-20" vacuum is actually closer to 10 psi you may see 29 psi fuel pressure on the gauge (20" vacuum = -10 psi pressure + 39? psi programmed deltaP = 29 psi). These numbers are approximate but I 'spect you get the idea.

The tune can also change the fuel pressure depending on load. So the programmed deltaP in the previous example can be non-linear such as:
20" vacuum = -10 psi + 39 psi programmed deltaP = 29 psi
0" vacuum = 0 psi + 45 psi programmed deltaP = 45 psi
10 psi boost + 50 psi programmed deltaP = 60 psi
18 psi boost + 60 psi programmed deltaP = 78 psi

[Zack]

^^^What Fordnut said.

Anytime you hook an Autometer FP gauge to a returnlessfuel system, it reads about 10psi lower than what it really is.

[mnxtrik8r]

The fuel pressure seen by the PCM was 47 psi and the fuel pump duty cycle was 67%. The only mod is a Trilogy. I changed plugs yesterday from 32 platinums to 22 copper (motorcraft). I am leaning a little toward a fuel quality issue. The car sat all winter and this spring I am only on my second tank of 93 octane minimum non oxygenated no ethanol. I have had no problems in the past but we'll see. The detonation was worse before the plug change and the passes were short because of the detonation. thanks.

[sailsmen]

If you have the stock Tril Tune I doubt it is fuel quality. It is probably air in the IC coolant or a malfunctioning IC coolant pump.

2 bottles of Lucas Octane Booster per 12 gallons of 93 raises it to 103.

As I am sure you realize you are currently on the road to a "blown" engine.

There was a car on the West Coast prof installed Tril and the swore no air in the IC coolant. Jerry and Lidio flew out there only to find air in the IC coolant.

[mnxtrik8r]

I am reasonably sure the system is bled. With the key on it is circulating coolant like a banshee in the tank. Using a stethoscope, I can hear coolant on both cooler hoses at the core. The water is warm to the touch after running a long time, no where near hot. I need to check the fuel, there is a strong possibility that the fuel is a winter blend. I am going to drive it easily and burn off the fuel and try another tankful or two. The car isn't acting lean, as well as the data that I hve logged so far indicates the same. Hopefully it is just crappy fuel, time will tell. Thanks again.