2011 F-550 6.8L Running Lean/Surging, Need Help

The fuel trims on my truck are reading very lean. Both banks read about +10-15% at idle and light load. Under heavier loads the fuel trims easily go above 20%. The truck is driveable, but when going up a hill the truck will surge. It acts like it's not quite getting enough fuel, but then it finds the lost fuel and gets a surge of power. The trims under load seem to reflect this too. They go from +20% when low on power to practically 0 when it gets the surge of power.

I've checked the fuel pressure. It reads around 35 psi at idle and light throttle. If you give it enough gas it will jump up to 50 psi. I also did a leak down test on the injectors and didn't find anything odd. I replaced the MAF sensor with no change in behavior.

I don't have a lot of experience with the specific fuel system used by this truck. Hopefully someone here has experience with this sort of thing. The fuel pump is controlled by a pwm signal from the pcm. Usually these electronic controlled fuel pumps utilize a fuel pressure sensor on the fuel rail to monitor fuel pressure. This truck does not have such a sensor, so it must use other means to calculate the signal.

If I look at the pwm signal from the pcm, it stays at around 31-32%. The only time it changes is when I push the accelerator down half way or more. When I do this, it will shoot up to a 50% pwm signal. I believe this means the fuel pump is receiving full power. Also, the surging always happens when the pcm jumps between these two duty cycles.

I find it odd that the pcm doesn't command any other duty cycle except for what is uses at idle (~32%), and what is basically full power. Does anyone know if this is normal behavior for this type of fuel system? This is the only thing that has struck me as odd so far.

After replacing the maf you should have reset the keep alive memory

Thank you for the reply.

I reset the KAM. After a short drive the fuel trims went right back to where they were before.

Have you checked fuel quality?

I have ran it through several tanks of fuel. The truck is my own. It is driveable, but the surging going up hills is quite annoying.

Sounds like fuel flow issue to me ---- restriction either before or after the pump, it could even be internat to the tank.

That what I thought at first also. One of the first things I did was measure fuel pressure. Then I took the fuel line off and jumped the fuel pump. It seemed like a reasonable amount of fuel was flowing.

I decided to do an actual flow rate test today. A hose was connected right where the fuel line meets the fuel rail. The pump filled up a 5.25 gallon fuel jug in 4 minutes and 40 seconds. This is about 67 gph. This seems reasonable to me. However, all I did was google fuel pump flow rate per horsepower, so someone feel free to correct me.

I used a battery I had laying around to jump the fuel pump for the test. I'm gonna check the current and voltage at the fuel pump to make sure it's getting enough power from the actual power wires. I'm not sure where to go next after that though.

After ignoring the problem for a while, I seemed to have figured it out. I decided that the fuel pump volume measurement was too low after looking at specs on aftermarket pumps. I was getting around 67 gallons per hour in two different locations on the fuel system. The lowest spec I could find was 85 gph. I didn't think this would cause a problem at idle, but the volume measurement was taken using 100% power. At idle the pcm only commands around 60-70%. The fuel pump must have been a little weak.

I replaced the fuel pump with an oem pump and the fuel trims went from +10% at idle to -5%.