Hello, I drive a Supercrew F150 5.4L XLT 02 since 9 years, I put a Fuelflex modul since 8 years, since some months I Misfire on cylinder #3 (P0303) but only during towing with Ethanol (over 2000/2500 rpm), when I drive on gas all is fine, I changed Flexfuel modul, plugs, coils, header gasket, without any positive results, it's very curious to have the problem only with ethanol (E85), I can drive very hard and at high speed with just the truck, but with a trailer as soon as I ask more power i lose one cylinder, with misfire...but like I said only on ethanol, someone has already had same problem? I know engine needs more ethanol than gas, so I go check fuel pressure and maybe try to change fuel injector...
all ideas are welcome....thanks

 

I think your from France.
We do not use such a modification on these year trucks here in the US.
They are not designed to run on Ethanol originally.
Not being familiar with the module you are equipped with so I cannot comment on it.
However, Ethanol will run leaner that straight petrol.
When you put towing load on your engine a richer mixture is required for all cylinders as dictated by the Oxygen sensors verses accelerator opening, engine ignition timing and road speed..
Obviously this is a problem with one cylinder in your engine.
My suspect is all your cylinders are running on the Lean side and risking misfires and even over heating.
Only way you might change the misfire in your one cylinder is to select an injector that happens to spray more fuel than normal from the same pulse width from the computer.
This only a hope of sorts because the fuel injected is controlled by open periods rather than just continuous volume.
You might try a 21 pound per hour in place of the stock 19 pound per hour injector.
The most proper way is to alter the operating program in some way so all injector operate richer by wider pulse widths as a normal consequence will be richer.
One possible way is to run a colder thermostat so the computer gets a colder signal that normally tends to richen up the fuel injection.
Or the Cylinder Head Temperature sensor signal telling the computer the same thing.
You may also have a faulty ignition coil on that cylinder.
You can also use a Scanner to look at the operating conditions and should see what happens to the long term fuel trim tables as a way to determine what richness is being supplied to the engine .
This is an interesting situation and needs to be looked at in a technical way, for an improvement solution.
Here in the US, some forms sport auto racing uses straight Alcohol for fuel. This requires much more fuel volume the fuel control must provide in volume or the engine overheats and melt piston crowns damaging the engine.
The absolute fuel mileage under these kinds of conditions is very low.
Basically your running a module, E85 and towing are a worst case 'stackup' of operating conditions that has very little tolerance in the system..

Good luck with it.

 

Thanks for your reply Bluegrass, you right I'm french, with a gas at $5.9 per gallon, we've a great motivation for become green and put Ethanol box for have flexfuel trucks..
In the past I had problem with fuel pump, and lose pressure during a drive but seem different I lose only one cylinder, for example on highway during a hill, after some second at 2500 rpm, I feel misfire so I let the rpm down under 1300 rpm some second and I can use it normally until 2000 rpm, but sometime it's little dangerous to drive under 50mph in the traffic, injector seem a good way...I go check that more closely.

Thanks

 

Some additional comment for you to consider.
There is a lot going on within the operating program over the RPM range.
If the Ox Sensors are still calibrated for Petrol, that is one area that is causing the lean condition.
Perhaps a change in them to units more calibrated to alcohol would respond better to the E85. Maybe ones form a later Flex fuel truck might work better.
The gas sensors are calibrated for 14.6 to 1 optimum A/F ratio.
Alcohol ratios are a bit richer.
At throttle positions greater than about 3/4 open, the Ox sensors are not in control.
The engine reverts to fixed fuel tables that may or may not be richer because these tables also are moved in value over time.
Another way to richen up is to install an adjustable fuel regulator and adjust fuel pressure about 10 psi higher.
This should make the injectors meter more fuel per the operating pulse width.
Some times we wonder if burning more fuel from these off straight fuel mixes defeats the original purpose trying for cleaner air quality.
Good luck.

 

but this truck worked very well at ethanol during 8 years without any trouble at all, so I think it's really something which lost performance

 

Misfire on a cylinder that is reported as a code 30x is detected by the crank sensor measuring rotation time.
If the coil is low on output, the boot leaks voltage at the top of the plug, the plug tip is eroded, moisture in the plug well, the fuel injector meters less fuel or there is a cylinder fault, all contribute to cylinder pressures at time of combustion.
If the pressure developed causes slower rotation time under load as compared to a table developed for the average of all eight cylinders, the logic sets a misfire code when out of table limits.
Good luck.

 

What about swapping that injector with one from another cylinder to potentially narrow down the problem?

 

That's what I was going to suggest.

 

If swapping the injector doesn't solve the misfire and assuming you originally gapped the new spark plug to .054, reduce the spark plug gap to .045 and see if that clears up the issue.