Installed a Flowmaster 40 muffler on my 5.4 (manual) and MPG dropped from almost 15 MPG to 11 MPG!!!! Granted, it sounded so good that I probably was driving a bit more agressively, but after 2 tank fulls and very careful driving now, it remains just as low.

I'm wondering if the loss in back pressure (exhaust exits forward of the rear tire) could be sending the processor crazy. Any thoughts or similar conditions????

Truck is a 2006, SD reg cab, 4x4 with manual 6 sp and 3.73 LSD with 94K ,

Thanks,...GAP

 

Havent delt with flomaster in Quite a few years, when the 40 series come out there were several issues with them. Not sure if they ever resolved it. " sounds LIKE NO "

Back about 7 to 10 years ago, the 40 series where nothing. it was like putting a cheery bomb on your truck. and yes I think there was some talk of MPG. do some more research and see if you can swap it for a 60. " if there still making the 60.

Again it's been several years for me. I know the 70 and 80 series were OK.
the 40's were sold to any Joe Blow that wanted a rumble. in short some guys were drilling holes in there stock mufflers and getting better results than a 40.

Best of luck

 

did you change out the entire exhaust or just the muffler? back pressure issues really only exist when people start running dual 3'' exhausts or a single 4 inch or something equivalent. I swapped my stock muffler out for a flow master 50 series and i didnt notice any drop in mpg.

 

Sure hope not. I am limited to aftermarket exhaust, unless I want to get custom bends,,

 

Just forward of the stock muffler. Cat still in place. First thought was open-loop like running without O2 sensors, but no trouble codes returned from OBD-II.
My best guess now is that I lost the ability to hear when any secondary throttle body opening occurs. Before the muffler swap, the truck was so quiet that I could tell when those secondaries were operating. Now I may be using fill opening without knowing it. There is that much of a difference in gas mileage, I would think....

 

you don't have secondaries. its a fuel injected motor.
most likely you are running it more agressively without knowing it.
any decrease in back pressure is a good thing as far as MPG is concerned so its not that.
i doubt that your back pressure went up with the new muffler.
if after 2 more tanks of gas your mileage is still that bad, put the stock muffler back on.


as long as your O2 sensors are still in the right spot and are functioning properly you will be in closed loop (assuming nothing else changed)
you could go ahead and buy new O2 sensors if you wanted...

 

Some performance mufflers are directional. Installing backwards causes excessive back pressure. Not sure about the Flowmaster 40, but you might want to research that,

 

Yes. I know it's fuel injected. Maybe secondary circuit is not the proper term and secondary fuel mapping is, but there is a big difference when you're accelerating at an easy pace and WOT. Before the muffler, I could hear the difference, and stayed out of that range. Now I can't. We'll figure it out. The muffler definitely was the start... so probably just driving style I'm assuming. Thanks for the notes.

 

..........

 

there is no secondary circuit either.
its a smooth transition from idle to full power.

 

"IMTV?
From the Ford Manual:

The IMTV is a motorized actuated unit mounted directly to the intake manifold. The IMTV actuator controls a shutter device attached to the actuator shaft. There is no monitor input to the PCM with this system to indicate shutter position.

The motorized IMTV unit is not energized below approximately 2,600 RPM . The shutter is in the closed position not allowing airflow blend to occur in the intake manifold. Above approximately 2,600 RPM the motorized unit is energized. The motorized unit is commanded on by the PCM initially at a 100 percent duty cycle to move the shutter to the open position, and then falling to approximately 50 percent to continue to hold the shutter open."

Sounds to me like this may be the changing intake sound I refer to when accelerating at a greater pace. Not wanting to continue this debate over terminology... so what do you really think is the issue in losing 25% fuel efficiency?

 

Perhaps with the less back pressure, the oxygen sensors or ECU is seeing a lean condition due to less pressure, and it's dumping fuel (richening) the mixture to make up for it. I know this was the case with the camaros when they did exhaust systems and had to install a management system like the MAFT to tune the enrichment down.

 

That's what I am thinking. Unfortunately, I'm not sure if I can compensate for that without changing the exhaust again... I wonder if any aftermarket ECU controllers would be able to compensate?

 

You could get a programmer like SCT or the EDGE with custom tunes.

 

the 05+ mod motors use a variable size intake... thats what that peice is discussing.