My gas mileage has gone down lately. I just had a Magnaflow muffler installed and I'm thinking the computer may still need to adjust. Anyone else come across this?

Also, has anyone tried to keep track of mpg differences between driving smoothly vs. having a heavy foot? I'm thinking my right foot may be enjoying the new muffler a little too much.
Any help will be appreciated.

 

Hi there- a couple of things...

1. Turn on your headlamps, and then disconnect your negative battery cable for 5 minutes. Reconnect, and then drive at partial throttle for 10 to 20 miles. Give it a full downshift, full throttle dose of gas every now and then. This will set your computer up.

2. Catch here- if you're not running at least an OEM replacement air filter like a K+N, and a full system is advised, you're not getting all you can out of your exhaust. An engine is simply an air pump, and what goes in, goes out. As far as your computer goes, with at least a $40 K+N replacement filter, it will run much better on the new, fuller MAF readings, and thus push your exhaust a bit harder. I recommend the Volant cold air system (not their open one), but any will do. If you're short on sheckels, just pony up $40 and get the OEM replacement. You'll still get 2 or 3 HP.

Good luck, and have fun...

 

I have a K&N replacement in there right now. I also have a
FIPK that I pulled off my F150 4.6 before I sold it, just haven't had a chance to put it on.

Thanks for the reply.

 

In your case it could be some of both, but IMO the heavy right foot is usually the biggest mpg killer.

I got what appeared to be a very slight increase after installing my Gibson cat back system. At that point I wasn't hoping for much more in the way of mileage increases, as I had already got gains from other mods (pulleys and ignition upgrades).

I did a lot of mileage tracking at first with the other mods, but didn't notice any large change with the exhaust. Having kept better records might have showed a better improvement than I thought, but I was satisfied that I got a little more punch with no mileage loss.

 

I have a 2002 5.4L 4X4 with about 4100 miles as of today. I got a flowmaster si/do at 1000 miles and did'nt notice an increase in mpg (13-14) but did notice a little more throtle response and HP.

At about 3300 miles, right before I took a road trip, I installed a K&N replacement filter(not the FIPK). Even taking a road trip I only got 15 mpg. I'm not going to expect any more of an increase. I believe the engine is broken in enough to notice.

It runs real well with the flowmaster and the K&N filter combo

 

I reset the computer and so far that seems to be doing the trick. Thanks for the replies.

 

As my father's 97 F-150 ages (engine), he has been getting better gas mileage. He just recently came back from So. California and got a pretty consistent 17 Mp/G. I, on the other hand, have been getting 10 Mp/G during my city driving. I have been heavy on the accel. At times though especially on long trips I can attain my fathers Mp/G.

I think the only way to keep your gas mileage "economically" in the green is by servicing your vehicle regularly, use Techron or Redline SR-1 every once in a while.

Just my opinion...

P.S. It is a good idea to reset your computer after a mod.