Sharp MPG decline since Y-Pipe install??

[KyngofPop]

Wats up all? Just a question for ya all. I recently installed a y-pipe on my 94 mtx that I bought from a forum member and am quite happy with the results of the performance and sound but the drop in my fuel economy was sharp and nearly instant. I travel about 60(+/-) miles roundtrip highway daily for work with very few in town trips and/or stops.

When the car was stock i consistantly got at LEAST 485 miles per tank(26/27mpg) with the cruise control set at a steady 55 mph and waiting until the needle hit the bottom of red before refueling.

Since the y-pipe install and under the same EXACT conditions the miles per tank had dropped to about 370(20 mpg)??

All maintenance is up to date, change only came once the stock y-pipe was switched for the more free flowing design(not sure of manufacturer). No 7k runs, and no "dogging" on the car at ALL during the "controlled" circumstances. 02 sensors are 3 yrs old and functioning properly, and no engine codes.

Does the computer require a relearn time before I will see my mpg's increase once again to stock level or is this something I'm just gonna have to live with? TIA.

 

[zach44102]

Did you change o2 sensors? or did you use the ones already in the y pipe?

Or perhaps you love the performance gain and your right foot kicked your mmmmpeegeee's ass.

 

[kevinspann]

I'm gonna go with your driving Freddy.

 

[KyngofPop]

I changed the 02 sensors with the ones that were in my stock pipe. I did a few heavy runs but that wasn't under the "controlled 55mph" circumstances, they were off the record...and yes I like the way it performs with it but I don't drive like that all the time so no "mmmmpeegeee ass"kickin is goin on at all...guaranteed

Maybe my sensors are wearing out? and whaddya mean Kevin haha

 

[bpd1151]

Although a different application as far as model years.....

But after I installed my catless down pipes (my 2010's DD for now) I drove for about 3 weeks with the old tune file and I threw a CEL as expected.

Once on the dyno, I was told my two remaining front o2's were bad (even though I never threw a code).

Apparently they were running excessively rich, and wouldn't lean out, even when commanded to do so.

Now that everything's all in order, my MPG seems to have decreased as well.

But it's only most noticeable under WOT runs.

If I baby it, and use cruise control sporadically, my MPG's seem just ok.

Point is, I too wonder if the more free flowing design has adverse, undesired results in the MPG department?

 

[zblackbeast]

Reset the computer and try again. If anything, I saw an increase.

 

[RonPorter]

I just had a new y-pipe installed (nothing special, the Eastern Catalytic from Rock Auto) and I put in new O2s. Car definitely feels better! Can't tell about mpg until I get the speedo fixed, though. Although, if we have a western MI dinner meet this weekend, I'll check mileage using the GPS (too damn cold to work outside........snow on the grass today!!).

 

[jayro]

Reset the computer and try again. If anything, I saw an increase.

This. If the MPG drop continues then I would think your O2's are on the way out. They have to get pretty bad before they will throw any codes and will cause you to run very rich.

 

[pejohnson]

I think mine are going out as well. Gas mileage is down and every so often the engine light comes on for just a second or two then it goes off. I have an aftermarket y-pipe as well. Any preferences or recommendations on O2 sensors? Or all O2 sensors the same?

 

[kevinspann]

Bosch/Denso/Motorcraft

 

[zblackbeast]

Also, are the cats clogged? This could make a difference.. I know you said it was used

 

[RonPorter]

I think mine are going out as well. Gas mileage is down and every so often the engine light comes on for just a second or two then it goes off. I have an aftermarket y-pipe as well. Any preferences or recommendations on O2 sensors? Or all O2 sensors the same?

My CEL flickered at idle. Then the shop told me one cat was rattling.

I got the Eastern Catalytic y-pipe from Rock Auto, and two 13942 Bosch O2s. Turns out the 13942 is too short for the rear O2, so I returned it and got a 13950 Bosch, with a 5" longer lead. You could get two 13950s, but the 13942 is $13 cheaper, so get one of each.

 

[zach44102]

Clay its the magnaflow y pipe off the orange car. They are definitly not clogged.

 

[KyngofPop]

Nah, the cats aren't clogged at all, this thing runs like a beast since the swap. One other thing I also did was to swap in my spare set of 215/65/16's in place of the 225/55/16's that I was running. Not sure if that would eat up mileage but wouldn't think so since its more close to stock size than the 225's.

I'm gonna reset the computer and if things don't change a freshening of the 02's is in order sometime in my future. Thanks a bunch.

 

[Irish Pride]

Nah, the cats aren't clogged at all, this thing runs like a beast since the swap. One other thing I also did was to swap in my spare set of 235/65/16's in place of the 226/55/16's that I was running. Not sure if that would eat up mileage but wouldn't think so since its more close to stock size than the 225's.

I'm gonna reset the computer and if things don't change a freshening of the 02's is in order sometime in my future. Thanks a bunch.

235/65/16? Is that what you meant to put?

Stock size is 215/60/16. Using a 225/55/16 would make your mileage look better than it actually is and should be. 235/65/16 would be bigger than stock and make you mileage appear less than it actually is and should be.

I think you need to compare your speed on the guage with a gps and see how far off it is. You may not have been getting the mileage that you thought you were and switching tires just threw it off even more for the worse.

 

[KyngofPop]

Sorry, typo. I meant 215/65/16's.

 

[kevinspann]

215/65/16 is a pretty tall tire - taller than stock. So it should actually lower cruising RPMs slightly and POSSIBLY increasing gas mileage.

 

[platoribs]

Also make sure your thermostat is working per spec. if it doesn't heat up it won't drop out of closed loop and will cost you about 6-7 MPG...

 

[AREA 91]

Check to make sure there are no leaks where the Y pipe meets the manifolds.
This can definately make the O2's and milage suffer.

 

[KyngofPop]

Checked speed to gps today and with the current tires 55mph reads 60mph on gps. Area 91 I think u may me onto something buddy, I hear a weird almost whistle type noise if I shut off the ignitions while idling. Had a hard time with the far bolt on the ypipe when installing but thought it was snug enough. Gonna retighten them after work. THANKS1