Big Fuel Mileage Drop...anyone else?

[drledford93]

Hey all,

My MS3 has recently had a massive fuel mileage drop. The engine light came on but no noticeable problems from engine response. The only thing at all that is odd is that immediately upon startup after filling up, the idle is rough. Once it's given a little throttle input, it clears up. Nothing more after that. The codes that came up were for a possible vacuum leak and a possible misfire from cyl 4. I can feel no miss at all and see no problem in power output but the mileage has dropped from ~31 hwy to 22mpg. (Yeah, I have a heavy right foot but not *that* heavy...)

Have any of you guys seen this from your MS3s? Mine is an '08 with ~55k. Only mods are SRI, midpipe, BPV and engine mount. They were on the car long before this happened.

TIA,

Daniel

 

[MikeHTally]

What do you consider a "massive drop"? New plugs, maybe. If #4 is not firing reliably, it'll take more gas to do the work. Vacuum leaks can affect performance and therefore mileage. My last tank was one of the worst I've gotten. I wonder if it's the winter gas in non-winter conditions.

 

[drledford93]

...in power output but the mileage has dropped from ~31 hwy to 22mpg.

 

[taf0422]

with the after fuel up issue and it rough idle. Sound like it is a purge value.

My car got a check engine light. It was extremely noticeable after filling the tank up, went to start the car and it had a hard time starting and would idle really rough like the car was cammed. Recommend a Dorman.

1/2 the price of the OEM.

 

[MikeHTally]

31 mpg highway is not impossible, but if you have any kind of "heavy foot", you weren't getting 31 mpg in the first place. 22 mpg is about right. If you're relying on the computer, you're being mis-lead.

 

[imac]

with the after fuel up issue and it rough idle. Sound like it is a purge value.

My car got a check engine light. It was extremely noticeable after filling the tank up, went to start the car and it had a hard time starting and would idle really rough like the car was cammed. Recommend a Dorman.

1/2 the price of the OEM.

I second the purge valve. I had the same problem with mine not too long ago, got a really rough idle after filling the tank. Didn't notice a significant drop in fuel economy, tho. I had the purge valve solenoid replaced and I've been fine since.

 

[drledford93]

I put mine on Friday and had a 200 mile trip this weekend and the mileage went from 22 back to 31mpg instantly. It *was* the purge valve. Amazing stuff.

Thanks for the help, everyone. I appreciate it.

 

[Gertrude73]

What do you consider a "massive drop"?

 

[MSMS3]

Glad OP solved his problem.

But man, he must have a light foot. Keeping a raw egg between foot and pedal and not breaking it? I do drive a bit agresssively, I admit. But I've never gotten better than 22-24 mph in mixed city/highway driving and maybe 26-27 on the highway and I'm now at 66,000 miles.

 

[MikeHTally]

Glad OP solved his problem.

But man, he must have a light foot. Keeping a raw egg between foot and pedal and not breaking it? I do drive a bit agresssively, I admit. But I've never gotten better than 22-24 mph in mixed city/highway driving and maybe 26-27 on the highway and I'm now at 66,000 miles.

Agreed. I can't drive that slow. I got almost 31 ONCE during break-in. It was on a road-trip on the interstate - driving slow, varying the engine loads. I have gotten several 28s, but on day-trips and steady speeds. My overall is around 20.