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Well, I've had the car for 4 years now, and since the first day of owning it, I've suffered from poor fuel economy. I think it's time to cure this.

First off, it's not a lead foot. It's poor regardless of how I drive the car. It's always right around 12-13 MPG. If I'm on the highway, 12-13. If I'm romping on it 12-13. If I baby it around town, 12-13. It's a 6 speed, with stock gearing.

I dropped it off at the shop today, and they'll put it on the scanner and hopefully something obvious pops out at them, but he wasn't to optimistic.

It's got brand new wires, newish fuel filter, newish air filter, newish plugs (4 tip platinum--perhaps that has something to do with it.) Brand new AC Delco O2 sensors when the Headers were installed.

The mods on the car are Mid Length Headers, and a Borla Catback.

If the shop does not find anything, what all could I have them look at, or try myself?

Thanks.

LiENUS

Check your mass airflow sensor and mass air temperature sensor. Also while great for power a 160 degree thermostat reduces fuel economy slightly. What temperature is your car at when driving around? The higher the temperature the better for fuel economy (though worse for power). Generally anything to improve power worsens fuel economy. Also is your clutch slipping? a minimal amount of slipping can severely impair fuel economy. Have them check to see how much knock retard you are getting if you are getting too much your car will go into low octane mode although this wouldn't explain the huge drop in economy my bird with an A4 (and 2.73 gears mind you) gets 27 mpg highway with 93 octane running full timing and 21 mpg in low octane mode.

TobyZ28

Check your tire pressure, steering alignment, clean your injectors, toss those bosch plugs

Severous01

yeah...it's the plugs. get some NGK iridium

also, have you replaced the 02 sensors recently? they foul and wont throw a code but will give bad mileage. you may also have a leak in the exhaust somewhere. i have one where my egr's meet the headers. i have pace setters and the bumg wasnt supplied...and the stocker was too small. i called them and they told me everything should fit.....POS customer support.

any way, look for air leaking into exhaust and electronics.

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Clutch is fine, it was replaced last year. No change. Tire pressure is defiantly not it, as I've gone through 3 sets of tires on the car, no change. Not the steering alignment either, as that was done when I lowered the car.

I'll have em look at the mass airflow sensor and mass air temperature sensor, also have em take a close look at how much knock it's detecting.

As I said in my original post, it has brand new O2 sensors. I do not hear any exhaust leaks.

Ironxcross

Even with a tune?!

LiENUS

Especially with a tune. Lower coolant temperatures == lower thermodynamic efficiency. but its not enough to drop you to 12-13 mpg.

Ironxcross

I thought the 160 t-stat mod put the engine closer to its ideal temp point (like 180 ish or something), so that would increase power and efficiency.

The tune was to cancel out the poor mileage from the car running in open mode constantly (or whatever the mode is called where its trying to warm up)

LiENUS

Nope a 160 tstat tune just bumps the timing a few degrees and sets the fans to come on at an earlier point. a 160 tstat increases volumetric efficiency which means your car ends up burning more fuel at any given rpm.

Ironxcross

Damn it.

LiENUS

Its not a huge difference maybe 1 to 2 mpg at most. Properly tuned maybe a half gallon? You might actually see a gain in mpg when you get a tune because of the increased timing. But if you took a stock car and got a mail order tune checked your mpg then put a 160 tstat and got another mail order tune for your car you'd see a slight drop in your mpg.

zx1216

damn, you must like to drive, 3 sets of tires in 4 years? Check all of your plug wires again, i was runing on 7 for a while and that droped my mileage a good amount. Have the coil inspected and see if its in spec, if it runs smoothly the opti is prolly fine. My car is cammed and still does 20's on the hw so idk what the deal is with yours...

Ironxcross

What I did was I get the mail order tune for the 160 stat before I put it on, then I put everything on at once (pcm, t-stat). I haven't really noticed a difference in the mileage, but then again most of my stuff is city, so its going to suck either way.

DR.ZED

I noticed REALLY poor fuel economy when my plug wires (MSD routed over the top) got old.

I am wondering IF no cat has anything to do with economy. I'm going to do a test with my straight pipe.

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Unfortunately I don't think it could be the wires. It was bad with the stockers, and bad with new heat shielded expensive thingys.

Thanks for all the tips though, I've got a lot to recommend to the shop!