About a month or two ago, my car had a sudden drop in mpg according to the car's computer. Typically at 40-45 mph in 5th, the active mpg gauge in the instrument cluster would indicate about 32+ mpg. Now, it indicates from about 20-32.

After it occurred, I cleaned my intakes, maf sensors, and throttle bodies but the gauge still reads the same.

Does anyone have any ideas of what could've happened? Or am I just overreacting? I haven't noticed a significant loss in power or anything. My car has stillen gen 3 intakes and a stillen catback exhaust for about a year now with no issues until a month or two ago.

 

Not sure what mileage your G is at but could be a few things outside of what you said.

-Worn or dirty spark plugs prevent your engine from efficiently burning all of the fuel in the combustion chamber resulting in wasted gas.

-Dirty or clogged fuel injectors will cause inefficient combustion, wasting gas.

-An inaccurate or defective coolant sensor or engine thermostat will send an incorrect signal to the engine computer causing the computer to think the engine is cold and supply more fuel.

-Having bad oxygen sensors

Hope this helps. Try running obd2 scanner and see if there are any codes or pending codes.

-Brandon

 

There is only one accurate way to calculate MPG
Fill tank, set odometer to zero.
Reset computer
Drive 350 to 400 miles at a steady 70-75 MPH no cruise control, tire pressure 35lbs when cold
Remove junk from trunk
Refill tank and divide miles traveled by number of gallons to refill tank
Reset computer again

Four fill-ups from NJ to Florida
Two fill-ups for local driving while in Florida
Four fill-ups on the return to NJ



Frequent starts and stops, short trips kill MPG on these vehicles

Telcoman

 

The mileage is not too high. She just reached 57k.

I think it could be the O2 sensor. I did have a cel come on for that a while ago but it has since been gone. I convinced myself that it was intake related because of the somewhat poor condition the cai came in. Completely forgot about the O2 sensor.

I'll definitely check for codes whenever I have time. Thanks for the response!

 

Ok, not bad mileage. I would deff check the O2 sensor and you can run LUCAS fuel treatment + fuel injector cleaner product, it is pretty much the best and cleanest additive I have used (I only use LUCAS in my fuel tank/especially during the winter months by me) I also would calculate your mileage manually, that is the most accurate way as Telcoman also stated. The computer I believe works off most recent trips and is never 100% accurate.

If you are in the market for a new scanner, I just bought BlueDriver scan tool. It is Bluetooth also. My G is a 2013 and it qualifies for the additional features when the tool syncs with the vehicle, some models wont.

Keep me posted!

 

Yeah. The computer is not the most accurate way to calculate the mpg but seeing it change so suddenly makes me worry. I haven't calculated the true mpg before it happened though so I have nothing to compare it with to see if there was an actual change.

 

I'll take a look into some fuel additives too. O2 sensor definitely seems to be the culprit now.

I was planning on getting a bluetooth scanner but ended up deciding against it. Didn't feel like I would be using it enough to be worth it. And Autozone is like a 5 min drive.

 

I understand! I was not going to but then i did since it was compatible across many models.
I have not had any lights come on but I ran the scanner on mine and got a ABS code for irregular battery voltage. I am not sure what that means entirely regarding a remedy for it.

I hope the O2 sensor fixes your issues. I get pretty good mpg on my G so I feel you when it declines suddenly!

 

Quick Update:

Went to check for any codes but nothing came up. I don't have time to check the o2 sensors or anything because I go to college away from home. I guess I'll mess with the car during breaks or something.

 

Ok, hopefully you can resolve that with the sensor.

 

Another Update:

Still haven't checked the o2 sensor but I did re-tightened and re-positioned the intakes and added some thread locker to the engine cover bolt and it seems to be better.

 

Thats good! Let me know if you change the o2's.