Sandy741

Vetteman Jack

VetteDrmr

Sandy,

Two questions: How are you measuring your mileage, and have you changed your driving conditions when you saw the difference. For example, stop&go driving will kill mileage over a short distance. To really find out if you're having a problem first I'd measure your mileage old-school, miles driven on a tank / gallons pumped.

HTH, and have a good one,
Mike

vader86

Double check mileage the old fashioned way, and other than a change from more cruising to more city driving, suspect something like the oxygen sensors. These do not necessarily set a code until they are dead-dead, instead of on-the-way-to-dead.

PatternDayTrader

Winter blend fuel may be out already. Maybe.

Patman

Not likely, especially in Georgia. We don't even get winter blend here in Ontario until October at the very earliest.

NSC5

I have seen reports of significant mileage drops with other GM engines due a sensors and actuators in the VVT system. It appears these items can be "sick" for a long time before they degrade enough to set a DTC. If this is the issue you are more likely to get a failure and set one or more DTCs along with a check engine light by operating the engine over a wide variety of conditions. Now is the time to drive it a little more aggressively including harder acceleration and also shifting so that it sees a higher RPM range (use manual mode if you have an automatic transmission). This type of driving will typically expose a "soft" error sooner rather than later.

My 2014 Cadillac ATS with the 3.6 direct injected engine had a slight but noticeable drop off in MPG and after two weeks it finally set a DTC and turned on the CEL; one of the cam phase sensors was failing and once replaced mileage returned to normal. I never noticed a change in power level with that engine and I do a lot of passing on two lane roads but the mileage drop was readily apparent.

I would also switch fuel stations because your current supplier may be feeding you regular octane through the "high test" pump and if so your engine is often running with severely retarded timing courtesy of the protective knock avoidance system and sensors.

MMD

PatternDayTrader

Yeah today would be a few weeks early, but it explains a sudden drop in mileage with no other symptoms.

c54u

Check alignment.
Check tire air pressure.
Brake(s) may be dragging.
Change gas station for different gas.

nightroddersp

Doesn't it kill the computed MPG when you remote start and let the A/C cool the car before getting in it?

NSC5

Winter blend gas has between 95 and 98 percent of the energy content of summer blend gas so even if he is burning winter blend already it won't account for the 20-25% drop in mileage. The primary reason winter blend seems to give so much worse mileage for some people is because operating most cars in winter is simply far less fuel efficient. Warm-up is longer, a lot of people needlessly idle the vehicle, tire rolling resistance increases, a lot of owners don't properly maintain tire air pressure when the temperature drops, etc. I live in Central IL and my year around daily driver (ATS with 3.6 engine) loses at most 1 MPG average over the very cold months (far less than 5% from summer average).

Diesel fuel can have a far greater BTU per gallon spread between summer and winter in areas where much lighter distillates (like kerosene) are heavily blended in to prevent fuel gelling but even then a 25% drop in mileage would be largely due to driving conditions and not just the fuel.

orca1946

Change gas station 1st.

jimmbbo

Agree with confirming the mileage using the "old school" method if not done already. Any of the other suggestions could contribute, but a confirmed drastic change in mileage driving equivalent routes points to a problem with the car instead of external causes. Interested to hear the result and fix!

Rebel Yell

That was the first thing I thought of orca.

Sandy741

VetteDrmr

Well, sounds like you've got a problem then. You can try changing stations, but over a 4 tank run, if you're using the same station, they should have run through at least one run of fuel. Also, as mentioned above, I don't think a change in formulation would account for the difference.

Assuming you don't have any other symptoms, it's almost as if you're running E85 (not saying you are, just trying to visualize the scope of the problem.

Could you make a highway run for 200 miles or so and see what your mileage is? Just trying to minimize the variables in city-style driving.

Good luck, and have a good one,
Mike

Thunder22

Is the wife going for joy rides without you and killing it?

I would take it in for a check. Have you checked for any codes?

Markss

Did you get a tank of E15?

aj98