highway mileage drop-Why?

[Russell]

My highway mileage has dropped from about 29 mpg to about 25 mpg. Same roads etc. City mileage is about the same over the same period, perhaps slightly lower. Any ideas why?

Keep in mind this car has had new plugs, oxygen sensor, knock sensors, air and fuel filter over the past 6 months. No codes shown with my peake tool.

I am puzzled. I am thinking partially due to ethanol in fuel. Other thoughts include leaky injectors, weak coils, transmission lockup issues, old fuel pump.

[Tiger]

Snow tire? Fuel pump is my suspect... along with poor ground strap.

[Russell]

Snow tire? Fuel pump is my suspect... along with poor ground strap.

i have 4 snow/winter tires. however, the mileage drop was apparent before I changed tires. How can a fuel pump lower mileage? There is no stumble or hesitation??

[genphreak]

Can you not avoid ethanol laced fuel? It will corrode your fuel system and clog the injectors eventually.

In M50, the valve cover gaskets go bad, these can cause oil to leak adn air to get in- also, many pipes and connectors under the intake can go bad over time. I replace all mine on both M50s, was well worth it.

Perhaps injectors or FPR. Is drivability perfect at all times?

[Tiger]

Air filter?

[Ross]

i have 4 snow/winter tires. however, the mileage drop was apparent before I changed tires. How can a fuel pump lower mileage? There is no stumble or hesitation??

Too low fuel pressure means poor atomization at the injector nozzle and less efficient combustion.
Worth checking pressure. SB 3 bar on an M30 me thinks.
do you have winter fuel blend in your area?

[ss2115]

My highway mileage has dropped from about 29 mpg to about 25 mpg. Same roads etc. City mileage is about the same over the same period, perhaps slightly lower. Any ideas why?

Keep in mind this car has had new plugs, oxygen sensor, knock sensors, air and fuel filter over the past 6 months. No codes shown with my peake tool.

I am puzzled. I am thinking partially due to ethanol in fuel. Other thoughts include leaky injectors, weak coils, transmission lockup issues, old fuel pump.

If you think it started around the time of changing the spark plugs, then regap them at about 0.36.
Off the shelf they come at about 0.30 or so, but a larger gap exposes more spark to the air/fuel mixture and usually returns slightly better economy.

It was a trick in the old days to increase the gap for better torque as well as fuel economy and with coils per plug nowadays it works even better as there's less likelyhood of missing.

[Tiger]

That is still a valid way to tune an engine. By having a larger gap, you force the coil to produce a hotter and harder spark to ignite the fuel mixture. With independent coil packs of modern engine, you can widen the gap for better performance with no problem.

[Russell]

Can you not avoid ethanol laced fuel? It will corrode your fuel system and clog the injectors eventually.

In M50, the valve cover gaskets go bad, these can cause oil to leak adn air to get in- also, many pipes and connectors under the intake can go bad over time. I replace all mine on both M50s, was well worth it.

Perhaps injectors or FPR. Is drivability perfect at all times?

Ethanol is everywhere here in kY with no labeling-no choice. Valve cover gaskets only had a small leak when I changed plugs. No vacumn leaks. my indy and I checked and repaced at least one hose. No help

[bubba966]

Have you changed your tires? Different tires have different rolling resistances. That'd contribute, but I doubt it'd drop the MPG by 4.

Have you checked your tire pressure? Low pressure also adds rolling resistance.

How are the fuel & air filters?

Would guess it's mostly ethanol & lower tire pressures do to the colder weather.