Fuel Mileage is below EPA rating

[Merkava_4]

LINK

According to that link, I should be getting 19/30. My current in town fuel mileage has been consistently at 18.1 MPG. When I first bought the car from the original owner back in 2005, I used to consistently get 19.6 MPG driving around town.

I know this is going to sound crazy, but ever since I replaced the spark plugs, fuel mileage has been less. The factory original spark plugs were ACDelco double platinums. I replaced them at 85,000 miles with ACDelco Iridium plugs. The double platinum plugs were no longer available from ACDelco. And. . . it seems like I had better acceleration with the double platinum plugs too. I know this all sounds far fetched, but I'm just telling you like I see it.

What is your assessment for the drop in fuel mileage?

Thank you all for your comments.

 

[AVB]

It could be anything, did you replace the wires, has the fuel pressure regulator been replaced, tires aired up? Dirty MAF, dirty air filter, vacuum leak, O2 sensor

 

[Merkava_4]

- wires were replaced with new Delphi units
- fuel pressure regulator tests good at 40 PSI idle
- tires are maintained at the factory recommended 33 PSI
- MAF sensor has never been cleaned
- air filter is a Fram AirHog that never seems to get dirty
- no vacuum leak that I'm aware of
- O2 sensors are still factory original
- check engine light is off and no codes stored

I'm thinking increased back pressure from the original catalytic converter could possibly be the culprit.

 

[NHGUY]

For a car that size,engine,its a miracle it gets close to 20 in town at all.I would say slowly degrading 02 sensors,converters,injectors....are leading the cause.

 

[AVB]

When you check those fuel pressure regulators, you need to also check to be sure they aren't leaking fuel into the vacuum line.

 

[kam327]

Did you replace the tires?

 

[hypervish]

Originally Posted By: kam327
Did you replace the tires?

+1, I lost 10% of my mpg when I switched to new tires.

 

[Vikas]

This is very easy! Dig up the original spark plugs from your stash of old parts and put them back in.

 

[Eosyn]

Originally Posted By: Vikas
This is very easy! Dig up the original spark plugs from your stash of old parts and put them back in.


+1

 

[Corvette Owner]

Not that significant. Differences in winter/summer gas could make a difference.

 

[aquariuscsm]

Cheap [censored] ethanol gas is probably the cause too. I find some places fuel seems to burn up faster than others.

 

[faramir9]

Note that the revised EPA city mileage figure is 17 mpg.

GM sedans tend to have tall gears, designed for highway driving. On my '04 Chevy Classic with the 2.2 Ecotec, I got the "old" higher EPA estimate on the highway, (or 1 mpg more when careful, on summer gas), but closer to the "new" lower EPA estimate for city, especially in winter.

Bottom line is these cars were not designed to get good fuel economy in town. Still, if you noticed this right after changing plugs, why not pull the ones in there now and replace them with double platinum plugs, like the OE ones? Delco is not the only game in town. See: http://is.gd/B4bYw7

Worth a shot. At least that's something you can control, unlike winter weather/gas, traffic, etc.

 

[dishdude]

Maybe the gap is off on one of the plugs? If you feel strongly that there is a correlation between the plug change and fuel economy, it might be worth throwing a new set at it. It's possible a plug got damaged during installation.

 

[JamesBond]

Originally Posted By: Merkava_4


I'm thinking increased back pressure from the original catalytic converter could possibly be the culprit.


Uhhh, I wouldn't start there. 1.5 MPG over a tank of "in town" driving is statistically insignificant.

Maybe the new plugs and tires have it running so smooth and quiet that you don't notice giving it more throttle.

 

[JHZR2]

Never underestimate the power of your right foot. We beat the new EPA numbers every time, and I am a firm believer that it is driving style.

Could be something broken/botched, but I think it is user error typically, unless there are known issues with mechanical issues, way underinflated tires, etc.

Still smart to be proactive and check everything out.