*UPDATE* HELP! My car milage went from 35mpg to 25mpg

[JEDI]

I have a 2002 Chevy Prizm w/15k miles. A couple of weeks ago, I noticed that my gas milage on highway went from 35mpg to 25mpg in 1 tank. I changed the oil and put in a new air filter. Same problem.

Then I used STP super-concentrated fuel injector cleaner. I went back up to 35mpg. Then I filled up again, and I'm back down to 25mpg. I'm using Shell gasoline.

WHAT'S WRONG???

EDIT: Took my car in to the dealership. they did a diagnostics. found nothing wrong. o2 and temp sensors ok. spark plugs ok. they have no idea why it dropped 30% and couldnt offer any suggestions.

sigh..i've changed gasolines (Exxon), but still the same. i've added some more super concentrated fuel injector cleaner. lets see what happens

To recap: this started happening about 3 weeks ago (early jan). winter gas comes on the market in late nov. tire pressure at manufacturer's spec. any other ideas?

 

[iamwiz82]

you probably bought fuel injector cleaner meant to be used every tank, instead of every 6 months. Good fuel injector cleaner costs around $8-$10.

EDIT: found a good read on the subject

 

[JEDI]

congrats on being a lifer

the super-concentrated version is the one to use every oil change, not every tank

EDIT: ahhh..there are 3 levels of fuel injector cleaner.

1) The first type is the "one treatment" product that contains a large amount or high concentration of detergent to clean injectors with one tankful of gasoline. It cost between five and ten dollars.

2) The second type of fuel injector cleaner is a reduced effectiveness product that cleans most fuel injectors with one tankful, but in some cases, may take two or three treatments to get the proper cleaning to eliminate driveability problems.

*This is probably what I bought*

3) The third type of fuel injector product is a lower concentration product that primarily keeps injectors clean.

*This is the one to use every other tank*

There's also a 4th. They call it fuel system cleaner. It cleans cleans the entire fuel system, including injectors, carburetors, intake valves and the combustion chamber. Anyone have any more info on this?

 

[HannibalX]

Sounds like a bad air/fuel ratio. Your car is running rich, could be a bad o2 sensor. Does the service engine light come on? Take your car to AutoZone and have them pull the engine code (it's free!). If its a bad o2 sensor you may need to replace it at the dealer - OR - clean it yourself with noratic acid (spelling), you can get it at a pool supply store.

 

[BDawg]

You've been buying really cheap gas, haven't you?

 

[rahvin]

Something is wrong with your engine. You shouldn't need your injectors cleaned this soon, AFAIK the injector cleaner boosts octane so higher octanes will improve your gas millage but I would take the car in for warranty service.

 

[Adul]

I'm using Shell gasoline.

try a diff brand then

 

[JEDI]

Originally posted by: Trinitron
Sounds like a bad air/fuel ratio. Your car is running rich, could be a bad o2 sensor. Does the service engine light come on? Take your car to AutoZone and have them pull the engine code (it's free!). If its a bad o2 sensor you may need to replace it at the dealer - OR - clean it yourself with noratic acid (spelling), you can get it at a pool supply store.

nope, no check engine light.

yes, will take it in to the dealer

 

[zCypher]

Originally posted by: Adul

I'm using Shell gasoline.

try a diff brand then

lol

 

[HannibalX]

Originally posted by: zCypher

Originally posted by: Adul

I'm using Shell gasoline.

try a diff brand then

lol

That sounds funny but its true, I used to use Chevron gas and it sucked bad and now I use Exon/Mobile and car runs allot better.

 

[JEDI]

Took my car in to the dealership. they did a diagnostics. found nothing wrong. o2 and temp sensors ok. spark plugs ok. they have no idea why it dropped 30% and couldnt offer any suggestions.

sigh..i've changed gasolines (Exxon), but still the same. i've added some more super concentrated fuel injector cleaner. lets see what happens

 

[Grey]

Its the winter gas, I am surprised the dealership didnt know that. Where are you located? In NJ I lose about 8-9 MPG when winter rolls around because of the additives.

 

[JEDI]

Originally posted by: Grey
Its the winter gas, I am surprised the dealership didnt know that. Where are you located? In NJ I lose about 8-9 MPG when winter rolls around because of the additives.

ahh..but this started happening about 3 weeks ago (early jan). doesnt winter gas come on the market in late november?

 

[0roo0roo]

low tire pressure lowers mileage. a little atleast

 

[JEDI]

Originally posted by: 0roo0roo
low tire pressure lowers mileage. a little atleast

nope..tire pressure at manufacturer's spec. besides, there's no way that tire pressure can cause a 30% drop