I've got a '99 Z28 A4 with a SLP cold air intake which i installed this summer, but recently my gas mileage has started to drop. I used to get around 20-22 mpg with my everyday driving but the last few times i've filled up i've averaged around 18-19 and then today it was down to 16 mpg I don't know what is going on with it. A friend thinks my o2 sensors might be going bad, but i haven't had any warning lights come on or anything to give me a hint as to what might be happening. I'm a tiny bit low on oil, but i've never heard of that dropping gas mileage. (and yes, i'm changing oil here as soon as it comes). my tires all have the correct air pressure in 'em... i just have no clue why this is happening.

my friend also seems to think that my car idles wrong. to me it seems normal cuz it's done it since i bought it, but it shakes. not constantly, but it is repetitive. i don't know really how to explain it.

any ideas as to what might be going on? i don't really want to go spend 100+ dollars on o2 sensors just to have it be something cheap and easy to fix.
any help is much appreciated. thanks

 

how many miles is on the car? maybe its the plugs if you have a good many miles on the ones that are in it. my friend has a 98 F150 and a 02 sensor of his was going bad during the time it was bad his service engine soon light would come on and off on and off and he lost gas mileage and it sounded a little different. he solved the 02 sensor problem with cutting his cats.

 

it's got 115,7xx miles on it. i bought it used with 92,xxx. so i've put a good amount of mileage on it and i haven't changed spark plugs or anything. only stuff i've done was the cold air intake and switched to Amsoil - both engine and transmission. my oil light comes on when i start up, but that's not a big deal and as soon as my amsoil gets here i'm putting new oil in, so it's not that...

thanks

 

try changing the plugs i really cant pin point what would be the problem maybe the fuel filter is clogged? idk hope some gm tech guys see it this they could tell ya.

 

would it have anything to do with how cold it is? it's in the teens here in the mornings and doesn't warm up much during the day. it's been this way for the past few weeks. the only reason i say this is because my dad says he's been noticing a drop in his gas mileage in his passat recently too. personally i don't think that should have much to do with it. i could be wrong, but for the cold to make a 6mpg change?

anyways, maybe i'll try changing the plugs. if nothing else they probably needed to be replaced anyways. don't know the last time they were.

 

You have a high-mileage car with an uncertain maintenance history. I would do a complete tuneup (plugs, wires, belts, filters, seafoam, ect.) , and go from there.

 

ill go with what he said ^^^

 

note that a lot of gas stations have switched to their "winter blend" of gas. cars typically get worse mpg on this winter blend, and pair it with longer warm-up times every day and you should notice a ~2-3 mpg drop when winter comes around.

 

thanks for the replies guys. i'm now going about finding all the info i need for doin a good tune up. seafoam - which choco you wrote a great story on - new plugs, wires, etc... thanks for the help.

 

Definetly do the folowing. Its not that expensive & you will notice a difference.

1) new belts
2) new air filter
3) oil change & filter (if not done already)
4) fuel filter
5) new plugs

And most importantly the FULL seafoam treatment. There is a really good write-up on here for it. I did it & noticed a BIG difference.

 

another reason for a slight mpg drop could be the ethonol content of the gas. i know in mass they use 10%. that will kill it a little but not that much.

 

ugh, he speaks the truth.

 

thanks for info guys. just dumped my first bottle of seafoam into my car today. so we'll run with that for a little bit. hope it helps at least a little bit.

 

do the seafoam in the brake booster line with a funnel too! I works awesome, I did mine with 39k and it smoothed out some and i was just doing it for regular maintainence

 

yeah, that's coming up tomorrow when i have some free time to do it.

 

tune up (plugs wire,filter,air and fuel
clean maf
clean tb

the common things that cause low mpg
and running low of oil is stupid.have you ever priced an ls1 longblock?

 

I had 108,000 miles on my ride at the time & was getting about 19mpg mixed driving.....I couldn't figure out why everyone else on this forum was getting around 21-23mpg, so I did some research & found out that the front o2 sensors were the cause of my bad gas mileage, funky idle issue's & also I was experiencing dead spots when accelerating.....I changed out my front o2's & all the trouble i was having was because of "lazy" o2's.....found out they are only good for about 80,000 miles, anything after that, they still will work however not like they should.... If I were you I would:
1. Change plugs & wires
2. Change front o2's
3. Change fuel filter
IMO, you're having the same exact issues I was having......Hope this helps
Oh, 1 other thing, also if your experiencing cold weather it is normal for your gas mileage to drop because in the cold your engine will run richer because cold air is more dense, therefore the IAT sensor tells the PCM to drop more fuel so u won't lean out FWIW Never heard of a "winter blend" type gas

 

when i say low on oil, i'm not talking like a quart low. it's still in operating range, but a little on that low side. i don't feel that i'm doing anything to hurt it and i will be changing my oil in a few days here anyways. and no, can't say that i have ever really priced out an ls1 longblock...

-miamifan- thanks for the input. it does sound like you were having some of the same problems that i am. i don't have the dead spots while accelerating, but lots of the other stuff sounds similar. i'll take my camaro into where my buddy goes to school and hook it up to their computer - see if they can tell me if my o2 sensors are bad or something else. and yeah, i kinda expected a little bit of a drop in gas mileage in the winter, but i was thinkin that 6 mpg is a little bit much.

gonna finish seafoaming my car tomorrow. still gotta figure out the money situation for the plugs, wires and o2 sensors (if i need 'em) and all that fun stuff.