Bad Idle/Gas Mileage

[James2gdsm]

Been having this issue with my car for about 2 years now, getting pretty sick of it. Its a 99 GS, Figured it was time to reach how to the community for answers .

The problem is: It has a faulty low speed operation, I get very bad gas mileage in the city. Not really sure what it gets but I'd guess around 10-15mpg; at a stop it has a rough idle; sometimes when you come to a stop and take you foot off the clutch it will stay revved around 2k and you have to give it a little gas to get it to go down.

But everything runs fine on the interstate, I get around 28-30mpg (Actually calculated this)

I was told tonight by a friend that it could be the IAC, I've done some research and nothing says anything about it causing bad gas mileage

What I've done/changed; Lucas fuel treatment, plugs/wires, fuel filter, I've taken it to a local tuning shop to make sure the timing was correct. I am sure I have done a lot more I just can't think of it at the moment .

Hope you guys can help, and thanks in advance!

 

[TSI_AWD_FTW]

is you BOV vented? And what exhaust are you running?

 

[James2gdsm]

Its a 420A Non-Turbo, and it has stock exhaust.

 

[FearlessNick]

I get very bad gas mileage in the city. Not really sure what it gets but I'd guess around 10-15mpg;

So do I, I get around 15- 18 and that's not revving it up or anything just normal driving under 3k. Someone will come out with a answer. Oh and bro. Set up your profile so we know what all you have done to your car and what kind of car it is. Thanks

 

[Palmetto_TSi]

check the butterfly bushings on the intake. if they are worn out, you could have a vacuum leak there, and they could be causing the butterfly to stick open sometimes. Just a start... Actually, I would check all the vacuum lines and intake gaskets.

 

[Fotowntalon]

Vacuum leaks is a big culprit of surging idle. IAC and TPS effect idle as well, getting those tested/replaced wouldn't hurt either. Also any CELs?