Gas Mileage Problem. Bad City Good Highway

[BuNcHeSs]

I have a 97 GST with 120000 miles on it and I am getting very low city mpg, around 16, but and pushing over 30 on the highway. The car runs fine, but I do have the IAC code registering. I replaced it so I am assuming it is a vacuum leak.

I replaced the o2 sensor, fuel filter, spark plugs, air filter, and I drive like a grandma in the city trying to keep my boost levels down.
The only mod on my car is my type-S blow off valve which I replaced the stock one with after the vacuum line coming off the throttle body broke off. I checked all the hose connections, tightened them and tried listening for a vacuum leak.

Why would I have such good high way gas mileage, with such terrible highway? And also, is there any way to test for a vacuum leak?

 

[Jesse2012]

Is the blow off valve recirculated?

 

[1990AWD]

Your driving is the biggest part of fuel economy, probably room for improvement there alone.

Btw, I see a 10 mpg difference between city and highway, mainly cause using my right foot makes me grin.

 

[BuNcHeSs]

It is recirculated and whenever I drive and measure my gas mileage I try to drive as conservative as I possible can. Before I started doing the minor tune up things on my car it was down to 16, and the only thing replacing every thing that was noted in the original post seemed to have only significantly improved my highway driving. Before I did the stuff I was getting lower 20's on the highway.

 

[DSM1G90]

I am getting very low city mpg, around 16

Welcome to city stop and go driving. This happens with ALL cars - it just takes added energy to get a heavy mass going from a standstill along with rowing through the gears in both manual and automatic where the motor's RPM's are not constant as with highway driving.

I have close to the same figures with my LASER, but can almost muster 20mpg in town and close to 30mpg on the freeway with the cruise locked on 80mph.

Another factor is the usage of the 'winter gas' that is prob still in the tanks at the filling stations, which also kills mileage during the cold seasons and gets gets better when we switch to 'summer gas' during the hot seasons.

Just something to live with sports compacts. If you want to change all of this, stay out of the DSM and buy a VW or a small HONDA CIVIC, and then your city mileage will drastically improve.

-DSM

 

[Alpha]

You probably have some boots leaks that are not bad enough to cause driveability issues. I am guessing toy TB shaft seals might be a major factor in this. 100% of my DSMs have had boost leaks coming from my shaft seals.

 

[BuNcHeSs]

Will a boost leak make the the a P0505 code for the IAC. Ive had this code for awhile and I replaced it and also checked for any vacuum leaks and fixed what I could.

 

[sleeperX2013]

iac is what helps a factory car maintain a smooth idle. if there is a error in this system it will throw off your afr at idle which in turn would effect fuel economy . directly city mileage due to the fact that the iac isnt used during cruise conditions (highway driving). if you have replaced the iac i imagine you put a new gasket on it. if not do it. also check that the air passage to the throttle body is clear of debris. these tend to get clogged with carbon and oil resedue which cause it to malfunction. another route is to install ecmlink which has a iac simulation option that offers guys that no longer have iac because of intake manifold and throttle body upgrades the benefit of this control. if you want really good gas mileage you bought the wrong car. your posted numbers are not abnormal.