poor MPG after tune up

**christopher8911** · 07-12-2010

Having some issues with my 98 GT. After installing new LIM (GM aluminum replacements) gaskets, new Shure fix UIM and gasket, new heat senser, new 180 t-stat, new tps senser, new SLP intake, new pvc valve, and replaced vaccume line from throttle body to the intake (EGR maybe?)my gas MPG is around 12. The car seems to be running good and I don't know what is killing my gas mileage. Car does idle around 900 rpm, seems a little higher than normal. Before the swap I was getting around 20 mpg in town. I have checked for v leaks and everything seems to be tight. What should I do next?

**TheOtherNick** · 07-12-2010

Check the spark plugs. Check your fuel pressure regulator.

**christopher8911** · 07-12-2010

I did clean the tb and the fuel rails, changed the o rings on the injectors. Maybe I took it out of adjustment when I removed it from the car.

**christopher8911** · 07-12-2010

Originally Posted by 01GTP

Check the spark plugs. Check your fuel pressure regulator.

How do I check the fuel pressure regulator?

**Biohazed** · 07-12-2010

would be interested seeing this resolved, MY GT does the same; crappy gas mileage and also idles at 900, I really haven't had the time to get to the bottom of what is wrong.

**TheOtherNick** · 07-12-2010

Originally Posted by christopher8911

How do I check the fuel pressure regulator?

Was there some gas in the vacuum line connected to it? Do you have a vacuum pump? If you do put about 25" of vacuum to it and see if gas leaks out.

**christopher8911** · 07-12-2010

I can't remember. The vaccume side is the bank closest to the radiator, right? Do you think it could bt the tps? I cleaned the tb to an almost polished state and replaced the tps while I was at it. Car ran great before the upgrades, only had a lim leak that needed fixing. Feel like its something to do with my updates. Also, no rich exhaust smell or smoke coming from exhaust.

**SuperSport** · 07-12-2010

Originally Posted by christopher8911

and replaced the tps while I was at it..

The TPS is spring loaded and installs with the lip under the tab. So you gotta twist it under that, then twist the body of the TPS to line up with the screw holes.

If you just dropped the TPS in, its installed wrong.

Also, a new SLP intake...more air means more gas, but im not sure that it would warrant only 12mpgs...

**christopher8911** · 07-12-2010

Originally Posted by SuperSport

The TPS is spring loaded and installs with the lip under the tab. So you gotta twist it under that, then twist the body of the TPS to line up with the screw holes.

If you just dropped the TPS in, its installed wrong.

Also, a new SLP intake...more air means more gas, but im not sure that it would warrant only 12mpgs...

Ill re-check the tps first and try that since it will be the easiest to fix. I feel like its a fuel issue more than a vaccum problem. Ill change the spark plugs and wires while I'm under the hood. Thanks guys I haven't had much experience with the 3.8, I'm a V8 kind of guy

**christopher8911** · 07-13-2010

OK I removed the tps and pushed it in as far as it would go, then rotated the body to line up with the screw holes. Still no luck

**Smokin2000gtp** · 07-13-2010

How old is your O2sensor? Oh and you said you just put on a SLP intake, did you oil the filter yourself? Maybe try cleaning your MAF.

**christopher8911** · 07-13-2010

not shure about the o2 sensor but thats my next corse of action. getting frusterated with this car.

**christopher8911** · 07-18-2010

Changed o2 sensor, still no luck. Going to change the plugs and wires, what heat range should I get? Car is stock except for the cai and I will be adding headers soon. Want to get the mpg issue fixed before I get too far on mods.

**SlowNA06** · 07-18-2010

Stock heat range.

**christopher8911** · 07-20-2010

Finished installing my digital dic and my mpg in town is 13.4, open road is 18.8. How far off is that from normal for a pretty much stock car?

**Fivefingerdeathpunch** · 07-20-2010

i get 18-19 city, which really only the 3 mile drive to work and the 3 miles back home. On the open road like a county road or the freeway I can get 28-32 depending on speed.

**Bio248** · 07-20-2010

you should be in the mid 20s for highway at least.

**christopher8911** · 07-20-2010

Damn, that is what I figured. What does your n/a cars rpm show at idle without a/c? With a/c? I'm around 900 either way. Seems high to me.

**Fivefingerdeathpunch** · 07-20-2010

i know your asking for N/A, but i think the idle for n/a and s/c should be about the same.

my gtp rpms at idle are:

without a/c: 700-750

with a/c: 800