Problem With My Gtp

Hey Guys, Im New To Grand Prixs, And I Have Owned My 2000 Gtp (no Mods) For 6 Months Now, And I Was Getting 18 Mpg And Then The Heads Started To Pass Water To The Engine, I Already Fix The Heads Replace The Gaskets, New Oil, Filter, Spark Plugs Ac Delco Iridiums, Wires, Fuel Filter, Radiator Cap, New Hoses, Thermostat 180, K&n Drop In, And Removed The Cat Conv, And My Mpgs Now Read 9mpg, And Im Getting Aprox 125 Miles To A Full Tank, I Dont Now Waht Else To Do Any Comments Will Help.

sometimes mpg problems are over thought....

-Whats your tire pressures?
-are any of your wheels very hot after driving a short distance (indication of a sticking caliper)
-is your tranny going into overdrive? (1800rpm highway cruising at 60mph)

You sure you reset the DIC after refueling?

Why did you remove the CAT converter? That is illegal especially in California. Also, it may affect exhaust backpressure (not enough) which may cause a drop in fuel economy.

Another question: What octane gas are you running?

NO PROB GOING INTO OVERDRIVE, My Tire Pressure Are 32 Psi And Im Running On 89 Oct Fuel ($$$), And I Removed My Cat Conv Because It Was Damaged And It Was Almost No Air Flow And The Guy Didnt Have The One I Needed At The Time (OBD II) And Also Had To Remove The Resonator Because I Was Leaving Pzs Of It All Over The Freeway, I Already Ordered The Cat. And Another Thing I Have The SES Ligth SINCE THE BREAKE DOWN AND DIDNT WENT OFF WITH THE REPAIR. Thanks For Your Comments. IT HAS 86,000 MILES BY THE WAY

1st off, you are not really saving any money by running 89 octane over 91($0.10/gal). 89 octane will produce more detonation, so the PCM will retard timing to compensate. This makes the engine less fuel efficient, reducing gas mileage and can also lead to engine damage. I would get the car scanned and post the resulting codes pulled so we can better help you determine your issue.

Best thing to do without going off the deep end or wasting any more time or money is get the replacement CAT converter installed.

Your SES light is on because the rear 02 sensor sees a converter efficiency problem with the CAT removed. This is normal unless you connect a 02 simulator which may or may not be legal.

Most of us with GTPs get better fuel economy running premium at least 91 octane or better.