99 grandprix gtp 3.8l sc gets 9/13 mpg!!!

hey guys wats up. I have a 1999 grand prix gtp 3.8L supercharged. im fixing up EVERYTHING to make this car like new...( 92,000 miles).. i was wondering how i can fix my gas mileage, because at the price of gas im paying $12 just to get to work everyday, my car gets NO mileage, 8-9.7 mpg city, and up to 13.2 highway. i reset my dic at every fill, but i dont need the DIC to know i have never in any car seen so much gas used. we tested my cousins escalade gets WAY more mileage out of 10 gallons than my grand prix does (10 gallons i got 96 miles/escalade got 175 both city andhighway combined) now this is going to sound stupid but i kno some of it is attributed to the fact i have 3 16" chrome factory wheels, and 1 15" steel rim on the car. im working on all stock aluminums. now besides for that, this car has new spark plugs/wires, oil filter, crank & cam sensors, resistor, new resonator, air filter, FUEL FILTER/PUMP... i was wondering wat else i can do in combination with same size rims to increase my mileage up to at least average.
maybe 02 sensor, anything else?..i just want to make sure i can get everything mileage-related tuned up...the car drives great, im not a heavy driver, and i dislike taking cars to shops because im a girl and those guys think i dont know anything and try to tell me anything and any price they want, i would rather be able to walk in and say i need this, this , and this fixed....sorry for the long post just making sure i give u guys all the details..thanks AF!!!

bump...does any1 ever respond here?

bump...does any1 ever respond here?

It might help to post in the right forum which is Pontiac Grand Prix where there is plenty of expertise to provide make/model specific assistance or answer your questions.

Also, as advised earlier when you post do not make multiple posts on the same issue at different AF forums. In other words don't make the same post here at Engineering/Technical and at Grand Prix.

This thread has been moved from Engineering/Technical to Grand Prix in order to properly assist the member and at member's request.

Are you getting a SES light?

Which axle do you have the 15" rim on?

no SES light, and the back driver side...

with the different size wheel on the rear or your car it wont effect anything except for maybe your abs or low tire pressure system.

Like you stated, next change the O2 sensor before the cat coverter. Dont waste your money replacing the one after the converter as its only there to monitor cat efficiency and has nothing to do with fuel trim. Also only use genuine AC delco parts.

Any engine modifications done to your car? Your fuel pump resisitor isnt bypassed is it? Are you filling up with quality top tier premium unleaded gasoline?

Next maybe want to go rent a fuel pressure guage from advance or autozone and run a quick fuel pressure check to verity that a return line isnt pinched or anything causing higher than average fuel pressure.

I feel for you on your horrible mileage. On all the the part you replaced, what brand plugs and wires did you install (hopefully not Bosch)? Have you actually done a fuel pressure test? If you bought a Master or Airtex pump from one of the box stores, they are junk and could still be contributing to your issues. Also, as tblake asked, are you running premium (minimum 91 octane) fuel? Running 87 octane in a GTP causes KR (knock retard) which kills both mileage and performance.

Rich ..Lets say for arguments sake the owner is running cheap regular gas in his GTP. If he had a duel bug problem, borderline CAT(partially plugged) plus the issue of the PCM compensating for the 87 octane could that be the cause of such lousy gas mileage?

The question that comes to my mind is why go after the upstream O2 sensor if there is no erratic/lousy idle issue attributed to this problem. :dunno:

Appreciate your thoughts....

Hey thanx for replying.....Um to answer some questions i had mileage problems before replacing the parts..ive been replacing MANY things on this car trying to rid myself of problems...to the last post, the car does idle badly sometimes and there is sometimes hesitation in acceleration....

tried unplugging the MAF sensor yet?

Although the MAF sensor could be suspect and cause a mileage drop it may not be the sole reason for her really bad gas mileage. I ran with my MAF sensor disconnected for a weeks until the part arrived but noticed no significant loss of gas mileage either at the DIC or via manual calculations.

I'd look at testing the CAT convertor by running a backpressure or engine vacuum test and replace the front 02 sensor just for general principle. These are the usual suspects when the mileage drop is significant and unexplained.

Especially looking at the highway mileage figure she posted. :shakehead

On the miss at idle and acceleration hesitation, does it do this only when the car is warmed up, or all the time? Since the problem existed before the parts were changed, I agree with Bob that checking the converter would be my next step, but I would also suggest doing a fuel pressure test and a compression test (if they haven't been done), to rule out a bad cylinder or low fuel pressure from a bad new pump.

Eppiermx- the combination of junk plugs/wires, a bad MAF and 87 octane could easily cause this bad of mileage, the "perfect storm" if you will. Since the OP now admits the car has a bad idle and hesitation, I would switch to a possible bad cat, bad fuel pump or a bad cylinder. Further testing will be needed to narrow it down.

Other areas to look at for fuel economy not mentioned would be to ensure the 4th gear - OD is working and the torque convertor clutch (TCC) is locking/unlocking when it should at speeds above 45 mph in OD.

Just annecdotal info but I recall an old post where neither of the above was working properly which causes poor gas mileage. Doesn't cost anything to check but should be checked to rule it out.

Also, check tire pressures especially cold inflation pressure, make sure the 4 wheel alignment is good and tire wear is acceptable.

Final thought is to make sure the thermostat is rated correctly (stock 195 degrees) and works properly. Long engine warm up times although it may appear normal on the IP Cluster temperature gauge can cause poor economy and not allow the PCM module to switch out of open loop mode to closed loop mode soon enough causing a lean trim (rich condition) to the fuel injection system. :2cents:

Have you had the car scanned? I just moved from Phoenix to Dallas. When we got here, my GTP was running very badly, but no check engine light. Never really gave it much thought to look for the light when the key is on but the engine is off. Anyhoooo, went to Walmart and got a code scanner. Low and behold, 5 codes came up!! Got em all nailed down except one.. it's listed on another post. However, my mileage was never that bad. Even with 5 codes going off (MAF, O2 #2, EGR, Evap System, and the check engine light, yes there is a code for that! LOL) I was still getting 18 around town, 27 on the drive over to Fort Worth and back last week. For your mileage, my first guess would be catalytic coverter. Second guess would be a rolling resistance problem. Are any wheels getting very hot when you drive? I got a bad brake job one time that caused a lot of rub and heat, made the mileage drop big time. Good luck.

hey guys been reading everything and i appreciate the input...now as far as parts go, i dont remember the exact part names of my parts but i can let u know that all were bought new from napa and advance auto and i didnt buy the "cheapest" model of any part, rather i spent the extra money for the quality. i mean this gas mileage was like this before ANYTHING was put on, this has all been very recent that im fixing this car up...btw i changed my ignition switch, that resolved all of the electrical problems my car was having, THANX AF!!!!!!...as far as rolling resistance i had 3 of 4 wheel bearings changed and besides for the 15" thats on my car, i have all brand new 16" winter "blizzak" tires..also had all brakes done and all calipers work correctly, no sticking......now the continous suggestion that keeps coming up is my cat. converter..i got my resonator replaced and my exhaust is very quiet..the only engine code ive ever had on this car was about the cat. converter...guy reset codes and it hasnt came back on since, but the only problem my car has now driving-wise is the gas and the little bit of hesitation there is on acceleration and startup idle....and it seems like all these problems are linked to the cat....the problem is that the guys at cole muffler cant actually "test" the converter output, only codes and visually looking for cracks or leaks, which checked out good..i dont wanna shell out the $350+ to get this repaired unless i kno its faulty, but then again i like knowing the majority of this car is brand new...but it seems like this is my biggest problem, and i gotta spend money on labor to save it on gas lol!

Tha cat is the airflow choke point on a GP. I put in a Random Technologies high flow cat I picked up Grandprixstore.com and it's been awesome. I took it to a little mom and pop muffler shop to get it put on. That costs about $140 for the cat and about $50 to put it on back in 01. Summit Racing sells the Magnaflow cats for under $100 now.

Or you can just hack the cat right out, weld a straight pipe in, and buy an O2 simulator for the O2 sensor behind the cat converter.:grinyes:

Here's a couple free things to check.
1 - Make sure none of the spark plug wires are touching the O2 sensor. Try to position them at least one inch away.
2 - Turn the key to the run position a couple times then check for fuel in the vacuum line going to the fuel pressure regulator.

We need to know which spark plugs you installed. Pull one out if you don't know. I'd also check for vacuum leaks.

hey um i have ac delco plugs and wires...and as far as vacuum leaks, that check engine code said either cat. converter or vacuum leak somewhere, but i dont even kno how to check for one...

A great way to check for a vaccum leak is to take a spray can of starter fluid and spray it on and around the vaccum lines and connectors with the engine idling.. Just do little sections at a time, not the whole motor. When the the RPM's come up, you found the leak. Do this very carefully, started fluid is extremely flammable.

If your CEL is currently on, take the car to a local Advance/Auto-Zone, and have it scanned for free and post the codes stored (don't believe them on what the codes mean). We could speculate all day, but cannot help further until you post the codes and check fuel pressure. I would also do a compression test to rule out a bad cylinder.

hey guys been awhile just wanted to end this post...my car finally had a check engine light come on that pointed to the cat. converter..i ordered one offline from the grand prix store, paid $75 to put it in and now my mileage is around 24 city/31 highway......i dont know if thats what this car should be at but i dont care because ive cut my gas costs pretty much in half..i love u guys, thanks AF!!!