Thread: Horrible mileage

Hey everybody,

I have a 99 Pontiac Grand Prix GT with a 3800 Series 2 engine and it had sat for a while before i got it. Anyway, my problem is that i am getting really bad mileage seems like and i also noticed when i was checking some stuff out that its got a bit of hesitation just off idle, it also has no power to speak of and i can also smell raw gas sometimes when the engines cold. My question is could a plugged up fuel filter or a dirty fuel injector cause these problems and if not what could it be? Any help that can be given would be appreciated.

Thanks,
tgarison

Sorry about that , the car has 177,XXX miles, unknown on the plugs and wire i pulled one of the plugs out and it looks like the engines running to lean but that could just be my imagination, i didn't noticed the pressure regulator leaking when i was checking stuff but then again i wasn't looking for leaks. No SES lights but i do have a persistant brake warning light on but thats for another thread. I doubt that the cat is plugged up cause the engine will run all day till you run it out of gas.

Thanks,
tgarison

I did the seafoam treatment through the brake booster line and my little computer thingy in the car used to say 18.9mpg and now it went up to 19.7mpg. So it may be worth the money. Also it could be plugs or a fuel filter or clogged cat or O2 sensors going bad. It could be a lot of things. Also try running 93 octane in it for a bit that could help :-/

Lol i always run 93 octane in my gp

Because i can and 93 runs cleaner! :P

The way that gas is made is the higher octane rating means that it's more refined and that the fuel in question burns at a higher temperature there for less engine deposits. Talk about something you know about. I am in the gas business :P

Thanks for all the helpfull information i will try the new plugs, wires, fuel filter, air filter, and seafoam as i get the money. Thanks again for all your help.

Originally Posted by bluegtp91

That's wrong. If the car is not in need of such a high octane it CAN NOT burn off all the excess octane it isn't needing, thus depositing it in your engine and creating MORE carbon deposits than what you would have had for the 87 it was designed to run on. If your in the gas business you should know this.

^^100%.. if you're not running boost you dont need higher octane, as blue said the engine wont burn off the excess and you will infact increase carbon deposits, lower engine performance.

The liquid coming out of the pump may be cleaner and more refined, but what your engine does with it is the problem here.

Knowing gasoline isnt necessarily knowing automotive !

if it your cat converter you my notice poor shifting now i got a 01 gp gt and i need a new cat at 75'000 mi but im having the same problem almost mine car the rpm's drop mid acceleration to 2 k and i can step on it they don't move i'm going with the coil packs i've had the wires and plug changed this is my next step

Higher octane fuel burns at higher temps and pressures. The duration of burn is much longer in a low compression engine. At the beginning of the exhaust stroke, as the valve opens, you are sending burning gas past the valve. This increases the exhaust temps and can/will burn exhaust valves.

Hey everybody,

Thanks again for all the helpfull advice, and i thought i would give all of you a status update with a list of what i have done so far.

1. Spark Plugs changed Autolite 606 copper cores
2. seafoamed i don't think i used enough of it cause it didn't smoke much
3. Air filter replaced with spectre air filter

Just changing the spark plugs made a huge difference in the mileage and some in the power/ tq. The next few big things to take care of are the drivers front wheel bearing, recharge the A/C system, get the blower motor fixed and replace the fuel filter.

Thanks again for all your help!