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Car getting horrible MPG

Mercenary

Mercenary

New member
Hey, I bought my stock 00 GTP about a month ago and it was getting 24MPG on the highway and now I get 18. One day I was going about 80 going to work and my car started bogging down like it was about to die. Come to find out it threw some codes. A 305 ( 5 cylinder misfire) and a 103 (I think....it was something about the temp fo the car not reading correctly). I switched out the front plugs and wires (1 3 5) with some 606 autolite plugs and some NAPA wires. The service engine light finally went off a couple days later but it's still getting bad MPG.I took it to a shop and they said nothing seems to be wrong electrically.

The car had bad wheel bearings that were suppose to be replace before for I bought it. Is it possible that they weren't replace correctly and that's causing the bad milage? Any input would be appriciated.
 


mine mpgs were **** until i fixed an exhaust leak it was messing up my o2's how many miles on your car might wanna look into a bad o2....those can be bad with no code
 
Change your rear plugs. Do you mean fuel filter instead of pump? If not why are you replacing your fuel pump? Is your air filter clean? You can check an 02 sensor with a scan tool.
 


01GTP said:
Change your rear plugs. Do you mean fuel filter instead of pump? If not why are you replacing your fuel pump? Is your air filter clean? You can check an 02 sensor with a scan tool.
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Yea I tried changing to rear plug but they're not agreeing with me at the moment and yes I meant fuel filter not pump . Air filter seems fine. i don't have access to a scan tool at the moment.
 
Yeah. I had a leaky exhaust & got 15mpg cuz I was leaking at the headers & the Y-pipe. Deff' wanna seal up those leaks. How long has it been since you had an oil change? That can help some.
 
Un hook the front motor mounts and pull the engine forward with a ratchet strap makes the rear plugs a whole lot easier.
 
01GTP said:
Un hook the front motor mounts and pull the engine forward with a ratchet strap makes the rear plugs a whole lot easier.
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as a dude with fairly big hands and forearms I don't get all this rocking motors forward stuff. I mean I've worked on a lot of really hard motors to change plugs and the 3.8 liter in a GP ain't one of them. not even close. Of course I have a 3" 3/8 extension with a 5/8" head on it, but honestly, GP plugs ain't that hard to do.
 
I have big hands and arms too, yes I probally change the plugs with out rocking the motor forward, but why not takes about 1 min and makes it so much easier. I just re-read the first post the computer could be dumping in extra fuel due to the coolant temp sensor not reading right.
 


Im only getting 17 mpg in my GT...I just found out there's a hole in the resonator could that be it? Its at 108K miles.
 
Just a lil update my check engine light came back on and I had 3 codes this time the same 305 and t stat one but the t stat code was showed up twice. So on my next day off I'm gonna go ahead and try the back plugs and install that t stat and see what happens. Thanks for all the input
 
If you still have a cylinder 5 misfire you need to check your coil and injector for that cylinder. Since you have already changed the plug and wire for it. That is one of the major factors I would say for your bad mileage.
 
01GTP said:
If you still have a cylinder 5 misfire you need to check your coil and injector for that cylinder. Since you have already changed the plug and wire for it. That is one of the major factors I would say for your bad mileage.
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yea I'm thinking the next step is to change the coils. I'm gonna switch up the coils and see if the misfire travels with it. Hopefully it does so I can finally be done with this. I'd rather be modding

btw since I'd be changing all the coils is there a favorite brand for the gp?
 
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UPDATE

I finally changed the back plugs and wires and the car seems to run a lil better because of it. I switched the coil packs and around and took it for a drive. The same bogging/shuttering thing happend while I was accelerating. Took it to autozone and the same ol codes came up again but this time I got a extra special suprise.... a new code! I don't know what it is because it's still pending.

So this is making leading me to beleve that it has to do with the number 5 cycnider fuel injector. I'm trying to decide if I should get a new fuel injector or get a treatment done....maybe even get a dianostic. Any ideas?
 
bluegtp91 said:
Well what are the codes? How are they "pending"? They tell you the second they read them...
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Dunno, That's what the guy said but the codes are P0305 (misfire detected in cylinder #5) and P0128 (Coolant temp is always low). The coolant one should be easy to fix, it's either a bad thermostat or needs more coolant.The misfire is what's botherin me I can't figure out what's causing it.

Seaform was suggested to me but I dunno alot about it. It would be easier and cheaper than replacing the fuel injector.
 
Mercenary said:
Dunno, That's what the guy said but the codes are P0305 (misfire detected in cylinder #5) and P0128 (Coolant temp is always low). The coolant one should be easy to fix, it's either a bad thermostat or needs more coolant.The misfire is what's botherin me I can't figure out what's causing it.

Seaform was suggested to me but I dunno alot about it. It would be easier and cheaper than replacing the fuel injector.
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ICM? And are you sure it is the T-Stat and not the actual sensor in the radiator?
 
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