10.5 MPG ??? what is wrong with my car
 

I have a 05 g35 sedan. stage 3 clutch about to go back to stock. greddy catback exhaust and aftermarket headers that were on the car when i bought it. it worked fine when i bought it and slowly started acting up. when i go to crank it, it takes about 3-4 times for it to fire up and then it idles really really low for a few seconds then runs fine.. Unless i punch the gas it takes a second or two for it to actually go. Im also getting 10.5 mpg at the moment. i took it to infiniti dealership in dallas and they gave me a pricetag of $5-$6,000 to fix everything on the huge list they gave me. im sure they were just looking for money. Does anyone even have a clue of what this could be? i just want to know whats wrong with it. im not even driving the car because its that bad. sucks making payments on a car you dont drive!

The hard starts could be the result of a bad crank or cam position sensor. The bad gas mileage could be a result of a bad MAF or O2 sensors. Are you throwing any codes?

Get it scanned at an AutoZone, they do it for free. Don't have them clear the codes though, BUT get the codes from them and search them on here to see a fix for them.

While you're at it, pull your plugs and take a look at them. Have they ever been replaced?

its normal to get these miles on these cars. We don't have the most fuel efficient cars on the road. These cars drink gas like no tomorrow.

yeah check engine light is on. when i took it to the dealership they cleared the code but came right back on after i left the dealership. im thinking it might be an o2 sensor just from research ive done. but i still dont know forsure. and yes i just replaced the plugs 2 weeks ago and it ran a little better but it didnt fix the problem. it cost me $65 for plugs and $215 for labor to change them!

i used to get about 19-20 mpg before my car started acting crazy

According to the dealership, what needed to be fixed?

they told me that the aftermarket headers that were on there did not have an o2 sensor. and that it was messing everything up. they gave me a quote for changing the headers,02 sensors and full exhaust. and a few little other things that were messing up. like bearings and brakes.. spark plugs.. stuff like that. but the main thing was with the headers. im not sure if they just wanted to make money because headers are not cheap to change. or if the o2 sensor just needs to be replaced and everything will be fine

First things first, don't go to the dealership. Find a good independent mechanic to go to instead. Dealerships aren't particularly worried about relationships with a customer, but a guy trying to get his business off the ground WILL TAKE CARE OF YOU. Pick his brain about the O2 sensors in the headers. If there aren't any, have someone weld some bosses to the headers to run some.

Also, are you running stock cats, test pipes, or HFCs?

I don't know the area but have you gone to another dealers to see what they say? Sometimes they have dumb people working there.

are you ok? have you not been driving you car for a year now? sure these cars are not the best fuel efficient, by that being said. if you were getting 15mpg this answer would have worked.. thats not the case... this guy doesnt have a V12 in his hood... sht even a ferrari/lambo get better MPG.

from that being said. you need to take everything in steps, from as small as tire pressure to inspecting parts... hope you find a solution.. this really sucks. lowest ive seen is 15mpg. i get 19 atm and thats pretty average for most people. some lucky bastards in here see 22-24 mpg...

^ Trust me, my car is well taken care off and well maintained. Our cars are good only on highways, but in the city they are terrible even if you try your hardest to drive slow and not press the gas to hard its the same thing. My friend who drove his car hard was getting 8 miles to the gallon. I get about 12 and i drive gently.

To OP, is your car tuned with the mods you have? if not there is your problem, with headers and exhaust you need to tune your car. Also change your transmission oil, differential, and check your spark plugs as well. The trasmission oil and Differential oil's supposed to be changed every 30k miles. Do a full maintenance on the car and you will see your car improve in gas mileage. Btw, if your Maf would be acting up it would let you turn the car on so you won't damage anything, the check engine light is probably because of the O2 sensor that is from the headers if its not connected to it.

Well a majority of your price tag the dealership gave you was that they would replace them with OEM parts, the cats are about a grand a piece before you get taxed. You could replace them with good High Flow Cats for about $500 for a pair. So I am sure that the headers won't be cheap either along with their OEM exhaust. Sometimes, the aftermarket parts are the better(performance wise) and cheaper than the OEM stuff.

when was the last time you cange your spark plugs?

seriosuly.. if my car was giving me 12mpg let alone... 8 mpg.. my car would have already been sold. i like my car a lot.. but even now that i get 20mpg i think its low. im not driving a ferrari or a hummer... so i dont expect the same.

primary O2 sensors and vacuum leak are the main causes of this

replace the O2 sensors

if you need some OEM headers i have 2 sets that i'll give ya for next to nothing.