If he has a CEL, then it throws up a wild card.

Really need to know what the code is before speculating what it is.

10.5 MPG is not normal. I have a modified V8 that dpes better than that.

@RAY it had been on for atleast a month and a half before it really started acting up,

@G35NYC. i changed the air filter a few weeks back also and it didnt change anything, also should i get a tune up before i change the clutch and headers and o2 sensors or after?

i got caught up with work for a few days so i wont be able to get the codes until this weekend. but i will post them as soon as i get them so check back and help me get this figured out please. i dont know a whole lot about car mods and stuff so all the help is appreciated

Seriously, don't do a thing until you get the codes. Your mods are fine. You don't need to take them off.

Like I say, get the codes first, but my guess is Passenger Side Camshaft Position Sensor. That will make your car do everything you have described - bad mileage, hard starting, CEL, etc. You're lucky to have a CEL on - at least that way you know you can read a code. Mine didn't even throw a CEL when it started acting up.

The other thing you should do is clean your MAFS, using CRC MAFS Cleaner. A dirty or bad MAFS can cause a myriad of other symptoms as well, also including everything you've described.

Do both of these before changing any O2 sensors.

ok i got the codes this morning.

P0300

P1273

P0011

What. Are. Your. Mods.

im not sure i bought the car about two months ago. and the guy that had it said the guy that had it before him put all the mods on. but i know it has

aftermarket headers and the catback exhaust

and a stage 3 clutch.

i also bought a camshaft position sensor

and a crankshaft position sensor today and plan on putting them on this sunday.

hopefully that could be the problem!

P0300: Multiple Cylinder Misfire
P1273: Air/Fuel Sensor 1 - Running lean
P0011: Camshaft Position Actuator A - Bank 1

You dont need to get rid of your mods, but since you are going to replace the sensor. If needed Check the gap on the plugs and clean/replace MAF

Lastly, get a tune so you can take full advantage of the mods.

I'm sorry I'm a newbie. Check what gap? And also where can I get a good tune up

No offense, but I guess since you dont know what a gap is, then you probably will not be able to check the gap on the plugs. Check the link below for the DIY on Spark Plugs to give you an idea.

eturkov.blogspot.com/2010/10/diy-video-g35-spark-plug-replacement.html

I will help you out further by giving you the link to the sticky at the top of the sedan page. This will show you a number of DIY videos for service and a couple mods.

www.g35driver.com/forums/showthread.php?t=183421

A tune will remap your ecu to get the best performance out of your mods through out the entire RPM range. A lot of the members are running an Osiris tune, which gives you different maps/tunes to choose from (valet, econ, race, etc.)

Hope this helps, and good luck

Don't bother just checking the gap. It's the same labor to just replace the plugs. This is part of a tuneup.

Replace the cam sensor first and reset the ecu by unhooking the battery and then pressing the brake pedal several times. Then see how it runs. The tuneup is a good idea if you have over 100k and it hasn't been done.

The p1273 could be a vacuum leak or intake leak. Could also be due to a Bosch O2 sensor if any have been replaced. Our cars don't like them. Use Denso sensors.