Hello,

First, I just want to note.. My car knowledge isn't the best when it comes to this. Also, sorry for this long post... Trying to be descriptive, really hoping for some input. My car is an 03 g35 coupe 6mt.

I had noticed my car was beginning to idle rough, this came to my attention sometime around January. It would sit and cause a noticeable shudder that was easily felt throughout the car. Maybe a month later my cars CEL popped, solid and no blinking. I was sitting at 1/4 tank gas and thought that this could have tripped it. I went to a gas station, filled up to full (I always use 91 octane and the occasional 93 [a rarity around here]), and clicked the cap several times. The CEL would go off a couple of days later, but the rough idle was still in question.

Shortly after this, sitting in the car and talking to my girl, we noticed that my engine was particularly "noisy".. I don't notice it on my tachometer, but it would sound like the engine had a misfire and the RPM's drop for a brief second (it is still doing this). It would continue to do this intermittently as I sat idle. Driving the car felt fine, no delays or hiccups. There was no CEL at this time.

The very next day I decided to take out the K&N filter and MAF. I cleaned the MAF with MAF cleaner, hoping it would solve the rough idle. I cleaned the K&N filter and oiled it sparingly, I let this dry. After this, I was, and still am, convinced that the car idles more smoothly and the violent "shudder" is now gone. Cleaning the MAF did seem to help, but I did not take a q-tip to it and only sprayed it down several times with the canister.

Cleaning the MAF did not entirely rid the car of what would sound like a misfire at idle. No CEL light would again show for a month or so, not until last Friday night. The light has remained on (although, it had turned itself off briefly at one point, only to come back) and I ran the codes, P0300 random misfire.. Could the MAF be the cause of a misfire code?

Again, while driving the car, the car runs smoothly and there is no delay on acceleration, or fluctuation in RPM. It is only when the car sits idle that what sounds like a misfire occurs. Because I had cleaned my MAF and it seemed to tone down some of the original intensity in this, I am wondering if my MAF needs a more "deep" cleaning... or, a new MAF. I am not sure if I should go into the trouble of replacing the spark plugs at this time, or what my next step should be.

 

Coils and spark plugs is more of a random misfire. Not 100% on all maf symptoms but i do know a bad maf will cause fluctuating rpms at idle.

Have you had your plugs changed? If not try that.
Check for vacume leaks

I like to do everything on my car myself but sometimes its easier to pay to troubleshoot then to start guessing .

 

Thanks for the reply.

I do believe my plugs are original to the car (I can't say for certain) and, if so, they have around 81,000 miles on them. So, I will still be replacing these one way or another. It wouldn't hurt to start with them.

This morning, I had reset the MPG record on my nav unit for my 6.3 mile, 15 minute commute to work; In city driving, and once I had parked my car, it had estimated 22.3 MPG. Wouldn't my throttle and fuel consumption be poor if my plugs were going?

I have been watching the RPM's more closely over the last 24 hours and I noticed that I do not have any fluctuations in RPM while driving. Where my RPM does fluctuate is at idle. Once the car comes to a rest, the RPM's sit at 700 and a few seconds in, the car shudders and this then spikes down to 500. It doesn't sit at 500 long and then bumps back up to 600, but this is where it sits... at 600 RPM. Seems low.

Would that be caused by a dirty throttle body, vacuum hose, or both? I will need to check for leaks on the vacuum hose in the next two days.

 

81K miles on OEM plugs probably isn't the issue. I know when i took my plugs out at 115K miles, they were fine.

However, i'd probably be popping the hood, removing the engine cover for a few days and giving the entire engine a VERY good visual inspection.

P0300 is pretty vague. It means you have a misfire on multiple cylinder, but not always at the same time. This could be for MANY reasons and it's tough to pinpoint which one.

First thing i'd do is probably just pull the plugs and coils to inspect. Valve cover gasket leaking is common and the oil pools in the spark plug wells. I'd check for this as well as this can cause a misfire.

 

If Your going to clean the throttle body do NOT touch the plate or it will cause more problems.

When cleaning the maf sensor do not touch the resistor

Try to do the idle relearn . Not sure how to but look it up. I did some reading on maf sensor before and Your symptoms do point that direction but its tough to say for sure .

Do not go and buy any used sensors or aftermarket. Aftermarket sensors are prone to problems and a used one might also be bad.

 

Well.. It happened again, after several days of the light being on, it has now cleared itself again and it has been cleared for the last 24 hours.

Are you referring to the ECU rest? I am definitely considering doing that this weekend.

 

input you requested: you have a 12 year old car, problems/CEL's on and off since January, and admittedly have bad car knowledge........id keep going on the interwebs looking for magical answers and NOT go to an ASE mechanic

 

Thanks for your reply.

 

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