Hello All,

2004 G35 6MT Sedan with 116,000 miles.

Started running rough and misfiring under load about 5,000 miles ago, set code P0300 "Random Misfire". Had begun using a lot of oil the past 15,000 miles or so also. Pulled the plugs and #4 was oily and not firing well, and #6 spark plug well was full of oil. Typical valve cover replacement issue.

Replaced valve covers with new Nismo units approx. 5,000 miles ago. New NGK Iridium plugs all around, oil change with Castrol Syntec 5/30 and a K&N oil filter and we were good to go. Has run like a scalded dawg the past 5,000 miles, but noticed a puff of smoke when backing off throttle after continuous 80 MPH+ freeway runs, big puff of smoke when accelerating hard for the first couple of gears, and also occasional big puff of smoke when cold started after sitting several hours. Still idled smoothly and ran smoothly at all times. This indicated a problem to me like maybe worn valve guides (I'm hoping).

Running about 90 MPH this morning for about 10 continuous freeway minutes (no traffic), then came upon slower cars and backed off, coasting down to about 65 MPH - cleared the traffic and went to speed back up (gently without downshifting from 6th) and she started bucking and snorting like crazy - looked back and a big wall of smoke behind me, apparently emitted when I backed off (excess crankcase pressure venting through PCV valve? - it's gotta go somewhere I guess ). CES light blinking frantically. Downshifted and brought revs up a bit, smoothed out some but still misfiring and CES blinking, get where I'm going and pull off freeway. Restart car and it's running REALLY shitty now, will barely idle. I drove it back home (approx. 25 miles) at 75-80 MPH, keeping revs up a bit so it ran smooth, only ran rough at stoplights or under a load.

Pulled the plugs and checked the compression - a bit over 180 Lbs. per cylinder in every one except #4 - and the #4 plug was covered with wet oil and carbon deposits and had obviously not been firing. ZERO pressure in #4. Not even trying to build compression, just free spinning like there is no piston in the hole at all !

All indications are a big hole in the piston (in my past experience, if the valves were damaged or rings cracked or worn I'd get at least SOME compression). Its almost comical the way it spins without resistance.

BIG $$$ QUESTION? Do I drop the pan and pull the head and just put a new slug in #4 (with rings and bearings, of course), and then run it ??? Maybe a valve job if it's warranted (but then off course gotta do the other head to match).

Your thoughts, fellow G-Men ??

 

Well, how much tolerance do you have for ****ing around?

If you have the time, replace the piston. If it were me, I'd rebuild or replace the motor.

I haven't heard of this happening on a stock VQ before (i have heard of oil consumption, but not a total loss of compresssion). You sound like you run it hard, I'd get a slightly built motor and add an oil cooler.

 

If it were my car I would pick up another motor or rebuild the whole thing. Sounds like you were running lean in #4. Whatever you do get those injectors flow checked and find out what caused it so it doesn't happen again. Its possible you have a valve problem rather than a piston problem.

 

Rebuild the motor and stop driving it like you stole it.