Hello, I have been a member for awhile, but this is my first post. My 2005 G35 Coupe has a rough idle. I have looked at many threads regarding rough idles, on both g35driver and my350z forums, but have found nothing that has fixed my problem. So I thought it was time to make a thread on my specific situation.

The problem is that my rpms drop at idle, then the car shakes for about 2-5 seconds, then once the rpms raise back up it starts to idle normal again. This happens maybe about every 40 seconds or so, not in consistent intervals. When I am driving the car, I feel no power loss or hesitation. At Idle, when I turn the AC on i do not experience this rpm drop/ rough idle.

Things i have done/replaced:
Cleaned throttle body
Idle relearn procedure
Cleaned MAF
Replaced all coil packs
Replaced all spark plugs
Installed 4 new o2 sensors
Cleaned inside plenum (after installing catch can)
Checked for vacuum leaks

One last note, during colder temperatures in the winter, upon starting my car, the brake pedal is very stiff. Its so stiff that i cannot push it down and the car will not brake if i tried. After about a minute of warming up and pressing on it, it will finally sink down, and then i can brake normally. Furthermore, when I turn my car off, I hear a whistle/pressure being released around the brake booster area. I am not sure if this is related to my idle issue or not, but I am just mentioning everything I can think of.

Does anyone have any ideas? My best guess at this point is a vacuum leak, but i have looked and found nothing. Thanks for your time!

 

Hmm, when my g35 idled rough my engine threw the code P0113- Intake Air Temperature Sensor 1 Circuit High Bank 1. Is your engine throwing any codes/lights? If you have access to a scanner, the first thing I'd do is scan it and see what the engine is trying to tell you. Otherwise stop by an auto parts store, most of them do free scans.

The second thing I'd recommend is just going through and checking all connections to your sensors, and tightening all the connections between the air intake/throttle body. See if your air filter is dirty/clogged. Just the basic stuff. That's what did it for me.

 

Thank you for your reply. I do not have any engine codes, and have not ever since this problem started. At the shop where I work, we have a pretty high end OBD2 scanner. Ive used that multiple times to see if there was any codes that were not popping a check engine light, and it showed no codes.

Also i have checked many, if not all, connections of sensors and vacuum lines. I will go through once more throughly to rule that out.

 

So I had this issue happened to me once. Where are my car would rough idle and it would do this weird thing where it would kind of drop down for a couple of seconds and then it would go back to normal for I don't know maybe a minute or two and then it'll rough idle again if you haven't looked at your evap sensor the purge control valve. Or your PCV valve or where you hear the hissing on the brake booster that's a vacuum leak right there the brake booster drum there's a hose line that draws behind the secondary firewall in the main firewall there's that black tube line. And there was a few threads that I read about that that black two rhymes sometimes has split or crack somewhere due to excessive heat or sometimes just the car being old. But also where that hose comes out from the secondary firewall to the brake booster that's all a soft hose area and that stuff's notorious for cracking and bleeding air. Also if you can't push your brake pedal in during the winter time that's because of brake drum booster is getting ready to go out. And it'll cause freeze and breakups because it's too cold in the brake drum booster you should have free play on the pedal all the way through. Even if it's cold. I would replace the map sensor for the rough idle I'd look at your evap sensor valve if the purge control line is I want to say is at least above one on the EVAP roll for voltage chances are the evap canister purge control valve is gone or if you have Street zeros the canister control valve is straight gone there's nothing. But after that I would do the idle relearn after you do your math center and check your PCV and the EVAP sensor. Because those are also main causes and leadings for rough idles because the PCV valve can't regulate the cam case pressure properly. Cuz when that happens when you're driving normal and nothing's losing power or whatever you turn the AC on and all that blah blah blah blah blah chances are the throttle body could be going too but at the same time what really happens is that maf sensors saying it's on its way out and the check engine light will only come on when the map sensor is getting ready to drop voltage under three and a half so if you put your resistance meter on and it spikes up over on the temperature prong which I think is on the right side last pin your MAF sensor should be no greater than I want to say about 10 or 15 ohms of resistance if it's higher than that then the map sensors core is gone. I've been a performance mechanic for 10 years and I have an 04 G35 that I'm currently modding out.