Posting this here instead of the engine sub-forum for more exposure.

I have a 2010 G37 6MT and have been experiencing random misfires. Rarely the check engine light comes on but sometimes it does during cold starts and it shows P0300 random misfire. Once the CEL is cleared the car drives fine. Recently the vibration has gotten worse at idle. Other symptoms include the engine sounding a little rough. The vibration comes and goes at idle in waves and I only feel it in the car seat. With the hood up you can feel the engine vibrate.

Here's what I did:
Throttle body/MAF sensor cleaned
New OEM spark plugs
New coilpacks x 6
Battery is good

I just recently changed all the coilpacks and I noticed some oil in cylinder #3 around the spark plug. Less than 1 inch. So I believe I will need to change the valve cover gasket sometime. When I log my misfires, cylinder #3 always has the most followed by 5. I only get 1 or 2 misfires in the other bank, cylinders 2, 4, and 6. So it seems like the misfires may be caused by something on the right side of the engine (passenger side).


So the question I have; is it possible the valve cover gasket is causing the misfires?

Next I will check for any vacuum leaks with a smoke machine and replace the PCV canister/valve. I will check the injectors as well.
​​​​​
is there anything else that could cause this? After I changed the coilpacks I noticed it was running a bit smoother but now seems to misfire at higher rpms and rough idle has come back.

Thanks in advance

 

The misfire at higher RPMs could definitely be from the valve cover leaking oil into the spark plug wells, that's a pretty common symptom once you have confirmed a leak there. Change the valve cover gaskets and O-rings around the spark plug holes, and see if you still have misfires.

Also, being a 2010, it may be a good idea to check your oil pressure to make sure your oil gallery gaskets haven't blown out. Those can cause misfires and rough idle symptoms as well, and may not always trigger limp mode or other codes.

 

Your expectation is uninformed. You will get more exposure here than in the other forum.

 

Update:
Tomorrow I will replace the PCV valve, check for vacuum leaks with a smoke machine and check the injectors. If that doesn't solve it, which I doubt it will, I will replace the valve cover gasket and go from there.

I've now noticed that it misfires only during cold starts. Torque is registering anywhere between 5 to 25 misfires mostly coming from cylinder 3 and 5. Car feels great when driving no stumbles there might be one or two misfires but for the most part it's great.

At idle it continues to vibrate. I can feel it mostly in the seat but starting to feel a bit of vibration through the steering wheel. The vibration comes in waves. I checked with the Torque app my RPM fluctuates between 638-650 rpm. At moments I do not feel the vibration but then comes back.

 

Since you have Torque, what are your short & long term fuel trims for each bank and air/fuel ratio when at hot idle?

 

Car is stock and has 176,000KM about 110k miles.

Replaced the PCV valve on one side (didn't realize car has two) but the other one seemed to be fine. Checked for vacuum leaks and found a small leak on the driver side air inlet hose right after the MAF but it was very small.

Car still misfires and idles roughly. Next I will replace the valve cover gasket, spark plug tubes, and possibly the spark plugs.

No CEL but during cold starts but there is a pending P0300 code.

​​​​​​
​​​​​
​​​​

 

Air/fuel ratio fluctuates between 14.4-15, it isn't steady and fluctuates with change in engine RPM.

​​​​​​LTFT Bank 1 - steady at around -7%
LTFT Bank 2 - steady at around -5.5%
STFT Bank 1 - spikes between 10-15%
STFT Bank 2 - spikes between 2 - 10%

The STFT for both banks follow the same trend up and down depending on the vehicle idle.

Also when it misfires you can hear audible pops from the engine.
​​​​​​I'm thinking it's the valve cover gasket but I don't know. I will look into pressure testing the oil later on after eliminating the more common issues. I do not have any codes at the moment.

​​​​​​


​

 

^^^ Those fuel trims and AFRs look normal to me.
I don't think air or fuel is your issue.

One option is to use Torque's "Test Results" section to view the OBD mis-fire counters. This might lead you to a specific cylinder.

 

Newer cars the pcv valve is brass.

 

Thanks problem is it is random and some cylinders will show more misfires than others and then when I restart the car other cylinders will misfire. Sometimes it's as bad as 15-25 misfires in one cylinder usually 3 or 5 which is the side the valve gasket is leaking on. I have a feeling it's the spark plug gaskets that are messing it up.

The funny thing is once I start driving I get no misfires. So I'm wondering if at idle the vacuum created somehow sucks in additional air or the spark plug is fouled where there was some oil leaking into the tube. When driving the pressure seals everything so no misfire but I am not sure.

I will go ahead and replace the right side of the valve cover gasket and spark plug tubes (cylinder 1-3-5) and see how it runs. I've noticed the vibration more and more overtime so something is wearing out and causing it to vibrate.

 

Two thoughts:
• As you mentioned, intake vacuum is highest at idle and this may allow a small leak to have a larger flow
• Misfires are much more difficult to be detected at higher RPM, so they might still happen but you or the ECM can't detect them.

I follow your logic about the misfires being one once bank and replacing the valve cover gaskets, but the PCV system and the cylinder head oil drain holes are connected to both banks as well as the crankcase. As I see it, if one valve cover was leaking I am not sure the mis-fire will show on that side.

Please let me/us know your resuts!

 

What cars have brass PCV vales? I would surprised at that due to the cost.
The G35/G37 PCV valve is all plastic.

 

they dont clog like the older ones

 

Today I replaced the valve cover gasket and spark plug tube gaskets on both banks. Also replaced the plenum at the bottom of the intake manifold. Cleaned the throttle bodies and replaced all 6 spark plugs. Put everything back together and still getting misfires on the first start. 0 misfires in any cylinder while idling after start and 0 misfires when driving. Car drives great still a little bit of vibration at idle and the rpm fluctuates slightly between 650-675 rpm.

No cel. At this point I'm not sure what else could be causing the vibration. I don't think it's an engine mount. I'll have to drive it for some time and hope a cel leads me in the right direction.

Do you guys thing replacing the fuel filter or anything else is worth it at the moment?

Possibly a failing EGR valve but not enough to trigger CEL?





​

 

Check battery tie down