Hello all, been scratching my head at this problem for a little bit now. Bought a 09 G37x with 122k miles back in November, car has been fine this entire time until a month ago. Started running really bad due to the oem exhaust finally rusting away at the y-pipe (car would misfire while on cold start and idled fine after warming up). I fixed the leak but the idle never went away, next thought was spark plugs. Well all the plugs were fine expect for cylinder 5, the spark plug well was submerged in coolant. That would explain the misfire. I did some research to see if I could find anyone else with a similar problem but no luck. Anyone have any idea why coolant would be in the spark plug well? MAFs and TB's were cleaned a while ago, car never over heats, oil isn't milky, I'm not sure what it could be.

 

Have you compression checked that cylinder?

 

I have not, I guess picking up a compression tester is next on the list. I just did a glance over everything I could think of.

 

The only way I can see coolant getting into a spark plug well, is from above. For example, maybe when the throttle bodies were cleaned, the coolant tube was removed and it dripped/flowed onto the valve cover and into the well.

I don't think you have anything to worry about going forward.

 

Well, the misfire started happening before I cleaned both MAFs and TB's. It was the very first I tried before knowing the rough cold start was due to a misfire. I'm just crossing my fingers that it's not failing head gasket or cracked head. I took a poke around the coolant lines on the TB but I couldn't see any visible leaks. After I dried the well out and put 6 new plugs in, the misfire went away for 2 weeks but is now back.

 

And is coolant back in the #5 well?

 

I am almost certain it is, I haven't gone back in there yet due to how cold it's been recently. Coolant fouling out the spark plug/coil pack in cylinder 5's spark plug well gave me the P0305 code. After cleaning out the well and throwing in new plugs, the misfire went away. It is now back and the car throws a P0305 about 50% the time I turn the car on. I plan on diving into it tomorrow, i'm supposed to get nice weather where I live and I plan on cleaning it out again and thoroughly checking for cracking in the well or leaking hoses since last time I didn't notice anything.

 

Check/ clean it out as needed. Then drive the car to get it warmed up before checking again. While the engine is hot look for any signs of seepage. Even the tiniest pin hole leak should present when the coolant is hot.

A compression and/ or leak down test will help determine the condition of the head gasket. Some chain stores rent these tools/ kits.

It has been my experience that even the smallest of cracks in the head/ water jacket would present more serious, noticeable symptoms than yours. This is what leads me/ us to believe the leak is external/ above the head.

To state the obvious: use caution when working near a hot engine.

 

This definitely lifts some weight off my shoulders, not saying something catastrophic isn't possible still, but this brings some peace. Definitely going to preform both a compression test and leak down test ASAP. In the event of it being a failed hose, what is a good place/brand to look out for. Z1 has the full kits and what not but I have seen posts about them not holding up well over time.

 

If it is a leak, from a hose or otherwise, it obviously is not a major leak else other, more severe symptoms would present and you'd have considerable loss of coolant.

That said, diagnose first before throwing parts at it. If you need a hose(s) there is nothing wrong with OEM or quality off-the-shelf pieces, but that is just my 10¢...

 

Of course of course, thank you a lot for your input and help. This is the first car I've actually cared about and would like to keep around for as long as I can and a lot of this stuff is jarring to me.

 

Just tagging @BULL to see if he has any ideas on this.

 

Well, currently in here, coolant has definitely reappeared. It's at about the same level as it was last time. The coolant sitting on top of those two thicker hoses are the ones that connect to the TB. The drops are definitely from taking the coil pack out but where it is slightly wet where the little plastic clamp holding them together looks like it wasn't from the residual coolant on the coil pack when popping it out. I could that be the culprit? I'm not sure if that's even a hose for coolant. It definitely goes to the TB.

 

If you don't live somewhere that is cold, I'd bypass the coolant to the throttle bodies all together but that does look like the source.

Those are coolant lines.

 

I believe you found your issue. As a test, I would clean the hoses then wrap them with a heavy-duty tape. Drive the car for a while then check again. If there is a leak, the tape will tell and, in theory, should divert the leak away from the plug well.