2014 Maxima with 115K miles on it. CEL came on and pulled a P0306 code. Did the old swap test and determined it wasn't the spark plug or coil although there was a tiny bit of oil in the spark plug tube. Replaced the rocker cover to combat the oil leak. Cleaned out the spark plug well and everything ran fine for a while but then P0306 code came back again. Checked and replaced all vacuum lines and don't see a leak there. Runs and starts rough especially at idle. Pulled the plug and it was wet with what smells like coolant. I've been looking at my coolant level and haven't seen a drastic decrease nor is there any coolant on the ground. Checked around the upper plenum and lower intake manifold and cannot see any coolant leakage. Looked at the oil in the rocket cover and it's looks good like no coolant is leaking into the oil. Tiny bit of white smoke coming from the exhaust.

Should I replace the lower intake manifold gaskets. My guess is they get rusty over time and deteriorate.

Do I need to drain the coolant to do so? I just changed my oil and coolant but would it be smart to go ahead and change both again after the replacement?

Any other suggestions before I dive into that project. Any possibility it's my head gaskets?

An update: pulled the 6th cylinder spark plug this morning and could see coolant seeping into the engine just below the spark plug threads. Got a long Q-Tip and stuck it down there and sure enough.....green coolant smelling fluid. This doesn't seem like an intake manifold issue and seems much more indicative of a head gasket. I'm taking it to a shop on Monday to do head gasket test. Praying for the best but it's not looking good.

I too believe it is a head gasket as I dont know how else coolant could infiltrate the cylinder. If the shop says differently, please let us know.

Just as an aside, any idea how green coolant got put into your system? Oem is blue.

I'm afraid so too. Need to make a big decision on what I'm going to do with the car because it's only worth about $8k. I'm going to call around to some shops to get some estimates. Any members with rough estimates of cost would be fantastic. Basic understanding is they have to pull the motor to do the job so I might be looking at some other replacements as well.

I did a coolant flush at 95K and replaced with Nissan green coolant because that is all the dealer has at the time. Car ran absolutely fine for 15K with no issues but I did just recently take it on 3 consecutive long trips, two being through some pretty steep country roads and one being in heavy stop and go traffic through Chicago. Just never figured head gaskets were issues on this car.

I really havent heard much of head gasket issues, more to the contrary, these engines are pretty hardcore (I just went over 490,000 miles on the original engine and cvt with a few 1000 mile a day trips thrown in over the years thru the Ozarks.) I even had my engine go into limp mode when I had a coolant leak and I didnt watch my dash. That was over 50k ago and no issues. Have you scoped it out with a camera to see if it is a cracked block or ......?

I have not scoped it with a camera. Just honestly don’t want to throw any more money at it. Even if it’s a cracked block, I’m still looking at an engine pull/replacement which I was quoted about 3k just in labor. Most the local mechanics around here don’t even want to touch that kind of work. I’m convinced it’s the death of this car but it might not be a horrible idea to still pay the $100+ to get a diagnosis. Would rather put that $100 toward the registration on a new car.