Hello, I have ventured to change the valve cover gaskets, and after I took the Plenum off, I took a peek inside the valves, I found there is oil residue (not carbon build up) on all of them (see pic) where is this coming from? Do I have a bad head gasket?

For a bit of history, the car did overheat one time, but it did not show any dash lights, so I didn't know, then the engine shut itself off while I was driving, I was very cold outside as well. I repaired the coolant hose and filled with coolant. I drove it about a 1K miles more and did not lose any coolant, neither did the coolant seem to have oil, also, oil consumption is normal.

2010 G37 143k miles.

 

Oil consumption isn't normal on the VQ37s, but at 143K miles it could be present. But the fact that those black lines are perfectly bilateral seems to indicate that it's carbon (port injection, so the fuel sprays across the bottom of the valve) you see not oil as it would seem unusual to me that the head gasket would leak in the exact spot on both sides of the chamber. Maybe one of the others with more of an automotive background could comment.

In looking at this pic, where is the spark plug? six o'clock?

Also, are you the user on the Reddit sub that bought a used G from the Texas Drift Academy?

 

Here is another picture, some are valves are better than others. This seems to have the worst oil.

 

Thank you for replying. I posted another pic, and it appears to be oil instead of carbon.

The spark plug was 3 o'clock.

I'm not that Reddit user

 

Excellent. He was complaining that VQs are known from burning oil and used his whip as an example. He then disclosed a minor detail that he bought it from a "Drift Academy". There's no metallurgical magic that can keep cars which spend most of their lives being driven hard from consuming oil.

Let's see what some of the others say about the pattern you've discovered.

 

This is Oil consumption from bad valve seals. You can see in the close up picture that the valve guide has debris around the lip. This oil as it sees vacuum upon deceleration is forced into the intake port and this oil spray covers the back of the valve.
The injector then sprays and washes the front part of the valve however is unable to clean other parts so it wets the oil, that moist oily residue then dirties up and then attempts to dry. Because it cant dry it stays there.

Valve seals will be your only fix.

In my opinion, this fix usually shows age of the engine which to repair is much more money and work than replacing the engine itself.

 

The wonderful joys of port fuel injection.
Look at how clean the vales are after 143,000 miles.

No direct injected engine will look that free of carbon build up at 100,000 miles!

Anyways...
That tiny bit of carbon is nothing to worry about.
The PCV system introduces a little bit of oil vapour to the intake and when it hits the hot intake, it may collect a small amount.

 

As my Zed/4 is scheduled to have them "walnut blasted" (?) next month at 75k miles...

Most don't realize how well built and relatively problem free the VQ actually is...

 

I typed a long paragraph about how the VQ's don't need walnut blasting on your Z...

Then I re-read your post... Ohhh Z4, like BMW

In my mind, the VQ engines are the best engines Nissan has ever made. Good power and very reliable.
The more I read about the Q50 / VR30, the more I feel sad for the people who need to spend $4,000 on new turbos or $10,000+ on a replacement engine ON TOP of a monthly car payment!

 

How much does that run? Do you need a specialist or dealer? I ask because my wife's Honda pilot (17) has Direct injection and when I ask Honda about it they say, "you never have to worry about that on a Honda". That makes zero sense.

This is also one of the reasons I am leery of the VR30, well...this and the fact that Nissan and Infiniti are months away from Bankruptcy. Subject for another thread.

 

Lexus gets a cookie in this category as they use a combination of both port and direct, and they also use timing chains, not belts. Not sure why Honda insists on rubber timing belts. If you want a smoking deal on a Honda, find one that uses a timing belt and has around 100K miles on it as people unload them before that service is due and take a haircut in the process because most use dealers for service and this is a 3-4K job at the dealer. Independents will do it for less than half.

 

Most people just let the stealership or indy shop do the job. Depending on the shop, and engine, it can run ≈$500-1k. I have a indy shop that will do the work and on my Zed/4 with the N54T engine it will be about $650. I've neither the time or ability at this point to do the job. This post from spoolstreet shows the basics of what is involved.
Utter nonsense. By virtue of the design of DI engines, the valves WILL get gunked up- there is NO way around that issue. I left Honda decades ago after having owned 3 of them with zero issues or regrets- I just do not like what they have become.

 

$650 seems reasonable. Do people report a noticeable improvement in performance after this job is done? Is there any other work you need to do once the heads are off, like changing plugs/coil packs?

As for Honda, they are catering to folks that value reliability first, and cup holder and pocket book storage second. They are constantly giving me crap for changing my oil at 4K miles and transmission fluid every other oil change, air and cabin filters every 12K. "Let the computer tell you when it needs to be done". They also don't like that I disabled the VCM (put the back cylinders to sleep to save fuel, using the torque converter and slippage to control vibration) to save wear and tear on both the engine and trans/torque converter.

With that said, I bought my son a 2012 CRV and that thing is bulletproof. I see why the older CRVs have a cult-like following. Good acceleration, comfort and even handles well for an SUV. 25mpg in traffic. That thing is built for grinding out commutes.

 

I have heard it described as one of those "night/day" scenarios where most do not know how bad the engine was running until the work was done. Remember, with the dirty valves, the engine is essentially "suffocating" as the air can not flow to fully fill the chamber. Here is a before/ after pic of the valves from a BMW N54 w/ 140K miles (taken from above link)



In most severe cases, the engine will misfire, stutter, have vibration issues at certain points in the power band, have severe lack of power, and even just cut off at idle. This may/ not be accompanied by a litany of idiot codes. Obviously, it is one of those "it will only get worse over time" ordeals. So for Honda to tell you "it never happens" they are misleading their customers.

In my case, I am having it done more as a preventive maintenance item though I am feeling a slight "blip" mainly during cold starts/ fast idle. Hard to tell as it goes away once the exhaust flap opens so, who knows if that additional back pressure is contributory or not. No other issues and I'm still getting 25-28mpg so...

Either way, most would go ahead and replace the spark plugs (BMW recommends every 30-45K miles anyway), coil packs IF they are cracked/ damaged, replace the PCV valve (which is a known issue for my engine), VANOS solenoid seals, and whatever other gaskets/ seals that are involved. The indy shop will be doing the walnut blasting and the plugs while I will do the others as well as the belt, tensioner, and pulley at home.