Hey CantZ, sounds like you’ve now got a good understanding of this issue. Like you I’m surprised that so much water vapor can find it’s way into the engine. I’ve been told that modern oils can deal with it and when then the engine comes to normal operating temp the moisture is driven out through the PCV system.

Most days I only drive my Z a short distance, so after reading your post I was curious to have a look inside my filler cap. It was nice and clean with no sign of oil/water emulsion.

Looking over the engine, it seems to me that the cap’s position, and the fact it’s made of non-heat-conducting plastic, probably makes it the coolest running part on the engine. Could this be a reason that even though moisture is driven out of the hotter parts of the engine, it gets trapped in the recess inside the cap? If there is any validly to this, then maybe your new Nismo cap (being made of aluminum instead of plastic) will now pull enough heat from the engine block to drive off any moisture, and will cure this problem. Anybody concur with this.

 

It's true. Happens more often when you run E85 also.

 

Happens on my Nismo oil cap as well, condensation for sure. My Z has 24K miles on it so I know its head gaskets are superb. Happy Motoring chap!

 

I heard letting it idle for long periods of time could cause this...

 

Thanks everyone for their input and information. I do believe it's just condensation in the valve covers. I thought it might have something to due with me using Royal Purple oil, I dunno? My Z has 107,000 miles on it with no major motor work, so it has the oem head gaskets.

 

+1. It is water and likely from condensation. Checking your oil for more should be pretty easy! If it's clear (i.e. black), you're ok.

 

Best way to check if it's a head gasket is to open your radiator cap and look for air bubbles. If you find air bubbles come up then you sir have a blown head gasket. Good luck!

 

If it gets "peanut buttery" in texture, then worry about gasket issue, my.02

 

I usually get Jiffy-extra crunchy.