There are just a few random globs of black suddenly on the inside of the e expansion tank. Please don't tell me this is a blown head gasket. Is there anything else that could cause it?

Oil looks fine on the dipstick, not milky.

In recent history, I blew a chunk of front cover gasket and the oil light came on. Got home with a dry dipstick and a huge trail of oil behind me, but after taking the cover off (again) and redoing the gasket with The Right Stuff this time (screw those paper gaskets), not a drop of oil has leaked, no oil light, no overheating at all, and I just took it on a 400 mile trip and it ran beautifully. On a side note, I randomly pumped up with that new Shell V-Power Nitro+ and swear I got about 60 more miles out of my tank, and it seriously has been running much more smoothly.

Anyway, could it be a head gasket? Can a head gasket just leak a little but not bad enough to cause problems, just enough to leave a trace?

 

Love your optimism! OBVIOUSLY you forgot that you are referring to your Land Rover Discovery, so let me remind you. haha OMG that is funny....

 

Did you flush the cooling system after the repairs to the timing cover gasket, is it possible that some oil go into the cooling system?

 

but seriously, now that I have thought about it, are you using an aftermarket gauge to monitor your coolant temp? If you have a head gasket leak, it will be reflected in your engine operating temperature, and you should have figured that out by now. If your temps have remain unchanged, then the mess you found must be RTV silicone. You should have noted your operating temps in the original post, and the fact that you didn't scares me a little bit. So lets start with that.

 

Hahaha yes, it's that relentless optimism that makes me not break up with my high maintenance girlfriend and drive her off a cliff with myself strapped to the hood every month when something goes wrong. Hey I think Land Rover's have periods! That's what it is! They go crazy about once a month, with a few extras thrown in here and there just because.

Operating temps have maintained around 200, normal for me. I've got a new thermostat (well, year old) that's the 180.

The RTV kind of makes sense... when I did these jobs I didn't fully flush coolant since I had done that about a year ago, just a drain and top up.

Ok, so if head gaskets were blown, I would DEFINITELY have had overheating? Or not necessarily?

 

The cooling system is pressurized. A head gasket leak or otherwise would release said pressure with negative results. It may not necessarily overheat (depending on one's definition of such as related to the Disco) but operating temps would spike to values higher than what is normally experienced. Pull some of that junk out and make a confirmation.
Mine is on it's time of the month too. SoaB, this thing is driving me nuts. I agree with your theory of the menstruation cycle. No other explanation.

 

Suggest you attempt to remove one or two globs of black gunk and determine what it is. Liquid or solids? If liquid you should be able to use a paper towel to absorb a glob.

HGs can leak/seep externally or internally and when "minor" don't necessarily cause any noticeable performance issues or result in codes. Usually the only time a leaking HG causes an overheat condition is when coolant is not checked and replenished on an **** basis.

My first HG replacement was on a '67 Caprice with a 327, only symptom was constant steam out of the tail pipe due to coolant entering a cylinder.

May want to do a thorough flush of cooling system and be optimistic.
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