I'm in the middle of doing a cooling system refresh (new radiator, water pump, t-stat, etc.), so I drained the coolant, flushed everything with water, and added some Thermocure to circulate for a few days and clean the block before I put in the new radiator. Coolant was last changed/flushed just over two years ago. When cleaning out the overflow tank, I noticed a bunch of grey sludge in the bottom. It was almost like clay. Here's a picture showing it mixed up a bit with the water I used for flushing the bottle.

After bleeding the system, I took it for a 20-minute drive on a warm day and noticed some bubbles coming up through the overflow tank. This could be exhaust, or it could be some air that didn't escape when I first bled the system. After it had cooled completely overnight I took off the radiator cap and about a half cup of coolant burped out like there was still some pressure in the system. That had me worried. I took it for another short drive this morning (long enough to have the t-stat open) and didn't notice any bubbles in the reservoir but after letting it cool completely, it once again burped out that coolant from the radiator cap. It seems like the radiator hoses get pressurize more quickly than normal, but it was also in the 80s this week, so maybe I'm imagining that. This is on a '98 4.0, by the way.

The oil looks fine on the dipstick, and I don't notice any milky residue under the oil cap. No steam or white smoke from the tailpipe. The radiator cap is Mopar and was replaced about 2 years ago. I'm worried it might be a failing head gasket.

What's the next step here? Should I do a combustion gas test? Take it for a long drive again and see if I get more bubbles in the reservoir?

 

You could do the combustion gasses test.

But, this could be as simple as the cooling system had stop leak at one point and a bad radiator cap.

 

I believe that an oil analysis only cost about $40 and should detect the presence of coolant in your oil, as well as other good baseline data that shows various metals indicative of bearing wear

 

if that was in the bottom of the overflow tank, it could have been there for years. How was the fluid that you drained out of the radiator and block. Did it look like mud or was it clean and yellow

 

I thoroughly cleaned the overflow bottle when I flushed the system and changed the coolant just over two years ago, so it's accumulated since then. I've owned it for nearly a decade, and I've done 3-4 coolant flushes/changes since then, so I don't think old Stop Leak would have been an explanation per cruiser54's question. I've never added any.

 

Just guessing here.....could build up/sludge/other matter in the heater core finally flushed out after your years of maintenance?
The only reason I say that is that your overflow bottle was clean 2 years ago and nothing else has been worked on in regards to the cooling system since.
Not really sure if this is even a possibility.

 

I feel like you're overthinking it, and getting air back in the system every time you pop the cap off. Run it for a while and see if you keep getting bubble in the overflow.

 

I was! I finally got it all back together over the weekend and took it for a few long drives. Everything seems to be working as it should.

By the way, I managed to get my hands on a Mopar HD cooling radiator, and you can see how much thicker the core is.