Weirdness after cooling system refresh

OK so had some weirdness with engine temps today and wanted to get some opinions. FWIW I did a cylinder pressure test on this engine when I bought the jeep and all cylinders were 175-185 with no pressure loss. It has a supposedly rebuilt head within the past year. Apologies for the long post, I want to give as much info as possible.

Some background - To refresh the cooling system I did the following - I flushed the heater core since it was clogged, using a garden hose and flushing both directions until the water ran clear. I then put in a coolant flushing solution and let it sit for a week. I then did 4 complete flushes of the whole cooling system, filling then running the engine for 10-15 seconds with no thermostat installed. The first flush was with regular hose water, then with distilled on the next 3 flushes. I drained the system each time by pulling the radiator hose on the bottom of the block to get as much from the block and radiator as possible. After the last flush the water drained from the radiator and block was clean and clear. I then replaced the water pump, thermostat (Stant superstat which is supposed to bleed air without drilling a hole in it), thermostat housing, and heater hoses. I refilled the system with half dexcool (pink) and half distilled water.

Up until today I have put about 50 miles on it just driving around town and temps have been rock solid, though I had noticed that it gurgles (from I think the radiator, but I never really tried to totally nail it down) when I shut it down and it’s at temperature.

Today I took it on it’s first highway trip since the cooling system refresh. It ran at 210 indicated for the first 20 miles, then randomly the temp gauge shot up to 250, like instantly, and on a slight downhill section. I pulled over and popped the hood and everything looked fine, it was not boiling over or gurgling. I left it running while I checked things and literally a minute later the temps were back to 210. I said F it, and even though I was at the base of a significant hill I decided to go for it. Temps shot back to 250 (instantly again) pretty soon after getting back on the road for about 5 minutes, then to 260. It was not steaming or driving any different than usual, it ran completely fine and normal. I got to the top of the hill 10 minutes later and on the way down temps dropped back to 210 and remained there for the next 30 miles, which included another hill.

I got to my destination and popped the hood. There was no gurgling in the radiator like there has been. The overflow tank was just above the hot full line, coolant looked clean and pink. Engine oil was recently changed and still looks brand new, no milkshake, level was normal. There was some coolant on the hood and air box that appears to have been purged from the radiator cap. 10 minutes of shutdown and I turned around and came back home, over the same mountain pass. On the way home engine temp never got over 215 or so, even though the climb on the return trip is steeper and continuous, unlike the rolling hills on the initial trip when it ran hot. When I got home there was again no gurgling coming from the radiator and fluid levels were the same as when I had started the return trip.

Did the cooling system burp itself? I’m a bit confused, I’d convinced myself that i have a blown headgasket but none of the symptoms are there. Thank you for reading my novel.

Also, I replaced the spark plugs after the leak down test, original plugs were worn but had no evidence of head gasket issues.

The gurgling you hear on shut down is likely oil draining back into the pan from the valve cover are. And you are probably right on the money about it getting out an air bubble. I bet if you check the overflow tank its probably lower now. Also you didn't give the year.

Ah, sorry, it’s a 98.

This morning, engine cold - I added nearly 1/2 gallon of coolant/distilled water to the radiator. Expansion tank remains at the hot full level (is this system supposed to pull coolant back into the radiator from the expansion tank?) even when the engine is cold. Oil looks clean and level is same as yesterday, no coolant in radiator neck so I can’t check color and clarity, but what’s in the expansion tank looks good.

Hoping this was just a big air bubble that got burped out of the engine, I can’t really think of what else it might be. No evidence that it’s burning coolant or anything like that.

For sure the way to know if the cooling system burped it so see if the reserve tank level has dropped when the engine is cold. Did you do anything during the coolant refill to try to remove excess air?

Not really, filled it, ran it, topped off, etc. Have not done any long drives since then though, just short trips in town. I used a thermostat that’s supposed to purge air without having to drill the hole, but perhaps I should have done that anyways?

The late model system doesn't require "burping", just make sure you have the correct cap!

I noticed this morning tha the cap looks pretty old, the rubber is pretty hard and brittle so I’ll replace it today. Is 16PSI the correct pressure rating?

If the radiator didn't pull any fluid from the reservoir, the cap wasn't working, i.e., the suction valve must have been stuck. Late model's call for a 16 psi "vented" cap.

Replace the cap, and check that the line to the overflow bottle isn't damaged or plugged.

Replaced the cap and had the same problem. Pulled all plugs to see what they looked like and they're all perfect, even tan color. Pressure tested cylinders again and they all were stable at 180-185 psi. Decided to throw a radiator at it since it's the last part of the cooling system I haven't replaced, and now temps are rock solid at 190. I guess the radiator decided to take a crap after flushing the system. Not complaining, running great now with solid temps.

Good to hear thanks for the update.

Which radiator did you go with? I'm debating on which one to get.