Bubbles in the coolant reservoir w/ overheating

[bradleyheathhays]

'96 XJ 4.0 here, and I'm seeing something rather ominous. Although it's getting into the low 90s and slight overheating is expected this time of year, my dash temp gauge is getting up close to the red zone lately in the heat of the day (which it never has before), and I'm getting bubbles girgling into my coolant reservoir for about 10 mins after shutting the engine off. Sounds like boiling water. Now, just a few weeks back I had to remove some coolant from the top of the radiator (maybe about 100cc) at the cap as I disconnected the overflow line when I was changing out the alternator. It seemed to start overheating after that, so I overfilled the coolant reservoir so it could suck in as much as it needed. Since then, it's normalized the level of coolant in the reservoir by overflowing the excess after it got hot, but it's still bubbling after shutoff.

Is this possibly just air in the coolant system somehow, and I might be able to fix it? Or is this a leaking head gasket for sure?

Almost forgot to mention, but a symptom that started along with this overheating is my idle speed in park gets up to and stays at 900 after it gets hot. Not sure if this is a symptom of a leaking head gasket or not.

Thanks.

 

[JPNinPA]

To check for a leaky head gasket you first with cold engine and fluid take the radiator cap off. Then start the engine watch the fluid flow into the radiator. This is where you look for bubbles. To further validate you could pressure test cylinders. Or put a combustion leak test on.

Fluid flowing into the expansion tank is normal. The expansion tank should always have some fluid in it even when cold. This keep air from making it into the system that could lead to boil over.
The other causes for boil over after shutdown could be a bad radiator cap, air in the system or leaky head gasket.
The first two are easy checks and repairs.

If you are running hot consider verifying outside of radiator is clean. No mud packed in.
Second would be checking the clutch fan by spinning engine off. It should only rotate one to two times.
You could flush the system to remove anything blocking fluid flow. Especially blocked radiator fins.
Also be sure nothing is blocking the radiator like grill inserts a huge wench…



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