Percolating/bubbling sound at the thermostat
 

OK, we've done some stuff to our 1998 Discovery. Based on the suggestions given to me by several the last couple of weeks, thank you. Took it out for a test drive down the long dirt road we live on, drove slow, drove fast, stopped at the end and turned it off for about ten minutes, did fine, idled low again, as we approached home it started idling high again. Park, turn off, fan goes off, bubbling/percolating sound coming from where the new thermostat is.

My husband is thinking it's air, I guess it would have to be, is this just a bad thermostat or likely the head gasket? Thanks

The engine temp gauge did not show it being to hot but it did seem like it was to hot, really hot under the hood.

The dash gauge is useless on telling the temp. Get a scan tool to see what the temp is as you drive.

the thermostat for the Disco should have a hole in it to release the air that gets stuck behind it when you install it. If there is air stuck in there, the coolant will boil and give you the bubbling sound. The dash temp gauge isn't much good for checking temp asit should be just below 1/2 way. If it hits the 1/2 way point, it is too hot.

First you want to make sure the thermostat is in the correct way ...pointing in the correct direction. I've personally never put one in backwards (that I'm aware of), but scores of people do if they don't understand the situation. So, first of all, make sure it's in correct, or anything you do from this point on will be a waste of time. Also that it's working correctly. What I do on my 98 when putting new fluid in, is to find a hill and park it on with the reservoir at high in the air as possible compared to the rest of the engine.

Signs you have a failing headgasket, or cracked head/block:
1) You need to keep adding fluid (and you've ruled out any leaks)
2) You remove the oil fill cap, turn it over and see a whitish 'milky' substance on the bottom side.
3) You remove the coolant tank cap (don't scald yourself), and when someone gives the engine RPMs, you see bubbles or water shoots up ...or, when it's idling you can faintly smell combustion gasses in the reservoir.
4) You hear a 'bubbling' sound in your heater core as you accelerate.
5) You hear a bubbling sound in your heater core after you start it up in the morning ..or start to drive. This is because some fluid leaked back into the engine overnight
6) You see fluid leaking out the side of the engine (not that common)
7) Milky substance in oil or on dipstick (if you see this, stop driving it and fix the problem)
8) You can buy test strips at a parts store that you dip into the coolant which will tell you if combustion gasses are present ..that's a good indication.
9) Remove all the spark plugs and look at them. If you find one that looks super 'clean' almost like brand new, compared to the others, there's where your leak is. The steam created from the water cleans the cylinder ..and spark plug out nicely.

Hope it helps.