I have changed coolant in my 5.7 l by disconnecting the lower radiator hose. Put a tube of Bars Leaks Golden Powder into the hose. Added about two gallons of 50% green coolant to the reservoir.
Unfortunately, I did not open that bleed valve next to the thermostat. Probably because of air pockets the coolant started boiling after the engine ran for a few minutes.
I turned it off and to took the car for a short ride. The coolant temperature was not around 174F as usual, but rather in 180 -185 range. Added a little bit more coolant the next morning. Drove the car for four hours (a business trip) - still the same 180 -185 F. STRANGE! What happened to the cooling system?
By the way, the lower radiator hose is much cooler then the upper one. I could not detect sharp temperature drop as the thermostat opened up as it's been on my 4.9 liter 1991 Seville.

Any comments are appreciated!

For some odd reason, when I did the coolant flush on the S Class I had, it ran a bit warmer as well. Nothing abnormal, but it was about 15-20* warmer than usual.

For some odd reason, when I did the coolant flush on the S Class I had, it ran a bit warmer as well. Nothing abnormal, but it was about 15-20* warmer than usual.

15-20 degrees F seems to be a big difference. Mine's about 10 degrees and makes me nervous! One of my theories is that because the coolant boiled , the thermostat got affected and opens at higher temperatures. The other theory is related to the coolant supplement I have added. There was not any supplement in the coolant I have replaced, despite the FSM says it is a must.

You need to bleed the air out using the bleeder on the thermostat housing and check it again.

You need to bleed the air out using the bleeder on the thermostat housing and check it again.

Thanks, Jay. I will do that tomorrow and will report back. Another weired thing (is not it too much? lol). It took only 4 qtrs of motor oil with filter change instead of 5 qtrs. Checked the level two times when engine was cold and all three holes on the deepstick were covered. I used Penzoil 10W-30. I do not know whether it was a good choice though (some suggest only 5w-30).

You need to bleed the air out using the bleeder on the thermostat housing and check it again.

Opened the air bleed screw. There was no air whatsoever, only coolant.

This was after the car was warmed up with the thermostat opened? Might help to get the nose of the car up in the air.

This was after the car was warmed up with the thermostat opened? Might help to get the nose of the car up in the air.
That's what I wanted to do, but because of rain I checked it with cold engine. I do not think there may still be air in the system after 500 miles and several heating-cooling cycles, but I might be wrong since I am not familiar with pressurized cooling systems at all.

Have to check it hot.

Thanks, Jay. I will do that tomorrow and will report back. Another weired thing (is not it too much? lol). It took only 4 qtrs of motor oil with filter change instead of 5 qtrs. Checked the level two times when engine was cold and all three holes on the deepstick were covered. I used Penzoil 10W-30. I do not know whether it was a good choice though (some suggest only 5w-30).
Is Penzoil a good brand? Never used one before.