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I feel there's still air trapped in the cooling system causing this water gurgling sound behind the dash and only warm heat.

1991 190E, 2.3 -I took the radiator cap off, filled with MB antifreeze, I was a pint low, turned the heat to high and turned off the off button. Started up the engine and waited till it warmed up. I then started squeezing the top, bottom radiator hoses and the hose at the rear of the engine, the top hose didn't seem to have much antifreeze in it going by feel (is this normal). I revved up the engine to 2500 rpm for a time and what happened next was the antifreeze started boiling and shooting out of the coolant reservoir.

Do I leave the radiator cap on while trying to purge the air out?

Any words of wisdom on how to purge this air would be greatly appreciated.
 

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The radiator shouldn't have a cap unless you installed an aftermarket one.
The coolant reservoir should be the main point of entry and purging.
 

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I meant the coolant reservoir cap, do you leave it on when trying to purge air out of coolant system?
 

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Leave it open, keep the heat in the cabin on full blast and let the car idle. Fill as needed.
 

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When I filled my 2.3, I undid the bolt located near the temperature sensors/switches at the front of the head. I also disconnected the line running from the thermostat to the coolant reservoir, the small one. I believe I replaced the bolt at the front of the head after the initial fill, and then replaced the small line once the car had warmed up and was holding temperature fine. Lots of squeezing of coolant hoses helped, too.

Also, when the car cools off completely (overnight), remove the coolant reservoir cap and replace it. Otherwise, the system will not build up pressure correctly which lowers the boiling point of the coolant.
 
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