Changed coolant now engine overheats
 

I'm baffled.
Today I simply drained the radiator, then refilled with new coolant. Now the temp climbs quickly to 120c
What the heck happened?
Everything was perfect before this.
I did nothing else.

Only clue as what it might be: while the top part of the radiator is very hot, the liquid in the expansion tank- and bottom hose are still very cold. So I'm thinking, almost certain that the thermostat is stuck closed.

But why all of a sudden after just changing the coolant, would the thermostat decide to stick? Maybe some slight difference in coolant formula or pH cause this? Oh, and the thermostat is fairly new, replaced just about a year ago.

1987 300SDL

You have an air pocket in the head.

Raise the front of the car. Carefully (due to the plastic neck) remove the upper radiator hose from the radiator. Using a funnel fill the head with coolant mixture. Reattach the hose, put the climate control on max heat, now try running the engine.

X 2
 

It's like a Genetic Flaw shared by all MB's...
If you don't fill the Head with the Upper Radiator hose with the front of
the chassis up on ramps it'll "Air Lock".

I was thinking air pocket also. I filled the new coolant in that manner like I always do- thought the top hose and the front of the car on a slight upward incline. Maybe not enough of an incline this time though. How much incline should I give it? Is there a sweet spot?

For clarification sake, when filling, does the top hose stay connected at the radiator, or at the head?
I usually leave it connected at the head, and fill it though the hose until coolant comes running out of the radiator top port.

Interesting.

I have a radiator cap with a hole. And a funnel that fits in perfect. With a little rubber seal.

Fill the funnel with the car running. Keep it full. Run for like 20 min. And bubble out all of the air.

Yeah... That won't work on the turbo motors...

Hose stays connected to Head,Whilst filling
 

I'm Vapor Locked Myself this morning on whether or not to remove the
Coolant Tank ('Called the "Radiator Cap",Elsewhere) Cap while you're pouring
the Coolant into the Top radiator hose.

Well.
Go ahead + remove the "Rad Cap" as well based on
"You want all the possible escape exits for Burping the air out".

I don't think you can get the Front up "Too High" while Filling.
'Just don't scrape the rear end while lifting the front with a "Lull"
(Industrial Forklift) [This is LAME attempt @ Humor,Do Not Try yourself!]

You may want to remove the vent line on the water pump housing and verify that it is not clogged.

To vent air in coolant
 

I simply disconnect and unscrew one of the thermo sensors in the head behind where the upper radiator hose connects and fill the overflow tank until coolant bubbles out of the hole. Then reinstall and hook up the sensor.

1983 300CD.

Something I have done with 16 valved vws in the past is Rev them to 3k. Have you ever noticed that an overflowing coolant reservoir will stop overflowing when you Rev the motor? This compresses the water, making it denser, and along with the speed, it is better equipped to push any air pocket out.

I admit I have not tried this on a Merc, but I'd be surprised if it didn't work.

https://www.youtube.com/watch?v=ADPxM2SC6Mc

This is a youtube video of coolant fill using vacuum. It works by createing a vacuum in your cooling system (collapsing all the hoses), you then open a valve and let the vacuum suck the coolant into your cooling system. No bleeding necessary because there is no air in the system since it was evacuated already.

I don't know if this will work on 617.xxx engine. The limiting factor will be the expansion tank- whether it will withstand 25" of HG without collapsing. I think it will, from the looks of it (I think it is internally supported with ribs), not as strong as the round globe shape expansion tanks of the VW's but maybe strong enough. I have a spare 617 expansion tank and may test it and see if it holds up to vacuum. Would be a cool way to do my next coolant change.

I just drained and filled my 300SD. I removed the hose at the top of the rad, and filled it into the hose holding it up high. My car was not jacked up at all but my garage is slightly inclined. I let the fluid slowly go down inside the hose and keep filling until it starts to come out of the top of the radiator. Then I reattach the hose, then I have a screw cap fill connector on my line near the firewall heading into the heater core. I fill that line up too and get the air out. After that it ran fine, had to top up the fluid a few times but that's it.

Changed coolant now engine overheats
 

Why not?

This compresses the water, making it denser...[I][/I]

I'd like to see how you do that. The only thing that makes water denser is bringing it to 39* F. You may be able to compress an air pocket, but not the water.

Prove it to yourself then; remove the cap on the reservoir, put water in until overflowing. Get the engine up to temp, And it begins to overflow. Then immediately Rev the motor to 3k or so. No longer overflowing. Is it compressing it because it is cooling it? On reflection, probably. Either way, it pushes out bubbles.