So, even though the radiator at that point is empty, I can start the engine and the system will suck coolant into the radiator from the expansion tank?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

nooooo.
don't start the car until all burping of the radiator/block is finished! make sure you have coolant exiting the vent line!
the expansion tank is connected to the bottom of the radiator with a 1" line, and to the top of the block with a 1/4" line. so air is pushed out of the block perfectly. just make sure you get coolant there.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

First, you definitely want to drain both the radiator (drain plug at bottom of radiator), and the engine block (drain plug-19 mm-on side of block forward of block heater...reach in with long extension bar. This will completely drain coolant (except for little in the heater core).

The great tip was this: To refill the coolant (after plugs are re-installed!), first disconnect the end of the large radiator/coolant hose at the top of the radiator, and fill as much 50/50 mix as it will take by pouring into the hose. Note that this allows you to fill engine block "behind" thermostat. Then reconnect the hose, and then fill as much as you can into the radiator by pouring into the expansion tank...this fills in "front" of the thermostat.

By doing this, you get coolant into the system on both sides of the thermostat, so you don't have the big air pocket, and the problem with running the car to over heating in order to get the thermostat to "release" and allow coolant to circulate.

I got this tip from an indy mercedes repair shop I happened upon while on the road today...I stopped to buy coolant, and we got to talking, etc....

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

This works fine on pretty much any MB engine, and will speed the fill process. Otherwise you have to fill slowly, with liquid getting to the top of the reservoir neck, to get liquid to flow through the vent line to the block. It's slow and painful. DO NOT start the car until you can feel liquid present in the upper radiator hose when you squeeze it! Turning the heater to "max" temp will turn on the auxiliary coolant pump, which will help circulate coolant through the heater core, even with the engine off (key must be on).

Here is the factory procedure - the location of the OM60x block drain is shown on the last page:
http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Maintenance/My81/2080.pdf

Ideally, you should drain block & radiator with a cold engine, then with the upper radiator hose and heater core inlet hose (near brake booster) disconnected, insert a garden hose (on low/medium flow) run through the radiator, block, and heater core with all drains open. This will flush ~99% of the old stuff out. Remove hose, wait for the water to stop flowing out the drains, then close everything up and re-fill with your preferred percentage of MB antifreesze or Zerex G-05. If you measured what drained out, you should be able to get that same amount (or within 1 quart of that amount) back in before re-starting the engine. Top off as needed after a test drive with heater running; best to wait until the next morning so you're adding to a cold engine. Watch the level for the next few days until it, uh, levels out.

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Check out my website photos, documents, and movies!

Dave, is it bad to drain a hot engine block?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I'm not sure that it's the draining that's the issue, but the filling part with cold coolant may be a "shock" to the warm components.

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting

Dave, John, et al....OK, let me see if I've got this. Please let me know if I've got anything wrong or am missing any steps.

1. Change when engine cold.

2. Put front wheels on ramps.

3. Remove cover beneath radiator.

4. Remove radiator and block drain plugs; collect and dispose of coolant.

5. Remove upper radiator hose.

6. Disconnect heater core inlet hose.

QUESTION: Does it matter where I do the disconnect?

7. Stick garden hose running medium force into radiator to flush.

8. Stick garden hose into engine to flush.

QUESTION: I’m unclear as to exactly what to do to flush heater core. It’s not like the garden hose can fit into that hose, right? And am a flushing toward engine or toward heater core?

9. Reconnect radiator and block plugs.

10. Reconnect heater core inlet hose.

11. Pour as much 50/50 mix as will fit into engine.

12. Reconnect upper radiator hose.

13. Fill radiator through expansion tank.

14. Turn key to get heater core circulating (but without turning engine on).

15. Start engine and check level, adding coolant as necessary.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Nate is correct. The problem with draining a hot engine is that you have to wait a long time for the block to cool off, before you can flush with the garden hose (which is typically about ~60°F water temp). With a cold block, there's no problem. I assume people know that you should never, ever, pour cool liquid into a hot (or even warm) engine, as it can cause serious damage. It's also no fun draining near-boiling liquid... safer to deal with it cold!

If you need to get your engine to cool down faster so you can work on it, turn the key on, set heater temp to max heat, and fan on high. If the aux coolant recirc pump is working, it will cycle coolant through the heater core and rapidly drop the engine temp. I do this for 5-10 mins at the coin-op car wash before pressure-washing the engine, for the same reason as stated above... cold water on hot engine (even externally) isn't a good idea. In cooler temps, you'd be amazed at how fast this trick will reduce engine temps. It's less effective when ambient temps are above 80-90°F, but it still helps. Don't run the heater/fan on max for more than 15-20 mins or so as it will rapidly drain the battery.