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

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83 300 D turbo 297K runs great. SOLD!
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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
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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, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

A note on coolant disposal. Very few places will accept used coolant; check and see if your county has a hazardous materials collection facility. Sometimes this is at the local dump, other times it's elsewhere. I generally save up 15+ gallons and take it all at once. If you live in a city with sewage systems (not a septic tank), you may be able to pour it down the toilet, and the waste water treatment plant can process it. Check with your local municipality. But please don't dump it on the ground, it will contaminate ground water.


Not really, but there is only one place that is easily accessible, where the hose goes through the outer (engine side) firewall approximately next to the brake booster.


Flush towards the heater core. This pushes water through the heater core and should get most, if not all, the old coolant out of the core. If you don't do this, it will mix with the new stuff (unless you want to flush the hard way: drain, refill, run to operating temp, drain, refill, repeat ad nauseum).

The garden hose is about the same size as the hose or pipe you're flushing into, just hold the garden hose up to the pipe and wrap your hand around it. You just need most of the water flowing into the pipe to do the job, it doesn't have to be a perfect seal. I also put the garden hose onto the rubber hose (or pipe) that goes to the engine. The diesels are slightly different than the V8 gassers (forget about 6-cyl gassers) so you may be holding the garden hose against either an aluminum pipe or a heater hose, depending on the model.



A step between 14 and 15: Do not start engine until you are CERTAIN there is liquid in the upper radiator hose, indicating the system is as full as you can possibly get it when cold. Another trick I use is to squeeze the hose as flat as possible to "burp" some air out the reservoir, just be careful not to push coolant out the reservoir top.


I really hate cooling system flushes, as they're time consuming and difficult to do properly. It's worse on the early M119 engines as those have two drain plugs on the block with no hose fittings and it makes a mess like you wouldn't believe. If your reservoir tank is nastly looking, this is a great time to replace it, as it contains a silica pack inside which helps control corrosion. If you change the reservoir, replace the level sensor at the same time (trust me). And unless the thermostat is recent (less than ~5 years old), it's a good idea to swap that as well with a new OE/OEM Behr/Wahler. When you're done, you shouldn't have to touch the cooling system again for 4-5 years.

I do wonder if, in my case (switching from the green stuff), it might not be a bad idea to get the system full of water, run it that way for a bit, then drain it again. It's a clean, well-maintained system (thus I've been advised I don't need to do the citric acid flush), but I'm wondering if having all water in there for a while might help clean it out. Obviously, I'd wait till the cold weather is behind us.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

If you do that, do not drive for more than a day or two with only water in the system. You'd be amazed at how quickly rust will start to form when there is no corrosion inhibitor present. People who run water-only in their system (i.e., due to racing regulations) have to use a separate corrosion inhibitor, such as Red Line Water Wetter.

If you were comparing to a drain & fill (no "flush" via garden hose), then it would definitely be advisable to run for a day or two and repeat the drain & fill process (maybe a couple of times) to get the old stuff out. However, if you can do the garden hose procedure, through the heater core and with the block drain open too, that should be adequate.

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.