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.

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.

:euro:

We're fussy around here about mix ratios because of our extreme cold. I do something that avoids having to "concentrate up" a full system... which is the waste of having to drain new over-diluted coolant from the car after doing a complete flush & coolant change to top up with pure coolant to get the desired ratio. This is pain when all you have is 50/50 premix, or if you've mixed all your coolant already to the same concentration, or you've run out of coolant and your system is not full yet!

Tip: Start the coolant fill process with a couple liters of straight coolant (non-mixed). Then start filling with mix. The pure coolant dilutes and mixes with the straight water that is sitting in the system.

It is less wasteful to "concentrate down" by adding straight water to an overly concentrated system. "Tuning" the ratio over a period of a couple days is easier by bleeding a bit out and topping up with water.


Other tips:

Check the RESULT ratio of coolant by sampling the overflow tank after a hot run or two. Don't rely on your bar tending mixology skills and jug juggling act to get the right ratio and never check it. Use an "antifreeze tester" (hydrometer) to be sure.


Never use 100% straight coolant.
Is that jug pre-mixed or not? READ IT CAREFULLY. Don't mistakenly dilute a 50/50 pre-mix to 25% coolant by blending with water.
Don't mix coolant brands or types, use MBenz only (or the Zetex).
Don't exceed 60% coolant to 40% water. The freeze point starts to rise again.

Scott has some good points there. If you know the actual system capacity, you can get a more accurate mix percentage by using the method he describes, rather than pre-mixing; especially if some water is left in the system. (I go a step further and blow through the heater core & radiator with compressed air to get even more water / old coolant out of the system, but this isn't practical for most folks).

One caution... the fill spec for the W124/M119 cars is incorrect; the FSM shows system capacity near 16 quarts, so you'd think that 2 gallons of a/f would make for a nice 50/50 mix. Not so. Actual capacity is approx 11-12 quarts, so about 1.5 gallons of a/f is needed for a 50/50 mix. I learned this the hard way. The specs for the diesels and 6-cyl gassers seem to be correct, btw.

Finally - when checking the freeze protection with a hydrometer, make sure the car has been running for at LEAST a half-hour (longer if possible) while driving (not idling) after reaching operating temp, with the heater on. It takes a surprisingly long time for the coolant to fully circulate and mix. Ideally you should drive the car for a few days before testing. This is of more concern to people who live in areas that get temps near or below zero F.


:balloon2:

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.


:batman:

FSM

http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Maintenance/My81/2080.pdf

shows capacity of 602 to be 9.5 litres. Is this correct?

AFAIK, yes, capacity is 10 quarts. If you don't need freeze protection to -34°F you could get away with 1 gallon of G-05 and fill the rest with water. The 40% mix is good to about -15°F. Although in the nor'east it's probably best to have the 50/50 mix... y'all can get pretty chilly up there.


:stuart:

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.

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.

:1eye:

If I flush everything with garden hose, what quantity of water would you guess is sitting there in system?

2 gallons

?? No, I mean how much residual water. Capacity of system is 9.5 liters in 602. Radiator is emty, block is empty, but there's still water sitting in the system.

I notice there are torque specs for radiator drain and block drain. Can I do these by feel (good-n-tight), like I do, for example, with the oil filter housing? Or do I really need to pay attention to the torque number?...which means getting another torque wrench.

I decided not to mention running the car with straight water after a flush, then re-flushing (again) before putting the new coolant in, as a "tip". Even though I do this.

The aluminum headed 60x are just too susceptible to heat damage, you have to be really careful not to overheat the car. Running straight water is a sufficient condition for overheating a car quickly, even if driven normally, and not hard.

During the flushing process, I run the car for a few minutes up and down the road on straight water. And when the needle climbs toward 100C I pull into the garage and finish the job. And careful that's BOILING HOT water coming out now. It's amazing what great heaters these engines make.

And torque by feel instead of wrench when twisting things on the radiator. This includes hose clamps and drain cocks. Plasty is just too cracky. Use a delicately firm hand with medium soft wrist and semi-rigid but relaxed fingertips and exhale as you do this (all told, about 11ft-lbs by my measure). When something goes CRACK! or snaps off when you're tightening on the rad, well, you over-torqued it. Get a new one.

And how 'bout the block drain? OK to do that by feel?

And I'm sure you've just started a flame war...the "inhale" crowd is about to pound you! ;)