Should I drain the block on my 300SDL or not?
 

A retired Mercedes mechanic suggested that I not remove the block drain when changing and flushing antifreeze as I was at risk to crack the block. Has anyone else herd of this or had this experience? Thanks, bob Smits

Never heard that one before.

I think it is a good idea to make sure all off the coolant is removed from the system before refilling.

Obviously make sure you don't cross thread or over tighten drain plugs.

I have had experienced MB mechanics tell me not to mess with the block drain....not necessarily because of a cracked block but just general unforeseen difficulties that could arise is removing a bolt that hasn't been touched in 30 years.

I have removed my block drain plug on my 240D and two Jeep Cherokees. I like to do this to get the crud out of there. But as stated before, a 15 to 30 year thread that is rusted in is a PIA to get out. I don't know about cracking the block. I would spray with PB Rust blaster or Kroil. Might aplly some heat. Not read hot, but a color change of blue, so 400F. A propane torch might get it.

I have experienced that if a block drain is locked in, then getting the engine to operating temperature and then attempting to loosen it makes for a successful job, you just need to loosen it slightly (half a thread) and then get out from under the car and then carefully loosen it all the way out.

2x on letting sleeping dogs lie. Flush with good quality water. The amount of water that gets left in the block is less than what's required for the correct coolant / water ratio. You know the total system capacity, so add straight coolant to start with. It's better to have a too strong mix than a too weak one.

-J

I drain mine at least once a year. We're required to run pure water (water wetter is allowed) at the track so if we puke coolant out of the engine it won't mess up the track (commercial antifreeze does not like to evaporate). This means that the block has to be drained every Fall as I don't heat the shop overnight or on the days I don't work in it so the block would freeze.

Anyhow, I'm sure the plug had never been removed prior to the first time I pulled it a couple of years ago (it's a 1985 engine). I've had zero issues. On the 617 the plug is hidden behind the turbo and is a PITA so I remoted mine and put a ball valve in a line leading away from the turbo area - I don't know what your engine is like.

Dan

when I flush a system I remove thermostat while flushing,and turn on heater.I use one gal vinegar,to clean rust and scale.Run in engine 15 minutes then drain and flush till I can't smell it.I fill system at thermostat hole,put stat back in last.aluminum heads I allow head to cool down before using cold water.

X2
My 300sdl block drain backed out with out any fanfare originally with 250k. Its located on the passenger side towards the rear.

If your thinking of using citric acid with the flush I hear its best to not run it through the heater core or one may end up with a leaky heater core.

I just looped a temporary section of heater hose at the block when doing the citrus acid flush. Then after several flushes with clear water I reconnected the heater core, set the heat on high, and flushed twice more.

Most importantly.... As the OP above suggests....don't put cold water in an empty hot motor. Your SDL head is aluminum and may crack with the intro of cold water.

Also you want to elevate the front end to allow the motor to burp any trapped air. I fill the motor through the top radiator hose...rotated 180* so the radiator end of the hose is up in the air and higher than the head. Reconnect the hose and run the motor for a minute and fill again.

It takes me almost an entire day to flush the system due to the need to have the motor cool down before refilling.

P.S. I also remove the thermostat and reconnect the housing. It is on the front passenger side of the block on a 603.961 so filling there is not probable. However there is a temp sensor / sending unit housing that looks like a T-stat housing on the front top of the head that could be a fill point.

The above poster makes a good point about the fill point having to be the highest point in the system. He details one good way - another is to take an old radiator cap and braze/weld on a standpipe and mount a funnel on top of that (I use a length of hose and hose clamps). Then the fill point is above everything else. I don't have to do that with Mutt the Race Truck as the Chevy radiator is the highest point but I have done this for other vehicles.

Dan

Thank You to everybody for the great advice. Bob Smits

I had no trouble removing the block drain in both my 300D's. I assume the designer's put it there for that reason. I loosened them while the block was hot after a drive, then waited until it cooled off. In my case, I was flushing and purging the blocks of water to switch to Evan's Waterless coolant. I don't recall much rust coming out, unlike my 60's U.S. engines where I had to loosen up the rust gunk to get them to flow.

But, I can see the concern. I tried to remove the head plug in one engine, to swap in the block heater from my failed engine, and I couldn't budge it. I bought a giant 17 mm allen wrench at ACE, used penetrant, a torch, long cheater bar, and hammer. The engine was out of the car, so plenty of room to swing. Finally gave up. Doesn't need the block heater yet and using one is a pain, so no loss. I was just amazed how tight that plug is on the engine.