1988 300sel w126

Need to replace the coolant on my w126.
Any advice on the best way to bleed the air from the coolant system.

When I replace the coolant on my 1979 300sd, I park the car on my inclined driveway, remove the coolant reservoir tank cap, turn on the heater and rev the engine.

Any advice is greatly appreciated

Hello - not sure if this will help but I read the following for the W126: The system is self-bleeding. The little hose that goes to the top of the radiator & over to the expansion reservoir is the way air gets to the highest point of the system.

IIRC the M103 has a plug (bolt head) or two on a horizontal surface at the front of the cylinder head by the stub hose from the thermostat. Leave a plug off while you fill and stop filling when coolant comes out of the hole. Refit the plug. That will reduce self-bleeding to almost nothing.

Sixto
98 E320s sedan and wagon

Have the front lifted up on stands or an incline.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

I noticed that there is a bolt that sits on top of the thermostat housing. I'm thinking that this is a bleed bolt.

Is there a bolt on the side of the engine block that can be removed to drain the rest of the coolant?

That should do it but it’ll take a long time for air to get past the jiggle valve in a closed thermostat. The plug I’m talking about is just inboard of the #1 injector. Looks like it takes a 17mm wrench.

IIRC all M10x and OM60x blocks have a coolant drain behind the starboard engine mount arm, It’s where Diesels have a block heater and turbo oil supply port. I suppose they’re generic bosses on gassers. Or do gassers have block heaters? The fitting is like a giant brake bleed screw that takes a typical garden hose so you can direct the effluent.

Sixto
98 E320s sedan and wagon

xaliscomex, the procedure you use for replacing coolant in your 1979 300SD should work fine. No real need to rev the engine. Simply let it idle for awhile to reach operating temperature so the thermostat opens and coolant flows freely through the engine, allowing any trapped air to escape.

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Fred Hoelzle

See post #2

Leave pressure cap - known as radiator cap to some off of the pressurized coolant tank. Start car after refilling with fresh coolant. Though it may not be necessary with this setup, I always turn on heater to max heat position and fan on low.
Let engine idle while it warms up. When temp gauge gets up to around 80C, blip the throttle a number of times. You might want to lay a rag over the opening of the pressurized coolant tank when you do this.
If you feel you have an air pocket, elevate the front of the car with a floor jack.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I always drill two tiny holes in thermostats to relieve air from system.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

I just top off with coolant every morning until it can take no more.

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95 E320 Cabriolet, 169K

This is one maintenance job that Mercedes made easy. The radiator drain valve has a nipple you can attach a hose to as does the block drain under the exhaust manifold, so you can do the job without spilling a drop.

The system should self-bleed, but may need a little help. I use a piece of old 3/4" heater hose with a few wraps of duct tape on one end so it will seal on the fill neck. Place it on the fill neck and blow multiple times. You should hear gurgling, which is fluid being forced into the block, and the level in the expansion tank should drop. Keep it near the full hot level.

Once you can't hear the gurgles, start the engine, and keep blowing. When the thermostat opens the level in the expansion tank should rapidly drop. Add as required. It may take a handful of heat cycles to bleed all the air, so watch the level and add as required until it stabilizes.

The capacity of my 190E 2.6 is about 12 quarts, so I start with 6 quarts of Zerex G-05 and top off with distilled water as required. Your 300E might have slightly greater capacity.

Duke