Refilling coolant - vacuum bleeding
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From: Emmett, ID, USA
2007 GL450
Refilling coolant - vacuum bleeding
I have noticed that refilling with coolant is unnerving: the motor seems to maintain pockets of air, and even with a burping funnel coolant will overflow, probably because of coolant boiling in certain pocket places. You can't leave the radiator cap off, or warm coolant will bubble up and out all over the place. That can't be healthy for the motor, right?
I noticed that WIS specifies vacuum bleeding and filling. The vacuum bleeder is cheap ($32), but requires a reasonable air compressor. I picked one up from Amazon and it seems to work well, despite missing fittings (I returned it).
Just thought I'd pass that along. Simply filling from the reservoir does not seem to work all that well.
I noticed that WIS specifies vacuum bleeding and filling. The vacuum bleeder is cheap ($32), but requires a reasonable air compressor. I picked one up from Amazon and it seems to work well, despite missing fittings (I returned it).
Just thought I'd pass that along. Simply filling from the reservoir does not seem to work all that well.
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From: Republic of Texas
'99 and '05 E55 AMG
Vacuum filling is the way to go but be advised an older vehicle's o-rings may be marginal due to age. I discovered this on my '05. Traced it to a heater valve o-ring that failed after pulling vacuum; ended up replacing the entire valve although replacing the o-rings would have been sufficient. Have a backup plan.
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2007 GL450
I've replaced the coolant reservoir, thermostat, and two idler pulleys, drained coolant in the radiator and re-filled with new coolant. Just the normal burping procedure. and then rechecked coolant level and refilled the reservoir after 1 day of driving. That was over one year ago and things are working fine.
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From: Emmett, ID, USA
2007 GL450
Vacuum filling is the way to go but be advised an older vehicle's o-rings may be marginal due to age. I discovered this on my '05. Traced it to a heater valve o-ring that failed after pulling vacuum; ended up replacing the entire valve although replacing the o-rings would have been sufficient. Have a backup plan.
The only question being, how close was that o-ring to failure anyway. It might be a good idea to trigger failure in the shop rather than on the road.
Last edited by eric_in_sd; Oct 9, 2023 at 08:24 AM.
Thread Starter
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From: Emmett, ID, USA
2007 GL450
I've replaced the coolant reservoir, thermostat, and two idler pulleys, drained coolant in the radiator and re-filled with new coolant. Just the normal burping procedure. and then rechecked coolant level and refilled the reservoir after 1 day of driving. That was over one year ago and things are working fine.
Running to warmup with the reservoir cap off seems to lead to overflow, presumably from steam expansion.
Maybe I have been overthinking it. idk.
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From: Emmett, ID, USA
2007 GL450
Running to full warmup and pressurization still burped out air, leading to a Top Up Coolant message, so I am a little skeptical of the benefits of vacuum purging. I don't see that it is a whole lot better than simply filling the expansion tank most of the way full while idling, then capping the tank, followed by running at a high idle for a few minutes to purge the air, until coolant temp does its customary temperature overshoot followed by stability.
The high idle seems to be necessary. On one of these go-rounds, I noticed the radiator fan kicking on. I checked the temp and it was above 220 - don't remember the exact number - and as soon as I revved the motor, the temp dropped back down. Conceivably that was the thermostat sticking, but I don't recall the fan going on full speed at idle warmup before.
It would be nice to have a known good conventional refill procedure. The vacuum apparatus is nifty, but it is a lot of work to set up, tear down, and clean up afterward.
The high idle seems to be necessary. On one of these go-rounds, I noticed the radiator fan kicking on. I checked the temp and it was above 220 - don't remember the exact number - and as soon as I revved the motor, the temp dropped back down. Conceivably that was the thermostat sticking, but I don't recall the fan going on full speed at idle warmup before.
It would be nice to have a known good conventional refill procedure. The vacuum apparatus is nifty, but it is a lot of work to set up, tear down, and clean up afterward.
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Mercedes gl450
It has seemed to me to be a struggle. You want to refill the reservoir with enough coolant that you don't run dry in first warmup, but not so much that it does not leave coolant expansion space during warmup.
Running to warmup with the reservoir cap off seems to lead to overflow, presumably from steam expansion.
Maybe I have been overthinking it. idk.
Running to warmup with the reservoir cap off seems to lead to overflow, presumably from steam expansion.
Maybe I have been overthinking it. idk.
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From: Albuquerque
'10 CL550, '12 GL550
Will revive this thread simply because the title could aid future searching and it's applicable to my current question.
My recently purchased GL550 has incorrect coolant (thanks Christian Brothers) and I'd like to get the correct blue G48 in there.
However, working at home there is no way I can access block drains and it looks pretty iffy even to access the radiator drain.
So, the question I've never seen answered anywhere (or even asked) ... will a vacuum refiller such as Airlift II serve to extract the current coolant? I note cryptic sentences stating there could be some residual that spits out when first creating the vacuum, but not sure this is equal to extracting, in this case, 12 liters.
Anybody?
My recently purchased GL550 has incorrect coolant (thanks Christian Brothers) and I'd like to get the correct blue G48 in there.
However, working at home there is no way I can access block drains and it looks pretty iffy even to access the radiator drain.
So, the question I've never seen answered anywhere (or even asked) ... will a vacuum refiller such as Airlift II serve to extract the current coolant? I note cryptic sentences stating there could be some residual that spits out when first creating the vacuum, but not sure this is equal to extracting, in this case, 12 liters.
Anybody?
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Thread Starter
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From: Emmett, ID, USA
2007 GL450
You're thinking to hook the vacuum bleeder up to the radiator drain, right? I bet you could. Hook up the exhaust fitting to a hose going into a jug. Make it secure; the output will be a mess! It will come out as a spray, mixed with lots of air.
But first I'd make absolutely sure the coolant is not compatible. Coolant is tricky chemically, and some are cross-compatible and some aren't.
But first I'd make absolutely sure the coolant is not compatible. Coolant is tricky chemically, and some are cross-compatible and some aren't.
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From: Seattle, WA
SL500 & GL450
Also worth adding since it never seems to get added to any instructions, remember to turn your heat on whilst refilling coolant.Turning on the heat opens the heater control valve allowing free flow of coolant through the heater core and avoids any undesired air pockets. Also, the GL isn't an excessively pressurized system, so if you get massive overflow when removing the cap from the overflow container, the there could be a slight blockage in the system. In my experience, MANY folks have failed secondary water pumps and sometimes this causes a restriction of flow. The secondary pump is located behind the drivers side headlight and is responsible for circulation of coolant after the vehicle is turned off and allows the "REST" function to operate when the engine is off. The pump is only about $35 and is easily replaced. You can check operation simply by placing your hand on the pump and feeling for vibration confirming operation.
Last edited by Aussiesuede; May 24, 2026 at 07:00 PM.
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From: Albuquerque
'10 CL550, '12 GL550
Plan to have heater valve open by selecting maximum heat and since I think it's electrical, leave ignition on during the process so it stays open. I suppose air conditioning switched off, too.
Perhaps I should refill twice, once with just water to dilute out any remaining incorrect coolant, and then the second time new coolant plus water. Distilled, of course. I have no idea what's in there currently except that it's not blue, so perhaps the "rinse cycle" might clean it up enough to avoid any compatibility issues. Book says it's 12 liters, so I can measure what I get out.
Thanks for your ideas.
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From: Kelowna, BC
08 GL320 CDI
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From: Emmett, ID, USA
2007 GL450
The vacuum device fits into the overflow bottle and draws a vacuum in the system from there ... my thought is in that process it will also draw out the coolant. Yes, a messy air/coolant spray.
Plan to have heater valve open by selecting maximum heat and since I think it's electrical, leave ignition on during the process so it stays open. I suppose air conditioning switched off, too.
Perhaps I should refill twice, once with just water to dilute out any remaining incorrect coolant, and then the second time new coolant plus water. Distilled, of course. I have no idea what's in there currently except that it's not blue, so perhaps the "rinse cycle" might clean it up enough to avoid any compatibility issues. Book says it's 12 liters, so I can measure what I get out.
Thanks for your ideas.
Plan to have heater valve open by selecting maximum heat and since I think it's electrical, leave ignition on during the process so it stays open. I suppose air conditioning switched off, too.
Perhaps I should refill twice, once with just water to dilute out any remaining incorrect coolant, and then the second time new coolant plus water. Distilled, of course. I have no idea what's in there currently except that it's not blue, so perhaps the "rinse cycle" might clean it up enough to avoid any compatibility issues. Book says it's 12 liters, so I can measure what I get out.
Thanks for your ideas.
The very first time I changed the coolant, I hooked up a shop vac to the petcock. (I didn't realize a vinyl tube would fit neatly over the output.)
Since then, I scrounged a Blue DEF jug, which holds almost the entire drain output, and fits under the front at ride height. If I were smart I'd be on the lookout for another one, because switching to another jug when the first one starts overflowing is a pain. People sometimes discard them at
This is not a DEF jug:
I've done coolant drains in public driveways without issue.
You should do your own research, but: My impression is that no coolants are cross-incompatible. Thus rinsing the system is unnecessary.
For what it's worth, my coolant is gold. Zerex G-05. And my 450 motor holds 10 quarts.
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From: CA
11 ML550, 95 Supra TT, 14 Audi S4
I got a vacuum bleeder for Christmas last year and put it for use the other day on another car (95 Supra). It does not suck out residual coolant - I still had a few quarts in the engine since the drain plug on the block is very difficult to access with turbos in place. However, while it quickly pulled a vacuum, collapsed all the hoses and sucked new coolant back in from the bucket once the valve was opened, it didn't do a very good job getting rid of air pockets in the system, which I thought was the point. The radiator was full, but after a test drive, it quickly became apparent the car was low on coolant and I had to add another 2.5-3 quarts while bleeding it the old fashioned way. Could be user error, or the no-name from Amazon vacuum bleeder, but I was less than impressed on attempt #1.
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From: Emmett, ID, USA
2007 GL450
I dislike using a vacuum bleeder on low pressure systems. Hydraulic systems can handle the vacuum but when you have hoses collapsing - that's not healthy (even when pulling a vacuum of, if I recall correctly, just 1/3 bar). So I remain unclear as to what a good bleeding method is. Fill the tank, run the motor until coolant starts to overflow the expansion tank, stop and let cool? It is aggravating.
Incidentally, I think the idea was to put the vacuum on the expansion tank, and open the petcock to allow air in so that air bubbles push the coolant up and out the vacuum bleeder. What a mess!
Incidentally, I think the idea was to put the vacuum on the expansion tank, and open the petcock to allow air in so that air bubbles push the coolant up and out the vacuum bleeder. What a mess!
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From: Emmett, ID, USA
2007 GL450
Over at BenzWorld, user WallyP suggested using the heater recirculation pump to clear air bubbles from the heater core: Fill the coolant tank, and leave the cap off. Turn the ignition switch on, but do not start the engine. You should hear a whine from the area between the tank and the headlight - this is a small electric coolant pump in the one inch (approx) line from the forward bottom of the tank. Let this pump run for some time, and you will probably see air bubbles coming up into the bottom of the tank through this line as the pump pushes coolant through the engine and both heater systems. It might help to squeeze the large upper radiator hose to push more air through the system. https://www.benzworld.org/posts/19055902/
I got my 3 gallon green can. I like that it has a spigot; I'm too awkward to manage an open catch pan.
I got my 3 gallon green can. I like that it has a spigot; I'm too awkward to manage an open catch pan.
