low coolant level
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replacing the water pump is the attempt of the first shop to fix my no heat issue. The coolant issue just go worse after the replacement. I was topping up almost every couple of days. I had the car back twice to the shop after. The first time they did the pressure test and found a leak in one of the pressure relief line and after the replacement it didnt get any better in terms of the coolant issue. Brought the car back again, they told me they performed another pressure test and the system was holding pressure with no issues for the whole day. Told me to keep monitoring it. After this visit, for whatever reason the issue gets better and like i mentioned i am topping it up every 350 miles or so.
i don't quite understand the last question. Is it supposed to hold pressure after it cools down during the pressure test?
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From: New Hampsha
'17 GLS450, '14 GLK250, Grandpa's Roadster
When you drive the car and the coolant heats up and expands, the system becomes pressurized. When it cools down the coolant "un-expands" and the pressure dissipates. If, after cooling, the system still has pressure, it means pressure is being introduced into the system beyond that due to expansion. A good head gasket leak can even result in the radiator hoses being rock hard the next morning.
Last edited by John CC; Mar 29, 2025 at 01:47 PM.
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When you drive the car and the coolant heats up and expands, the system becomes pressurized. When it cools down the coolant "un-expands" and the pressure dissipates. If, after cooling, the system still has pressure, it means pressure is being introduced into the system beyond that due to expansion. A good head gasket leak can even result in the radiator hoses being rock hard the next morning.
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From: unbegrenzt
2017 GLE350 4MATIC
You have an off-nominal, or rather abnormal, coolant loss problem.
You seem to have a strong view of what the problem is not. Is this wishful thinking on your behalf? And you seem to be unfamiliar with comprehensive diagnosis of coolant and engine sealing systems. The strong view and lack of familiarity are inconsistent. Do you agree?
An elementary coolant system pressure test indicates whether the cooling system holds pressure when the coolant reservoir, aka expansion tank, is pressurized. The tool is $30 and the test requires 20 minutes.
Combustion products test requires a $30-$40 tool and 20-30 minutes.
Compression test requires a $30-$40 tool and 30 minutes.
The three tests will tell you, or someone, what is going on. You have not comprehensively listed each test and its result, therefore you are like a unlatched door flapping in the breeze on this.
You seem to have a strong view of what the problem is not. Is this wishful thinking on your behalf? And you seem to be unfamiliar with comprehensive diagnosis of coolant and engine sealing systems. The strong view and lack of familiarity are inconsistent. Do you agree?
An elementary coolant system pressure test indicates whether the cooling system holds pressure when the coolant reservoir, aka expansion tank, is pressurized. The tool is $30 and the test requires 20 minutes.
Combustion products test requires a $30-$40 tool and 20-30 minutes.
Compression test requires a $30-$40 tool and 30 minutes.
The three tests will tell you, or someone, what is going on. You have not comprehensively listed each test and its result, therefore you are like a unlatched door flapping in the breeze on this.
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From: New Hampsha
'17 GLS450, '14 GLK250, Grandpa's Roadster
Thread Starter
Junior Member
Joined: Jan 2025
Posts: 38
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You have an off-nominal, or rather abnormal, coolant loss problem.
You seem to have a strong view of what the problem is not. Is this wishful thinking on your behalf? And you seem to be unfamiliar with comprehensive diagnosis of coolant and engine sealing systems. The strong view and lack of familiarity are inconsistent. Do you agree?
An elementary coolant system pressure test indicates whether the cooling system holds pressure when the coolant reservoir, aka expansion tank, is pressurized. The tool is $30 and the test requires 20 minutes.
Combustion products test requires a $30-$40 tool and 20-30 minutes.
Compression test requires a $30-$40 tool and 30 minutes.
The three tests will tell you, or someone, what is going on. You have not comprehensively listed each test and its result, therefore you are like a unlatched door flapping in the breeze on this.
You seem to have a strong view of what the problem is not. Is this wishful thinking on your behalf? And you seem to be unfamiliar with comprehensive diagnosis of coolant and engine sealing systems. The strong view and lack of familiarity are inconsistent. Do you agree?
An elementary coolant system pressure test indicates whether the cooling system holds pressure when the coolant reservoir, aka expansion tank, is pressurized. The tool is $30 and the test requires 20 minutes.
Combustion products test requires a $30-$40 tool and 20-30 minutes.
Compression test requires a $30-$40 tool and 30 minutes.
The three tests will tell you, or someone, what is going on. You have not comprehensively listed each test and its result, therefore you are like a unlatched door flapping in the breeze on this.
i am not sure what the last 2 tests you mention are about but the pressure test can at least eliminate the coolant system leak, right?
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From: unbegrenzt
2017 GLE350 4MATIC
Combustion products test detects exhaust (hydrocarbons) in coolant using a test fluid which changes color in the presence of hydrocarbons.
Compression test measures the peak pressure achieved in the combustion chamber.
All of these tools/tests can be purchased at Harbor Freight, or ordered online from a variety of sources. I am in the middle of this now with a 2018 BMW X3 with a hard-to-detect coolant leak, which is likely a failed head gasket.
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From: New Hampsha
'17 GLS450, '14 GLK250, Grandpa's Roadster
You can get the combustion gas test kit from most auto parts stores, Harbor Freight, etc.
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I am in canada so no access to harbor freight.
how does the kit from amazon look?
https://a.co/d/cIEpOao
so the test is to see if there is gas present in coolant. Is there any way that only coolant goes in to combustion chamber but not in reverse? I guess a leak is a leak and fluid has to go both ways.
how does the kit from amazon look?
https://a.co/d/cIEpOao
so the test is to see if there is gas present in coolant. Is there any way that only coolant goes in to combustion chamber but not in reverse? I guess a leak is a leak and fluid has to go both ways.
Last edited by nioero; Mar 30, 2025 at 11:14 AM.
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From: New Hampsha
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That test kit will work fine.
Last edited by John CC; Mar 30, 2025 at 11:36 AM.
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From: unbegrenzt
2017 GLE350 4MATIC
I am in canada so no access to harbor freight.
how does the kit from amazon look?
https://a.co/d/cIEpOao
so the test is to see if there is gas present in coolant. Is there any way that only coolant goes in to combustion chamber but not in reverse? I guess a leak is a leak and fluid has to go both ways.
how does the kit from amazon look?
https://a.co/d/cIEpOao
so the test is to see if there is gas present in coolant. Is there any way that only coolant goes in to combustion chamber but not in reverse? I guess a leak is a leak and fluid has to go both ways.
Cylinder head gasket failure should be done with multiple tests. It could be that the combustion gas test "passes" but there are other symptoms which "fail" and the gasket indeed is failed.
Do a compression test and cooling system pressure test. The three tests:
combustion gas test
cooling system pressure test
compression test
will give you a good basis to make a decision, in addition to your experience that you are in fact losing coolant.
Or skip the time and effort on the tests and buy a new car. Replacing a cylinder head gasket on an MB if you don't DIY will cost at least $5k, likely more, at an indy shop. After head gasket replacement you are on the same used "old" car trajectory with respect to future repair costs.
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Yes that kit is the same one I used, it is good.
Cylinder head gasket failure should be done with multiple tests. It could be that the combustion gas test "passes" but there are other symptoms which "fail" and the gasket indeed is failed.
Do a compression test and cooling system pressure test. The three tests:
combustion gas test
cooling system pressure test
compression test
will give you a good basis to make a decision, in addition to your experience that you are in fact losing coolant.
Or skip the time and effort on the tests and buy a new car. Replacing a cylinder head gasket on an MB if you don't DIY will cost at least $5k, likely more, at an indy shop. After head gasket replacement you are on the same used "old" car trajectory with respect to future repair costs.
Cylinder head gasket failure should be done with multiple tests. It could be that the combustion gas test "passes" but there are other symptoms which "fail" and the gasket indeed is failed.
Do a compression test and cooling system pressure test. The three tests:
combustion gas test
cooling system pressure test
compression test
will give you a good basis to make a decision, in addition to your experience that you are in fact losing coolant.
Or skip the time and effort on the tests and buy a new car. Replacing a cylinder head gasket on an MB if you don't DIY will cost at least $5k, likely more, at an indy shop. After head gasket replacement you are on the same used "old" car trajectory with respect to future repair costs.
or maybe i can just keep refilling the coolant or it will get worse over time.
regardless it still puzzles me that why all of a sudden it get worse after the water pump replacement and what the shop did to make it better during my last visit?
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From: unbegrenzt
2017 GLE350 4MATIC
i think the coolant pressure test and combustion gas test are fairly easy to do. I am not sure about compression test though. Does it involve removing spark plugs? I am not sure it is something i want to do myself.
or maybe i can just keep refilling the coolant or it will get worse over time.
regardless it still puzzles me that why all of a sudden it get worse after the water pump replacement and what the shop did to make it better during my last visit?
or maybe i can just keep refilling the coolant or it will get worse over time.
regardless it still puzzles me that why all of a sudden it get worse after the water pump replacement and what the shop did to make it better during my last visit?
- remove spark plugs
- remove fuel injector connectors
- install compression test gage with appropriate threaded adapter
- engage starter for 5 seconds, then release
- check compression on gage, note it
- release gage pressure, move to the next cylinder
- reinstall spark plugs and injector connectors
It's not too difficult.
Working through these tests will lead you to the root cause.
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Thread Starter
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MBWorld Fanatic!
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From: New Hampsha
'17 GLS450, '14 GLK250, Grandpa's Roadster
Pump it up to the same pressure as the pressure cap. It should hold at least half an hour. (Theoretically, forever.) Any significant change in pressure means something is leaking.
DO NOT run the engine with the pressure tester attached! It will not relieve excess pressure, and once it gets hot there's no safe way to release it.
DO NOT run the engine with the pressure tester attached! It will not relieve excess pressure, and once it gets hot there's no safe way to release it.
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Pump it up to the same pressure as the pressure cap. It should hold at least half an hour. (Theoretically, forever.) Any significant change in pressure means something is leaking.
DO NOT run the engine with the pressure tester attached! It will not relieve excess pressure, and once it gets hot there's no safe way to release it.
DO NOT run the engine with the pressure tester attached! It will not relieve excess pressure, and once it gets hot there's no safe way to release it.
as for combustion gas test, how long should i run the engine to get a good representation of sample? after driving for the day and when the engine cools down?
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so just an update on this:
so since my last top up in April, i drove over 1000 miles and to my surprise, as I opened the cap yesterday out of curiosity, the coolant level didn't drop at all. I expected to at least drop some based on my experience in winter. The only thing that I have done is basically hand tighten the cap as much as I can (I am afraid to damage the bottle if I tried harder). Another thought is that since weather got warmer, I didn't use the heat function at all for the last month or so. Not sure if that matters.
Things I have checked:
1. pressure test: pressure holding okay at 15psi after 30 mins.
2. Combustion gas test: no gas present in coolant.
I am not confident in myself to do a Compression test so that is not done.
so since my last top up in April, i drove over 1000 miles and to my surprise, as I opened the cap yesterday out of curiosity, the coolant level didn't drop at all. I expected to at least drop some based on my experience in winter. The only thing that I have done is basically hand tighten the cap as much as I can (I am afraid to damage the bottle if I tried harder). Another thought is that since weather got warmer, I didn't use the heat function at all for the last month or so. Not sure if that matters.
Things I have checked:
1. pressure test: pressure holding okay at 15psi after 30 mins.
2. Combustion gas test: no gas present in coolant.
I am not confident in myself to do a Compression test so that is not done.
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From: Aubrey, TX
GLS550, GLE450 Coupe
I had to replace the expansion tank cap (the main one, not the one for the turbos).
Mercedes Expansion Tank Cap - OEM Rein 2105010515 | FCP Euro
This is an OE part.
Mercedes Expansion Tank Cap - OEM Rein 2105010515 | FCP Euro
This is an OE part.
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I had to replace the expansion tank cap (the main one, not the one for the turbos).
Mercedes Expansion Tank Cap - OEM Rein 2105010515 | FCP Euro
This is an OE part.
Mercedes Expansion Tank Cap - OEM Rein 2105010515 | FCP Euro
This is an OE part.
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Another update on this:
as i fixed my heat issue in another thread by flushing the heater core and replacing the coolant reservoir, i had to top up three times with 24 oz each time after driving for 500 km. Almost exact same amount after the water pump was replaced last winter. Now the coolant stops going down.
this makes me think that the cap was not the issue neither. It could that there were air pocket in the system after each flush so it takes time for the air to get out.
as i fixed my heat issue in another thread by flushing the heater core and replacing the coolant reservoir, i had to top up three times with 24 oz each time after driving for 500 km. Almost exact same amount after the water pump was replaced last winter. Now the coolant stops going down.
this makes me think that the cap was not the issue neither. It could that there were air pocket in the system after each flush so it takes time for the air to get out.
