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2012 Mercedes CLS500 - mystery of the disappearing coolant
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PEagle
Novice
Sep 29, 2023, 8:07 AM
Post #1 of 17
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2012 Mercedes CLS500 - mystery of the disappearing coolant
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Hi,
I've been struggling to find the reason my CLS500 (M278, 4.6 litre twin turbo V8) is having its coolant disappearing for no apparent reason.
Car has 150 000 km (95 000 miles) and a very good maintenance history (had the car for not so long).
The coolant started disappearing as soon as I got it and I noticed recently, after investigating further, that the car doesn't need to run for the coolant to disappear. I may have added around 3 litres/0.75 gallons in 2000km/1200miles and around a couple of months.
Here are the findings so far :
- expansion tank sometimes seems to be below ambient pressure (air rushes in when opening)
- No pooling under the car
- No abundance of white smoke except for the occasional morning condensation
- Pressure tested at a garage and no pressure loss, no visible leak, no apparent level drop during the test
- Other garage found the same, tested for CO2 (head gasket leak) appears to be OK.
- No apparent level rising of the oil level, oil looks clean at the dipstick
- Coolant is clean even after use
I'm a bit at a loss on what to check for now and do not have a clue on where this coolant could be going, I hope you'll be able to point me in a direction that doesn't require to pull the car apart.
Cheers and thanks for reading.
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PEagle
Novice
Sep 30, 2023, 1:43 AM
Post #3 of 17
(710 views)
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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I know you feel you have done sufficient testing to eliminate all the possibilities but in reality you have not.
Finding slow, minor coolant loss can be very difficult.
The pressure testing has to go on for hours. While holding pressure for extended periods the car has to be in the air and examined very closely with a powerful flashlight. The heat of the engine can be burning off the coolant as fast as it leaks out.
I don't really have a way to do that and I'm not sure the mechanics are enclined to have a car take a lift for so long without me having to pay a massive sum... Any alternative way to check or find out if it's necessary?
Your chemical tests for heads gaskets are inconclusive at best. They cannot pick up small leaks and many engines have to be at operating temp for the problem to show up.
Is there another way to test it? (without disassembling the engine)
expansion tank sometimes seems to be below ambient pressure (air rushes in when opening) That's pretty much the way it works. Unless it was totally cooled down. A cooling engine pulls the expanded coolant back into the engine.
I'll try to do it from the same temp with the car not running in between.
Thanks for your insight, I'll see how I can find more answers on this issue. Fact is, it's not that slow a leak, I'll have to find a way to measure how much I lose over a period of time and if there's a max loss if I don't run the engine.
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Hammer Time
Ultimate Carjunky
/ Moderator
Sep 30, 2023, 4:33 AM
Post #4 of 17
(705 views)
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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I'm sorry if it seems like too much hassle but that's what we have to go through every day with problems like this.
If you are expecting a 10 minute solution, good luck with that.
Yes, there is one good, reliable way to test for a blown head gasket but you aren't going to like it.
Bring the system up to operating temp with the cap off and sniff the fumes over the coolant with an exhaust analyzer. That will tell you if there are any hydrocarbons mixed with the coolant.
Another way is to remove all the spark plugs and the pressure the cooling system for at least 30 minutes, then use a scope to see if any coolant got into any cylinders.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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PEagle
Novice
Sep 30, 2023, 6:39 AM
Post #5 of 17
(697 views)
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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I'm sorry if it seems like too much hassle but that's what we have to go through every day with problems like this.
If you are expecting a 10 minute solution, good luck with that.
I'm not, it's just that a solution that requires hours of pressurizing while on the lift would cost a ton as I do not have a lift home.
Yes, there is one good, reliable way to test for a blown head gasket but you aren't going to like it.
Bring the system up to operating temp with the cap off and sniff the fumes over the coolant with an exhaust analyzer. That will tell you if there are any hydrocarbons mixed with the coolant.
Another way is to remove all the spark plugs and the pressure the cooling system for at least 30 minutes, then use a scope to see if any coolant got into any cylinders.
I don't see why I would hate those, it's much better than the 25+ work hours to take the heads off. I just need to find a good mechanic to do all that now.
Would a compression test show a potential issue as well?
Thanks.
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PEagle
Novice
Sep 30, 2023, 8:49 AM
Post #7 of 17
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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Thanks for your help. I'll try to find a garage that accepts going for a long test and that has an exhaust analyser.
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Sidom
Veteran
/ Moderator
Oct 1, 2023, 9:38 AM
Post #8 of 17
(666 views)
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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With small coolant leaks, the 1st thing to do is, get under the car, take off all the splash shields and look around for evidence of a coolant leak. Usually there is a tiny white trail somewhere on the block, pan or even a hose connection. Some leaks only leak under pressure, so it only leaks going down the road, it won't leave a big puddle just sitting there with the engine off.
After that, you have to start looking at internal problems. With small leaks, chemical tests will never reval anything. I've had the best luck with a 100 psi pressure transducer and a scope, which most DIYer don't have available to them, but really the one thing that will really confirm it, is to just pull the plugs and see if one of the cylinders is way cleaner than the others. Water/coolant is one of the best carbon busters and cyls with gasket leaks are always cleaner than the other ones.
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PEagle
Novice
Oct 2, 2023, 9:40 AM
Post #9 of 17
(620 views)
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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With small coolant leaks, the 1st thing to do is, get under the car, take off all the splash shields and look around for evidence of a coolant leak. Usually there is a tiny white trail somewhere on the block, pan or even a hose connection. Some leaks only leak under pressure, so it only leaks going down the road, it won't leave a big puddle just sitting there with the engine off.
Car was inspected whilst under pressure and we found no white trail or leak. Though it was a small amount of time so could be not enough to get a visible leak.
It's doing it even if the car doesn't run so it's not hot run only.
After that, you have to start looking at internal problems. With small leaks, chemical tests will never reval anything. I've had the best luck with a 100 psi pressure transducer and a scope, which most DIYer don't have available to them, but really the one thing that will really confirm it, is to just pull the plugs and see if one of the cylinders is way cleaner than the others. Water/coolant is one of the best carbon busters and cyls with gasket leaks are always cleaner than the other ones.
I'll pull the plugs and do an endoscope inspection at the next garage provided they have one.
Also, I'm not sure I'd call that a small leak. Checked and lost 125ml (around 4 US ounces) over 24 hours.
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PEagle
Novice
Oct 2, 2023, 2:38 PM
Post #11 of 17
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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It's doing it even if the car doesn't run so it's not hot run only. That's not what is happening. It is merely drawing previously lost coolant back into the block as it cools.
Even several days after? Car hasn't run for 4 days and is still drinking coolant. I'd have expected coolant to have returned to the block in a day max (but my knowledge being limited doesn't help)
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Hammer Time
Ultimate Carjunky
/ Moderator
Oct 2, 2023, 2:46 PM
Post #12 of 17
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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If coolant were to leak while the car is parked it would be either hitting the ground or going into the engine and if it were going into the engine there would be other indications of that.
It's either burning the coolant or leaking it. There are no other options. If it is leaking from a cold engine, then the leak would be obvious.
If there is no white smoke out the back, no coolant in the oil, then it's not leaking when cold.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Hammer Time
Ultimate Carjunky
/ Moderator
Oct 3, 2023, 11:46 AM
Post #14 of 17
(545 views)
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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Yes, the milky oil would be very obvious.
Really, this is not rocket science. We deal with stuff like this every day. It's a slow leak and it can be found. You just have to keep it under pressure until it's found which shouldn't be that long.
Any reputable shop will know what to do and if they tell you they can't find the leak, they are just too lazy to put the effort into it.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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PEagle
Novice
Oct 3, 2023, 2:16 PM
Post #16 of 17
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Re: 2012 Mercedes CLS500 - mystery of the disappearing coolant
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Yes, the milky oil would be very obvious.
Really, this is not rocket science. We deal with stuff like this every day. It's a slow leak and it can be found. You just have to keep it under pressure until it's found which shouldn't be that long.
Any reputable shop will know what to do and if they tell you they can't find the leak, they are just too lazy to put the effort into it.
I will, I just need to find the proper shop. I'm not in the US and good shops here are very hard to find and often have their timetable filled to the brim but I'll persevere.
If you are letting the car sit, wait until the next day and then go over all hose connections with your finger. To see if you finger gets wet on any of them. If not, then after the car has sat for 24 hours, I would make sure the coolant level is at the correct level and then pinch off the heater hoses to see if that stops the leak. Then, systematically pinch off the rest of the hoses, including the reservoir supply hose..
If you have the reservoir blocked off and you are still losing coolant, then you have a bad tank.
It has been sitting for around 5 days without running, I'm having a hard time identifying and reaching to the connections and hoses (engine is rather cramped in there) but I'll make sure to try.
Wouldn't a bad tank leak be obvious?
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