I went for a drive this morning and the bad cap theory is starting to turn a few cogs in my head.....

I drove to my indy to pick up a part (That hadn't arrived yet ) {9 Miles backroads only}
Before I left there I popped the hood and noted that the hose was hard so I took the cap off and relieved the pressure.

Then I drove to Farmington {15 miles, some highway, max speed 65} I turned the engine off and let it sit for about 30 minutes....when I came back I popped the hood and checked the upper hose....it was soft. I took the cap off again (I had a 20 psi cap in the trunk that I wanted to try but it didn't fit)


Drove home again {11 miles, highway, max speed 75) This time on the highway the coolant light kicked on again for a minute or two. While writing this I have been letting the car sit again for another 30 minutes. Just checked the hose....it's starting to go soft again but I did notice that there is coolant around the bottom edge of the cap on top of the coolant tank so something came out.....



Another thing....assuming that it isn't a bad head/gasket but that there is still too much pressure....what could be the source? Apparantly air getting into the system would be the culprit so the question becomes how is the air getting in? I should mention that I do have one leaking injector and the front cover on my engine has been leaking oil since this happened and it was not sealed correctly.

OM603: Freak failure of timing cover. 1987 300D

Perhaps one of these two areas is letting too much air into an otherwise good system? (I probably have no idea what i'm talking about but if it helps to know I might as well post it)

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1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

Does not sound like combustion gas is leaking into the coolant. It does sound like you have a questionable cap. I'd go to Autozone and borrow the coolant system pressure tester and test your caps right in the store, also test a replacement cap that they sell, and buy one that fits your expansion tank and give it a try. The leaky injector and timing cover should have no effect on coolant system pressure.

As to the coolant level light, when it goes on, and you pull over immediately to check, what is the level in the expansion tank? The float switch is a magnet on a swing arm that acts on a sealed reed relay. There's nothing to clean. If the expansion tank level is not low, I'd replace the float switch. Don't buy a Meyle, or other "OEM" garbage float switch if you want a good one. Get a Mercedes unit. I bought one from Autoha@@@s and it failed within a year.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The light only comes on at highway speeds above 70 or so and even then only when i've accelerated. If I hold was steady speed though it will turn off. I pretty much always stick to OEM equipment (As I told a friend recently...Mercedes and aftermarket parts don't make good bedfellows) Interesting that you mentioned not cleaning the sensor....a few years ago the light came on and stayed on while the coolant was full...we pulled it, cleaned it and it was fine afterwards.....

Which brings up a question, how reputable are the caps that Autozone sells? The only thing I buy there are cleaning products (Meguiars)

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

I would run a non-OE such as Autozone cap as a temporary measure, but that's just me. Should meet the spec new, but I trust the OE cap to meet the spec for years.

__________________

Gone to the dark side

- Jeff

That is my thought as well. After I get the starter into the 280CE, right now i'm just waiting for the part. I'm going to head to the dealer and ask for a new cap. I'm going to give one of the lower pressure units a try first, probably the 120 to start off conservatively.


Also I just went out and checked the hose again, it's pretty soft now....which is really perplexing, one time it stays hard overnight and then the next day it softens up again within an hour? (Please let this is all be cap related) But again if I saw coolant next to the cap then the pressure was relieved but there still shouldn't be coolant coming out....right?

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

Your expansion tank coolant level maybe at the min mark, and when you accelerate, the sloshing cause the level to change momentarily and triggers the light. Maybe there is nothing wrong with the float switch? There really is nothing to clean other than the hinge that the arm swings on.

I am sure the Mercedes cap is a better made unit. But an aftermarket cap, as long as it fits properly, should work fine. It is not a very comples device! I do understand if you want to use a genuine Mercedes cap. My suggestion for the Autozone cap is as a diagnosis tool for the problems you are having. It certainly is a lot cheaper than a head gasket.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Does the 126 have an overflow tank in the fender like the 124? If so, the tank in the engine compartment is for expansion, if it exceeds the cap pressure it goes into the neck, down the hose, into the overflow tank in the fender. If that overflow hose is blocked for some reason, your cap will not vent properly.

__________________

Gone to the dark side

- Jeff

One expansion tank is on the left side only. Which hose are you talking about in this case, there are two. (One in the back and one in the front that goes to the radiator) Note this is an old picture...the expansion tank to radiator hose that you see zip tied is hanging loose and occasionally will find it's way on top of the support which makes closing the hood difficult (Which I always notice and subsequently move the hose back where it belongs) If your clogged hose theory is correct then I should probably check this one first as it might have been pinched a few times.








Haha sometimes at least I have several new ABS sensors . I have some testing equipment but not everything.

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

You don't have to own the test equipment. That's why I suggested Autozone. You take the 2 caps you have, screw it onto the tester, pump it up, look at the pressure gauge when it vents and that's the opening pressure. If it does not vent and pressure continue to build, the cap is crap. It takes a minute to test. If the cap is fine, I would clean the seat of the exp tank and check that the vent hose is clear. You can also put the vent hose into a soda bottle to see how much coolant is vented.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Sounds like it's worth a shot then. After I pick up the starter for this 280 i'll stop Autozone on the way back and find out whats what.

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

You can't expect the interface of the cap to the neck of the exp tank to be totally dry. If pressure is being vented via the cap, some wetness there is to be expected. Do you have a vent hose at the neck of the exp tank? Whatever coolant that is vented normally go through that hose.

I see you prefer trial and error by replacing parts rather than diagnose with test equipment?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The back hose leading from the filler neck.

__________________

Gone to the dark side

- Jeff

Are you three absolutely sure that a w126 car has an overflow catch bottle?
None of the cars I've looked at had one, and I know that my two w126 cars don't have one. If the cooling system burps beyond the main reservoir, the overflow hose just dumps it to the ground, routed either down into the shock tower hole, or down next to the starter, depending on where the hose has been draped.

For that matter, I don't remember seeing any recovery bottle in the one I parted out, or in the EPC for it, but I may have overlooked it in the EPC...

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...

Your SD was not equiped from the factory with an overflow tank, your SEC should have the overflow tank listed in my previous post and it should be located behind the fender.

my god four pages on this..

you just need to get a new cap.. if that doesnt work, test the oil and coolant.