Author Topic: The coolant expansion tank cap conundrum - 100 vs 120  (Read 147 times)

ptashek

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Folks, I need your collective brainpower :)

My 450SE came with the "100" coolant expansion tank cap from factory, including the warning sticker. I tried to source a replacement from MB during the resto, and was told they have been NLA for a number of years already, after being replaced with the "120" part number. I've re-used the old cap as it was still sealing OK, but it's deteriorating now to a point where I need to think of replacing it. So I've ordered a new cap, using the "100" part number, and was sent a "120" cap as expected.

The 100 is designed to open at 1bar / 14.5psi overpressure, while the 120 at 1.2bar / 17.4psi.
Reading this old thread it seems some people have 120s, some even 140s on their cars.

The question for the experts among you then: is it safe to use the 120 cap, instead of a 100?

I'm guessing in a well functioning cooling system (which mine is) it doesn't really matter, as the cap overpressure function only kicks in if the system overheats, or say the head gasket blows and dumps cylinder pressure into it.
1993 "Pearl Blue" W124 280TE
1979 "Icon Gold" W116 450SE (history, resto)

TJ 450

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Re: The coolant expansion tank cap conundrum - 100 vs 120
« Reply #1 on: 14 May 2020, 04:12 AM »
I can't say I've ever seen a 100kpa cap in person. My 6.9 is running a 120kpa cap and as you say a lot have the 140 which might be an issue.

I would be surprised if the system couldn't handle the extra 20kpa of the 120 cap.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

ptashek

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Re: The coolant expansion tank cap conundrum - 100 vs 120
« Reply #2 on: 16 May 2020, 04:54 AM »
Thanks Tim, that's what I'm thinking.


Purely for future reference, can I ask you folks to check what cap is on your car? 100, 120 or 140? Which engine?

Maybe there's some sense to it :)
1993 "Pearl Blue" W124 280TE
1979 "Icon Gold" W116 450SE (history, resto)

raueda1

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Re: The coolant expansion tank cap conundrum - 100 vs 120
« Reply #3 on: 16 May 2020, 07:16 PM »
Mine is 120.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

BCDC

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Re: The coolant expansion tank cap conundrum - 100 vs 120
« Reply #4 on: 16 May 2020, 10:49 PM »
Interesting thread this. I had a very experienced radiator mechanic, who has also owned W116 Mercedes replace the top tank on my car radiator that had pinhole leaks when I bought the car. He did a fabulous job. When were checking the system he mentioned that the original 120 cap was probably a bit high for my system. The hoses were rock hard with the pressure, so he recommended a .9 bar cap. Mine doesn't have an overflow tank, so excess hot coolant will just go to waste needing topping up occasionally. Thats just the way it is, but the car runs well with good temperature. Maybe the high pressure ones increase the boiling point slightly but also stress the hoses and clamps. The original ones were possibly high because if the cars were driven flat out, it may be helpful in some circumstances. 

BC
1978 280SE Cream with blue interior. With SLS

Randys01

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Re: The coolant expansion tank cap conundrum - 100 vs 120
« Reply #5 on: 16 May 2020, 11:18 PM »
The variability in pressure rating is more an issue of original ultimate vehicle destination eg Iran versus Nova Scotia....and allowing a bit of heat build up in those warmer climates before dropping its bundle. The only reason it is pressurised at all is to raise the boiling point. The additive over plain water raises it even further. A couple of pounds plus or minus is neglible. More modern cars seem to have even higher pressurisation.

Any W116 irrespective of engine spec and locale..with or without aircon etc etc should never ordinarily get beyond 190 degree F indicated......altho the system is designed not to boil before 220/230 we should never be in this territory.
In good condition the reality is simply this...you can run it with no cap.!
The coolant will only expand and overflow/dribble out and you will have an air gap in the header tank next morning.
So relax on cap value...it's all good.