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Expansion Tank Cap Basic Questions
Just trying to figure out some things related to expansion tank caps.
1. If the inner seal of the cap is compromised/deteriorated, what is the effect? I'm assuming lower of system pressure and potentially loss of coolant but wanted to confirm. 2. If the cooling system has a recovery tank that is in good working order, will that prevent a bad expansion tank cap from resulting in a loss of coolant? 3. Is there a certain risk in replacing a bad cap on an older cooling system in that the increase of pressure might cause something else to fail?
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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles 06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU 91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion 19 Honda CR-V EX 61k mi Fourteen other MB's owned and sold 1961 Very Tolerant Wife |
#2
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Answers
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1. A pressure cap that doesn't seal tightly will prevent the system from pressurizing as much as the cap is supposed to allow. For the most part, the purpose of a pressurized cooling system is to prevent the coolant from boiling at normal operating temperatures, especially at high altitudes. Using antifreeze also increases the boiling point. If the coolant never gets over 100C you could probably operate without significant loss of coolant even with a damaged cap. 2. The recovery tank may prevent loss of coolant depending on how bad the pressure cap is. However, coolant return from the recovery tank is based upon vacuum in the cooling system and a damaged pressure cap may not allow a vacuum to form. 3. There's no way to know whether pressurizing a system that hasn't been pressurized in a long time will cause leaks. Try it and find out. Jeremy
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"Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#3
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I can't really explain why, but in every case where I've had a car with a bad pressure cap, it's also made the coolant temps run hotter. My best guess is localized boiling in the cylinder head slowing heat transfer, but I don't know if that makes any sense.
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1981 Mercedes 300TD, 1994 Honda Civic Del Sol http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png |
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for #3 i would say if you dont pay attention and put a 20psi cap on there you will find a weak link the system is supposed to operate at 14psi almost every single replacement cap i have found has been a 20-25psi not 14 like the car needs, I was having coolant leaks due to to high of pressure... now i have a 13psi cap and it appears all of my leaks may be gone, still watching... i also have lower temps as well not having leaks...
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85 mercedes 300D-T 258k miles |
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Quote:
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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles 06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU 91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion 19 Honda CR-V EX 61k mi Fourteen other MB's owned and sold 1961 Very Tolerant Wife |
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A pressure cap is used to prevent the system from being over pressurize. There are many parts that might be fail due to the over pressure.
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"Positive attitudes create a chain reaction of positive thoughts." - CurtMaxwall |
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