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Old 08-10-2010, 06:27 PM
Old School Mechanic
Join Date: Dec 2006
Location: Auburn California
Posts: 127
AC Oil to add

'90 560SEL. I fixed an small AC system leak and vacuumed the system down and am ready to recharge.
1. how much oil will the vacuum pull out or does it pull any?
2. is there a rule of thumb on how much oil to put in (or any) for a slow leak or a big leak like a ruptured hose?

I've looked all over and can't get a good answer?

Thanks big time.
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Old 08-10-2010, 11:04 PM
Registered User
Join Date: Mar 2007
Posts: 158
The vaccum pump will not remove any oil from the system. The existing vacuum can be used to pull in any additional oil that you may need to put into the system. Adding oil to an existing system is a dead reckoning situation. If the compressor was running normal with no noises 1 or 2 ounces would be OK. If the leak area had no visible oil traces, it may not need any. If the leak was on the low side, there may not be any oil visible. The high pressure side will usually leak oil along with the refigerant. Make sure the oil is compatible with the refrigerant. If the system was at atmosphere, a new dryer is a must. The oil could then be added to the dryer.
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Old 08-11-2010, 11:19 AM
Posts: n/a
TOPGUN gives a great explanation. All I can add is that in this situation with an unflushed system, adding oil even for a seasoned a/c pro is nothing but a guessing game.

If you didn't replace a component, then one or two ounces will probably be okay unless you had an oily mess where you repaired the leak, then your guessing game might need a little extra added.
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Old 08-11-2010, 12:33 PM
Gilly's Avatar
Registered User
Join Date: Mar 2001
Location: Evansville WI
Posts: 9,616
There will be oil removed in a charged or partially charged system. The oil is dispensed from the charging machine (be sure charging machine collector is empty before recovery!) and measured and a similar amount put back in. In the case of a rapid "decharging" MB has a list of quantities to be returned, and also quantities to be put back in for component replacements.
Much of the oil will stay in, especially in the compressor.
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