Just finished replacing the evaporator in a '92 300D. Have a new receiver dryer to put in. Is it necessary to add oil to it if the system is going to be evacuated and recharged with R134 by my mechanic this week?

By opening up the system to replace the evaporator...not to mention the time it was run with it defective ..... all the literature would suggest that whatever oil IS in your system at this time has moisture mixed with it. We don't know how long that oil and water have been in a mixed state and so we don't know if they combined to make the acids which are so bad for the inside of the system.
When AC guys work on home systems after changes or burnouts they have kits to check the oil for those type things...
In our systems oil is both mixed with the refrigerant and circulates through the system...and POOLS in places.... it is very hard to even get all the moisture out of a pool of oil... and impossible to get out any possible acids which have formed..
So the accepted conservative ( read long life ) answer is that you need to flush your system.
Aside from that... any leak is assumed to have taken some oil with it.. so it was not even just the amount of oil in the R/D and the evaporator assuming you could measure those things accurately that may be missing..
So to have the right amount of clean oil in your system ... you need to flush the proper items , install new oil in the proper places, and install that new R/D and vacuum the system right before putting your refrigerant back in.
My MB books only go to 85.. so I am not going to try to be more specific.. have messed up assuming stuff in the past... perhaps you are lucky enough not to have the Delco compressor ? Perhaps your system is not a TEV ?--Greg



Just finished replacing the evaporator in a '92 300D. Have a new receiver dryer to put in. Is it necessary to add oil to it if the system is going to be evacuated and recharged with R134 by my mechanic this week?

Add 40 cc to the new evaporator and 10 cc to the new reciever/dryer.

Add 40 cc of oil if system suddenly lost refrigerant, such as in a line break. I assume you replaced evaporator because of a slow leak. If that's the case just the oil mentioned in first paragraph.

So what you're saying is to flush then evacuate the system, add the correct amount of oil, then charge correct? I assume there is no need to add oil to a new r/d if we flush the system? Had a week off from work. Changed out the ball joints on my '91 300D then did the evaporator on wife's '92 D. Wrapped up her interior at 7:30 this evening...glad it's done. It was a good week! Thanks.

Yes. No. It is not uncommon to add oil to the different parts of the system being replaced so that when you start up the system you have oil circulating back to the compressor faster than if you add it all at one place. You have a late model car so I don't know how much should be added into the actual R/D but someone will know that... Greg


"So what you're saying is to flush then evacuate the system, add the correct amount of oil, then charge correct? I assume there is no need to add oil to a new r/d if we flush the system? Had a week off from work."