Car is a '95 E300D, 124 Body (same as an E320). I have a relatively sloW AC leak, whose source I can't find either with a sniffer, or with a UV Dye. Logically I would have suspected the evaporator as the cause, as i know that its a common problem on these models. BUT, If the compressor is OFF, such as it was most of the winter, then the leak is rather very slow, In fact the compressor is still working after I filled it back about 4 month ago. I do however see lots of bubbles, so I am sure that I am missing some R134. probably still have about 1/2 capacity left. However if I run the compressor, then it will stop working after about 10 days, meaning that most of the R134 would be gone.

My question therefore is the following: With the compressor OFF wouldn't the pressure in the evaporator be much higher then when the compressor is ON. So if so, had the leak been from the evaporator, shouldn't I expect it to leak faster when the compressor is ON instead of OFF.

Am I right in my assumption? Based on what I described, can I therefore dismiss the evaporator as a possible cause of the leak?

Any thoughts?

Thanks

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Strike that. Reverse it.

I'd expect so too, but I don't know if this is the case. A crack could be sensitive to temperature, but then again, the evaporator is even colder in the dead of winter when parked.

You could have a leak at the compressor seal or the flexible high-pressure hose. Dye won't help for an evaporator leak, but it will indicate anywhere in the high-pressure side. I'd add some.

No. While it is counter-intuitive, the pressure in the evaporator is LOWER when the compressor is running than when it is not.

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2012 E350
2006 Callaway SC560

That's exactly my point. Since the pressure is lower in the evaporator when the compressor is running, I would then expect that with lower pressure in the evaporator, the leak would be slower. However it leaks much faster when the compressor is running, That's why I was thinking that its not the evaporator.

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'95 E300 Diesel, 264,000 Miles. [Sold it]

From your description I would expect that the leak is on the high side, but there is certainly no guarantee of that.

There is ONE good way to find such a leak, but you will have to spend lots of money or find a shop that is equipped to do it.

With the system empty, put in about 2 oz. of R22 and then pressurize the system with nitrogen. Then you can find the leak with an electronic leak detector. R22 is MUCH more easily detected by such a sniffer than is 12 or 134 plus the entire system is then at high pressure making the leak even more detectable.

Good luck,