I think I have isloated either a bad or plugged expansion valve.

Car (W124) has a history of intermittent AC (when it works, it is ice cold! and when it doesn't it is hot), so been barking up the wrong tree thinking my comrepssor was cutting out. Finally noticed that AC clutch always has the compressor engaged, so I got out the R12 manifold and took some readings:

-Static pressure on suction and discharge is around 125#. (ambient was around 100 degrees, so this is probably about right).

-Once the compressor engages I get a vacuum on the suction side! Discharge stays right at 125#.

So, after doing some reading on the internet, this looks like something is plugged, especially since I am seeing a vacuum on suction, and expansion valve is usually the culprit. So I start car back up and tap on the expansion valve and within seconds I start feeling ice cold air again (I had removed gauges, but it was pretty obvious what was going on).

So my thinking is that I either need to clean out or replace the expansion valve. Of course, this involves removing the refrigerant, valve work, flush, vacuum, and re-charge. I don't have all the equipment so will have to find a good AC shop.

Here are my questions:

-Why is this intermittent? Could the valve get plugged then unplug itself, then get plugged again, etc.

-Are there moving parts on the valve that can fail, or is it essentially just a restriction in the line?

-Since the cost of a new one is low, replacement seems the obvious choice. But if it is contamination, is there a chance that a flush could remove all the contaminants with necessitating removal/replacement of the expansion valve?

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K