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Tar like substance in STV?

newton5 on Mon May 23, 2011 10:47 PM User is offlineView users profile

Year: 1964
Make: Chevy
Model: Malibu
Engine Size: NA
Refrigerant Type: R12
Ambient Temp: NA
Pressure Low: 0
Pressure High: 0
Country of Origin: United States

A fellow on another forum (specific to his car) has been posting progress pics of his AC rebuild. He's doing a doing a nice job of it and I've been offering suggestions as I'm able to. The system was inoperative for quite some time but intact on the vehicle.

When he opened up the STV for rebuild, the diaphragm was covered with a black, tarry substance. The oil in the compressor still looked good when he drained it and there was none of this substance in the lines or TXV. Upon cleaning the old diaphragm, it was still intact. No holes, tears or signs of deterioration. (I can post links to the pics if it's ok to do so.)

Any thoughts on the source of the goo?
We're guessing that it's contaminated oil, but why only in the STV?

I've rebuilt a few STVs and have never seen this, so now I'm curious.
He also posted a link to a good article on rebuilding an STV. The one in that article had the same goo, but the diaphragm was known to be leaking.

I appreciate any thoughts,

Edited: Mon May 23, 2011 at 10:59 PM by newton5

JJM on Tue May 24, 2011 12:19 AM User is offline

My first impression was diaphragm deterioration, but the question is which diaphragm are you referring to that's good?

I assume you're referring to the diaphragm inside the body, not the external vacuum diaphragm. Does the external diaphragm (that rotates opposite the inlet end) hold vacuum? If not, that could be what's coming apart. Also check the the two valve cores.

Is the inlet screen clean? If so, reasonably good chance the contamination isn't coming from elsewhere in the system as the oil confirms, though that goo could be hose material too.

Either way, after 47 years, chances are good there has been some breakdown in the rubber somewhere in the STV - even if all "looks" good. I would just rebuild the STV, replace all hoses and seals, dryer, flush, and not give forensics another thought.


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