I’ve been pulling a vacuum on a 410 system and the compressor keeps grounding out (it’s not a burnout, I tested it for acid). The oil in the vacuum pump comes out black and has a foul odor. I can get the vacuum below 500 microns, but do not want to start the system until the oil in the vacuum pump is clear. I’m using a Yellow Jacket 11 cfm pump and Yellow Jacket oil with Yellow Jacket black hoses. Please advise.

Try changing the oil and run the pump isolated from the system for a while to see if the color and odor reappear. If it doesn’t reappear, then the problem is with the system. If it does reappear then the inside surfaces of the pump may need to be cleaned.