Superheat swinging

[hvac17011]

I am having a problem with a txv where the superheat swings between 0-40. The charge looks ok. 300 head 100 suction 10 subcooling. Sight glass clear. This is a new txv old did the same. I took the bulb off suction line valve opens. I tried changing bulb position. I had at 4 oclock as this is a 1 5/8 line. This is on a mcquay 40 ton split. This actually a liquid injection to control superheat. I am just looking for what causes swinging. I can describe unit operation if need be.

[jayguy]

possibly too much oil...has the unit had a lot of compressor changes (or 1 compressor change by poor techs!)

the oil on the inside of the pipe acts as an insulator for the bulb until the over-feeding washes it off, then the txv finally senses the true suction superheat (0F) and then swings the other way...can you time it with your watch?

[hvac17011]

I just replaced this tandem set. I replaced valve because it wasnt acting right. The pin was very tight when i took apart. The bulb is on suction line before tandem set after accumulator. We have had oil return isuues. I actually thought about maybe the pipe had oil in it. I did ttry and move bulb to top of pipe.

[jayguy]

if you have too much oil in the system, where the bulb is won't matter. try removing some oil and watch the system. if you need to have that much oil in the system to keep oil in the compressor then you have another problem.

[karl k]

Is the suction and head pressure fairly constant?

[hvac17011]

The head pressure and suction pressure are constant. Jay oil return we believe is an issue. It is one of those deals we cant just start fixing due to manufacturer/warranty. They are tandem compressors left compressor is full of oil the other sight glass is at very bottom. They have an equalization tube so that dont make sense. My question is sounds like if supoerheat is winging probably system problem more than valve problem.