Ive rebuilt many of these closers, they can be real finicky.
First I would swap the closers between the machines as its pretty easy, and see if the problem follows.
Next, if possible, Id run the drain into the tank without the second pump pulling, just the main pump feeding and see if the problem goes away. Basically dump down the drain to a tank to prove that the restriction is on the drain side.
And your "system" pressure is in line? Not way high?
Is your gear pump hotter than the vane pump?
OK, this is a little more involved, but ... and I have done this for years....
You could dump your closer oil into a 5 gal bucket, run uptown and fetch a sump pump and put in there, and run a hose back to the tank.
On my big twin turret lathe I had done this for many years after blowing up the 3rd scavenge (gear) pumps.
Only recently have I abandoned this as the pumps that I was using seem to be getting less reliable, and oil goes all over the floor. I was using Little Giant.
My main spindle still drains directly into the tank as the pump assy that I currently have only has 1 gear pump, and only the sub is drawn with the pump - AND the T fitting to ambient so not to suck bad stuff.
Not saying that you should doo this long term, but you could try this and see how it effects your issue.
You should NOT have that much oil in your return line!
The frothy could just be the heat tho too. Like as it is squirting out through the hydro-static gap.
Even with a leaky closer, I don't see why there would be extra heat.
Don't know where this heat is comming from...
We'll try to swap the closers and see if the issue moves. If it does, it sounds like we have too much oil passing through. We already completely rebuilt it to the extent that we can here (o-rings and seal rings). If it is the closer, my understanding is that they have to go back to the OEM for rebuild (Kitagawa I think).
If the issue doesn't move with the closer, we'll try dumping right into the tank. These systems are not that complicated so I have to think the issue is in one of these places.
I'll report back when we've tried this stuff - still trying to keep other machines making parts in the meantime.
The collet closer appears to be the issue. We swapped the good one to the problem machine and right off the bat, the system pressure is 80 PSI higher and the drain hose flow looks like the other machine. We are going to let things warm up and run our test program but we may have found the problem. The closer didn't look worn or beat up when we replaced the seals, but it's a 25-30 year old machine so who knows.
At this point, I am confident it is the collet closer. We ran our test program this morning on repeat for 3 hours and no issue. Flow is good at the drain and the whole system runs much cooler. There must be too much oil getting through somewhere in the other closer. I'll look into getting this one rebuilt and if we do, I'll reply here in case anyone has this issue in the future. Thanks for the help everyone.