There'll be no damage to the machine as long as it can drain a full tub of water within the allotted time ... and perhaps run the pump several seconds longer for good measure before spin begins.

Is it a Speed Queen toploader?

It's a GE toploader. I figure it's a fair assumption that flow control won't hurt the pump because you can always get a clog in the line - from lint, clothes, whatever. But yes, if there is too much flow control, the tub won't empty. The drain doesn't work by water level, but by time. If it tries to spin while there is still standing water in it, it could be grim.

I've had that problem for years with my newer Speed Queens. There's a another thread about this topic here, so if I find it I'll add it to my post. We've had the drain run a couple times, tried a rubber cap to hold the washer's drain pipe and retain back flow, but nothing seems to help. Maybe we need a bigger drain pipe or eliminate a bend in it. What has helped is using way less detergent, and I've found Kirkland to be the best I've tried so far.

Hmm. I wonder what the detergent has to do with it. Surely that doesn't do something funny like increasing the viscosity of the water.

What I'm going to do is to slow the flow a bit, and outfit the plastic drain hook-pipe with a seal that I squeeze into the PVC drain port. You'd like to believe you could buy such a fixture, that connects to your drain hose, and clamps into your drain pipe with a good seal.

Drain overflow occasionally occurred when we installed a new washer at the time (1980s) Whirlpool was transitioning from the old-style belt-drive design to their direct-drive design. Direct-drive drained at a faster flow rate. Some customers who were replacing old with new either had some restriction on the drain or the piping was smaller than the higher flow-rate required.

@DanInTheDirt, some washers nowadays with electronic controls monitor the water level sensor to provide a variable drain time, to a point. A fault code may occur if the drain rate is too slow.

You have not talked about the existing plumbing. Do you have a standpipe, and is the piping 2", and how does the pipe route to the outside? BTW, you must maintain an air gap between the washer discharge and the standpipe inlet.

Um, if there is an airgap between the washer discharge and the standpipe inlet, the water comes shooting out back through the air gap. That's the problem we're talking about here. This is a 2-inch drain line and it's about eight feet to the outside opening.

The problem began immediately upon installation of the referenced GE washer? Or it was initially OK with that machine and the problem developed later? What washer did you have previously?

I think the problem has always been there. Same washer. Many years ago I wrapped the exit port on the plastic drain line with foam and duct tape, and pushed it into the drain making a sorta-seal, but the seal never worked very well, and I've had leakage in the wall for a long time.

I'd like to figure out a good way to reduce the flow. I guess I could pinch the plastic drain line, but that's probably inviting breakage of that line. I could also put some kind of blockage in the line, but I'm concerned that might come loose and end up getting dumped out in the drain pipe.

I am completely aware of your problem and that you are trying to close the required air gap between a corrugated flexible hose and the standpipe as a jerry-rig solution. Good luck with that. Let me restate with a little more bluntness:

To restrict the flow of the washer's discharge pipe would be a bass-ackward solution. The pump is probably plastic, and allowing more backpressure on those parts than what they are designed for will invite failure. The timing as mentioned is a huge risk, are you going to test various levels of restriction to get the timing right?

If your plumbing piping is correctly installed, it should handle any typical washer's discharge which at the top end is around 21gpm. Depending on variations between UPC/IPC that includes in 2" PVC a standpipe 18"-30" long, a 2" Ptrap above the floor, and connected via a sanitary tee that utilizes a vent and then preferably connects as quickly as possible into a 3" main. Many jurisdictions still do not allow gray water discharge, so if this is a DIY setup you should be starting with looking at the plumbing.

That was my original question, as to whether flow restriction would damage the pump. It was suggested that it wouldn't.

My jurisdiction DOES allow gray water use in the yard, and actually encourages it. You need to be out of the aquifer recharge zone. I am. The output has to stay on my property. It does. It can't be used to irrigate root crops. It doesn't.

Yes, my plumbing SHOULD be able to handle the flow. It doesn't. Perhaps the P-trap or the steel beyond is corroded. That might be worth looking into. The timing isn't a "huge risk", in that once you decide that the timing is good, you keep it. It's a huge risk if you don't pay any attention to it. That was never suggested.

We put this hard rubber cap on, then waterproof tape to stop outflow from the little holes in it. Got it from a plumbing shop.

Soap bubbles seem to overflow as the water exits. I don’t think I was using too much detergent, but use even less now.

OK, I think I see the problem here.

The washer port on the drain line is about 4 feet off the ground behind the washer. The line runs down, under the floor, and over to the kitchen sink where it goes DOWN to the sewer, and up about 4 feet to the drain access port on the outside wall in back of the kitchen sink (and probably up beyond that to the standpipe). Now, what I didn't say is that the drain line behind the house to the sewer has been clogged. That was going to be a "someday I'll get to that" deal. So I just pipe the grey water out of the access port and it shoots out from there into the groundcover. The groundcover loves it. But the fact that the outside access port is 4 feet over grade means that when you try to shoot water through the system, there is a LOT of water sitting in the pipes that has to be pushed out. A big"U", four feet high and ten-ish feet long. Kinda like a giant P-trap. For low flow, that's easy to push. For a high flow rate, that's a lot of resistance. The access port is right at the kitchen, so from there (sink, dishwasher), there isn't much water to push on.

I didn't think this clog was a big deal in this context, because it's wide open at one end, and wide open at the other end.

Now, if the outside access port were down close to ground level, this would all work a treat. The line would drain naturally. Not sure why they put it up there.

So I'm going to get my sewer pipe cleaned out, and be responsible about root-kill after I do that. Sigh.