In message , Clive George writes
Highly likely, my next door neighbour asked me to have a look at his car because his washers weren't. After I'd replaced the pipework, flushed the bottle/pump out and ultrasonically cleaned the nozzles it all worked nicely, he claimed he'd been using it for years. Friend's wife also used it and had it sorted by the dealer as the car was under warranty (ouch). I suspect that pre-mixing it would work fine but they had just squeezed a bit into the washer bottle without mixing it so it accumulated and blocked the pipes.