This just happened to me last night on my 3880. I switch from PK to MK ran a few prints that appeared to be ok, then switched back and my head exploded (mine, not the printers). My prints were terrible. I started getting very worried that the switching valve failed. The last time I tried using MK was probably 6 months or more. Fortunately, I found a few posts that mentioned doing a nozzle check/head cleaning. First head cleaning showed great improvement, a second one did the trick. I've had this printer for about 6 years and never had a problem with clogging. It was even powered off for about a year and had no problems. I love this printer and I'm very glad I did not have to get it repaired, or have to purchase a new one.
