Just food for thought. The inline fours (at least JPN types) are designed to run two different flow patterns based on water pump flow rate. At low rpm they typically flow end to end through the head. At highway speed this transitions into a spiral type flow. That is the reason for the arced side passages in the head gaskets. If you can get access to earlier Camry New Car Features training manuals, you can see the diagrams in the engine section. I think it was '92 or '97 when they first showed us.
Once we saw that, we began to understand why replacement head gaskets for older vehicles were suddenly being redesigned!
Once this characteristic was discovered, a lot of development went into headgasket passage/water port arrangements. Without this planning, cold weather engines would have emissions problems at idle and a few other problems. Water pump position was also optimized, and thermostats moved to the inlet side to accomodate the improvements, which also helped minimize hysterisis in coolant temp.
I suggest caution when trying to change flow geometry. I would be tempted to run electric water pump and stay in the spiral/rotary flow rate area. I use a Dedenbear Sprint Car water pump (remote type) on my motorcycle engines. These engines also exhibit the "odd" coolant passages through the head gasket.
Perhaps this will give you some ideas on what to investigate next. Often, when I was struggling with some supposed complaint, that I couldnt find, my friend Nels would walk over to my stall, look at it, and say, "Jim, if it aint broke, you cant fix it!"
Regards, JimL
