After owning both gens I’ll throw my 2 cents in on this. Cutoff valves was first mistake, the car has finicky cooling system. It needs every inch of cooling hose and capacity available to not damage anything not seen to the naked eye, if you pull up the routing of the entire cooling system flow schematic no matter what that dccv is is full force dependent on if the engine is running, even when it is off it’s still being used by way of the secondary water pump. I would think with cutoff valves it sent a lot of fluid to the dccv and aux water pump to sit there stagnant with no where to go and as well as in the heater core increasing the rapid growth of coolant material bc it’s not flowing or getting hot it’s no chance to kill it.
Heater core prob needs a backflush, and a lot will disagree but I’ve found aluminum acid cleans the f out of the coolant system. I’ve done it on my LS’s as well as my 2016 MKZ 3.7L and I’m at 240k as a daily driver of 300 commuting miles a day most days. I’ve had no issues doing so except a cooling system that works no matter what even with an internal water pump.
I don’t recommend it I also don’t disapprove it take that with a grain of salt and decide what you think is best for your situation.
I would never recommend aftermarket eBay dccv, it’s rare you get one worth a shit, but I would recommend it as apposed to a shutoff valve on the system. Any day. You will change them a lot bc they don’t last long but it’s better than your current setup.
The plug also gets wet its in a bad location OEM. Take dccv out and clean the little motors gently it involves disassembly of dccv, that could bring it back to life. Clean water out of plug if it’s standing inside. Hooking an alligator clip to it with 12v battery will confirm operation before reinstallation.
If you have a gen 1 after you upgrade to electric fan that pump gone opens up a lot of room, I relocated my dccv up there which made it last longer bc it wasnt sitting under any kind of fluid droppage. This takes extra hoses but you seem to know basic car plumbing and it shouldn’t be an issue.
Careful back flushing core, it’s likely eaten thru it if it’s been not circulating which is probably ten times the cost of a dccv to fix that issue. I hope this helps and good luck with what you decide and keep us updated I’m curious how it turns out myself
