Standard EA839 Coolant Problems Take 2 - Lessons Learned (Repost)
*Repost to Edit/Add additional pictures since r/Audi doesn't allow commenter to respond with pictures*
If you're DIY-ing your EA839 coolant system, attached are some photos of the VAG1274/8 Cooling System Test Adapter that doesn't get mentioned or referenced in any DIY video or discussion, but I'd highly suggest having access to it if you're utilizing any similar vacuum filler. Reference the notes on the pictures attached regarding tools needed to vacuum fill the system and the prescribed Audi manual coolant bleed routine after vacuum filling (Note - it will take roughly 25-30mins /~210°F engine oil for the radiator fans to turn on with the engine at idle). Plus, added a reference on how to 'trick' the coolant shutoff valve to run full open during all conditions.
Background - TLDR
As the title implies, I was affected by the standard EA839 coolant migration issues (outside warranty, like all owners) on my B9.5 SQ5 at around 55K. After doing a full replacement (minus vacuum line and air reservoir, caught early), with parts and tools from FCP was able to get it replaced. After vacuum filling the coolant with a Schwaben filler, I noted I had issues getting the filler seated properly with the supplied inserts on the reservoir and issues with trapped air in the coolant. It was to the point of having CELs and audible warnings randomly, only with WOT on hills getting "coolant temp too high" but would clear once the car was level; so I decided to go to the dealership to have them run the scan tool coolant system bleed routine.
Fast forward 3 months / 60K miles I noticed the replacement coolant shutoff valve had started prematurely failing (coolant pooling/drying on the top vacuum cell), so to prevent any further issues/migration I capped the N489 shutoff valve vacuum line to run the shutoff valve full open and ordered a new valve from FCP (to utilize their Lifetime Parts Warranty), which was a good stop-gap as I waited for a couple of months since it was only weeping not leaking. Then I went down the rabbit hole researching EA839 coolant bleeding procedures, and luckily FCP / other YouTubers at that time, since I replaced my system, put out good videos, but were glossing over coolant bleeding procedures. So after finding a copy of the VAG mx procedures, it specifically mentions a VAG1274/8 adapter tool to pressure test/fill the system, and I was able to source one through VW.snapon tools (they supply North America, register yourself as a dealer/mechanic to order direct or pay huge markups on other websites). This week, I finally got 'coolant low' warning lights and was continuously filling the reservoir, so it was time to replace the failing shutoff valve, vacuum fill the coolant with the Schwaben & VAG1274/8 adapter, and manually bleed according to the Audi workbook - so far so good. Hopefully won't have to do that again BUT now have it down with the proper tools/adapters.