That was a loan officer, not a guy at the credit union, and fun money is exactly what ruins budgets.

You really think a citric acid soak is gonna fix 187k miles of baked on soot and scale? That cooler is probably half plugged with hardened junk that acid barely touches, and the 15 degree drop you saw is just the thin layer that did come off. Give it 5 or 10 thousand miles and that white smoke will be back, except now the customer is out the money for your flush AND the $1,400 cooler he still needs. The reason shops just swap these is because the failure is usually inside the core where you can't see it, cracked or eroded, and no amount of soaking fixes cracked metal. When it comes back and he's stranded on the side of the road, guess who he's gonna blame.