Sorry I don't have any advice, but this certainly does seem familliar.

Step 1: Take your car in to have X fixed.
Step 2: X is fixed. Y breaks the next day.
Step 3: Mechanic says, "you need a new Y -- not my fault, just coincidence"

If I had a dollar for every time something broke the day after the car was in the shop and the guy claimed it was just a coincidence...

My theory is that, in fixing X, they accidentally nudged or loosened a hose/wire/clamp/connection. Instead of looking for the $2 mistake and fixing it, they tell you that you need a new engine.