Rua1r1 is your leon a tdi? I have 1.9tdi and have cleaned my engine bay with the power hose on numerous occassions. If you left your car in for a full valet and detail including the engine bay thats exactly what theyd do!
Ive been fully autoglym valet trained and thats how they do it! If its really bad i use some diluted pure soap to soak it in first and in really bad spots some tar remover or tfr, then just aggitate with a few brushes and blast it all off, hot water is better but cold is fine. then you could you could use the likes of autoglym super sheen to spray on as it dries, the water helps it move in to all the parts around the engine bay and when dried leaves a nice glossy finish.
in the 2 years ive owned my car and over maybe 10 times doing this i have never had one
problem! nor have i had a
problem with any engine bays that ive done in valeting work or for friends and family!