It doesn't take a lot. I know of a 1994 V6 that was put into a garage for the water pump and timing belt to be changed.
Firstly the garage mis-timed the cam belt and secondly they refilled it with OAT (red) coolant.
The owner took it back to them because it was running rough, they re-timed the cam belt and away he went. Within a week or so both head gaskets had blown. Why? Because although the garage had drained the system some ethylene glycol remained. This reacted with the OAT and formed a gel.
The person who took the job on spent many hours just cleaning the gel out of various parts of the engine, cooling system, throttle body, cold start fast-idle valve etc without all the time he spent sorting the head gaskets.
Imagine it or not, it can and does happen! I'd be very glad to be proved wrong (and i hope i am) but all i'm saying is that it doesn't bode well if the drain taps won't issue fluid.
