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Q: Cap was not tighten correctly on the coolant tank, coolant leaked onto engine. on 1999 Porsche 911

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The mechanic did not tighten the cap on the coolant tank, hot coolant, steam came out, runs all over the place, and now my car runs really bad, it want start. Can you explain what happen?
Assuming you replaced the coolant before driving the car, I would assume that some of the coolant got into an electrical connector and the dirt that happened to be there, or was carried in by the coolant flowing over the engine parts, contained ionic particles. The ionic particles become suspended in the solution and then created a short circuit between electrical contacts. If the circuit is low impedance (high current like a power cable), the high current can zap away the short and evaporate the water. But if the circuit is high impedance (like a signal/sensor wire), the circuit might not be enough to clear the dendrite connection. Even after the water dries a permanent dendrite could be formed closing the two circuits. Depending what the circuit is, your engine could run rough as a result.

In many cases, allowing the engine and wiring to dry well will cure the problem, otherwise you may need to take the affected electrical connectors apart clean and/or apply some good contact lubricant. You need a non-conductive lubricant that will improve contact of connectors, repel moister, resist corrosion, seal and protect the connectors for the future so this wont happen again.

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