Why would it be "unwise" to drive a car after water has got inside the passenger compartment?

Question

The Guardian has a consumer piece on a hire company generally being awkward when one of their cars was caught in a flood. The final statement about driving it after the flood struck me as kind of the opposite of what I would have thought. Here is some context and the quote:

I collected a hire car from the Enterprise desk at Tarbes Lourdes airport last July, and drove it about 12 miles to my holiday accommodation in Bagnères-de-Bigorre. That evening there was a severe storm that caused flash floods across town. Water entered the hire car, which was parked on a street at the top end of town, and left a muddy residue. No damage was evident, and the vehicle was working perfectly when I drove back to the airport four days later. I showed the residue to the Enterprise staff, who thoroughly inspected the vehicle and drove it a short distance before assuring me that everything was OK.

[Enterprise tried to charge him for the writeoff excess but backed down saying] “We are confident that the extensive damage to the electrics was as a result of water getting into the vehicle. We also maintain our concern that it may have been unwise for the customer to drive the vehicle, knowing water had entered it. However, on this occasion, we will cancel the excess charge as a gesture of goodwill.”

In this circumstance, if I was driving my own car, I would have wanted to start the car and get it drying as soon as the water appeared to have drained away, specifically to prevent the electrics from corroding. The implication I get from the above is that it could be worse for the car to be driven than be left, or perhaps even be unsafe.

What sort of damage could driving do in this situation? What is the "wise" thing to do after your car has been in a flood?

Answer 1

If water had indeed entered the electrical wiring, then turning the ignition on would then apply power to the wiring. This would then cause electrolytic corrosion to start eating away at the electrical connectors and wires.

https://en.wikipedia.org/wiki/Galvanic_corrosion

If water had got into any ECUs, then again serious corrosion would occur internally.

It would be far better to disconnect the battery in a case like this and get everything dry before reconnecting power.

Answer 2

Depending on how deep the water was, it might also have gotten into the engine. Starting the engine causes the 4-stroke cycle, "suck, squeeze, bang, blow" to start.

The "squeeze" is the compression stroke of the cycle in which the fuel and air are compressed before the spark plug ignites the mixture for the "bang" part.

Water, though, doesn't "squeeze" and can break internal engine components like pistons and con rods, causing 1000s of $€£ in damage to the engine, perhaps even leading to the engine itself needing to be scrapped.

When my eldest decided to take his car on a joy ride through some flooded parts of a nearby town and got stuck, the first thing we did after towing, not driving, the car back home was to pull the coil packs and plugs, then crank the engine for a few moments to blow as much water out of the cylinders as possible. Fortunately, we didn't see any water or even mist blowing out, so he hadn't gotten that deep.

Answer 3

In addition to corrosion one of the main concerns is safety - wet electronics tends to go poof. Fuses are there for overload protection but that doesn't mean it will prevent damage to circuit boards, electric motors or other components when electricity runs through them, only that the fuse should blow before the car starts on fire.

It's possible that something vital may fry when you turn the ignition on, or with water sloshing around as you drive it could also go kaput while you're on the road, which could cause an accident or at least a lot of inconvenience. So, better all around to wait for it to dry.