It’s raining cats and dogs, here’s how to navigate flood waters

Cars on a flooded road
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Hi Gavin,

Keep up the good work! Now that it is raining heavily in many parts of the country, you need to write about the dangers of driving in a flood zone.

Anthony M


There are lots of added hazards when driving in wet weather, and floods can be the most dangerous, especially if the water is deep or flowing fast. If both those conditions apply in the same place, the first thing you need to do is stop at the shoreline. 

Floodwater can range from poor drainage on a city street to flash floods and rivers in the wilderness.  In town, the chances are you are on tarmac, therefore the water is most likely less than doorsill deep and relatively still. There will be plenty of other people and vehicles around, providing lots of visual clues.    

In this most common instance, the flood is likely to be safely crossable and the driving technique is to go dead slow, in low gear, and at high revs. Slow speed means you are less likely to inundate your own engine or douse anyone else in spray; low gear means you have the power to progress through the considerable resistance of deep water without stopping. Revs also ensure a powerful stream of exhaust gas to push the water out of the way of the tailpipe and help the engine deal with “back pressure”.   

With this technique, still water below the middle of the wheels should not be a problem. Most cars will plough through a short stretch of water almost twice that deep; a lot of power will be needed to push the water out of the way and the front end bodywork and radiator might create a bow wave that comes over the bonnet, but   there will be a cavity of air in the engine compartment, especially if there is a big sump guard. Over a distance of more than a few metres at that depth, the cavity will eventually inundate to near the level of the air intake and could submerge part of the radiator fan, which can suck itself forwards into the radiator fins.  

A water level higher than the floorpan will cause the vehicle to (at least partially) float, compromising traction for both power and steerage. A small degree of that is manageable on tarmac due to relatively good grip, in still water that won’t sweep a “weightless” vehicle sideways.   

So, in floodwater half-way up to your knee you can probably continue your journey…slowly, in low gear and at high revs…and with commitment. Just take your time, take care, and remember to test-press and dry out your brakes as soon as you are out of the pond. 

In moderate floodwater, the most likely hazard is getting the electrics wet (principally the distributor and HT leads to the distributor/plugs) causing the engine to stall.  Diesel engines do not need any electrics once they have started and are much more waterproof as long as their air intakes and fans are not submerged.  

If the floodwater is flowing at some speed (more than a gurgle) the depth is much more critical. The full weight of a car is usually able to resist quite a swift flow of water pressing against the side of its wheels (10 to 20cm) but if it is deep enough to reach the doorsills (and above) the bodywork acts like a sail and the sideways pressure is enormously increased. So, if in any doubt, just wait – the risks are higher and the consequences can be lethal.
On safari, the road surface under the water might be a lot less reliable – soft mud and/or large rocks might be on the menu; hidden washaway ditches are possible.  You need to judge that or test it on foot in relatively still or shallow water.   

If the water is quite shallow, some flow is not an immediate veto. If the water is deep and the flow is rapid, an immediate no-go decision is clear. In marginal cases and when progress is imperative, there are some judgment techniques using anchor ropes, or opening all the doors and entering the flow very gradually. This adds weight by flooding the footwells, and reduces the sideways push by letting more of the water flow through. I’ve only tried that in an old Suzuki LJ 50 in town. It does work, but while the passenger can crouch on the seat and only get wet shoes, the driver gets soaked to the waist. The primary consideration is how serious the consequences might be if it goes wrong. If your experience does not run to that assessment, don’t do it. 

Any “flash flood” flow can change, very rapidly, from a tidal wave to a trickle (and vice versa), and when in spate the water can be “loaded” with a battering ram of boulders and logs. 

If a well-beaten dirt track is flooded for a long distance like a canal, the safari maxim is to stay on it! Unless there is compelling evidence to the contrary, driving off the side onto apparently firmer ground could get you into very soft and deep mud. The fact that the road has retained water usually means it is well compacted. The absence of surface water on the sides could mean they are absorbent, and are now like porridge.    

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Why can’t mechanics fix my faulty fuel gauge?

Hi Gavin,

I love your motoring Q&A. Please help me diagnose my Toyota Belta 201. The fuel gauge doesn’t work.  Wiring mechanics have been unable to solve the problem. Sometimes the tank shows it is full, other times it indicates that it is empty even after fuelling.  Mechanics have even changed the dial, but nothing.  What could be the problem?


Get rid of one problem immediately: carry a 5-litre can of petrol in the boot. Motorists should constantly monitor their fuel consumption – always setting the trip odometer to zero when they fill up, and doing the Kilometres per litre sum when they refill. That way, you will always know roughly how much fuel is in the tank, even if the gauge misreads or fails. It will also alert you to other problems if your normal rate of consumption suddenly changes.

Dashboard gremlins often have a very simple cause (a broken wire, a loose connector, a sticky needle or a blown bulb). If you have already attended to those checks, remember that the dial only does what it is told.  By a Sensor. 

In your case, that lives at the opposite end of the car, in and on the fuel tank. Inside the tank is a float switch (a bit like a ball valve in a toilet cistern) which moves up and down with the fuel level. That might be broken or intermittently stuck. If the float is operating smoothly, then the sensor that interprets and sends information (not the dial that receives and relays it) is a likely suspect.        


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My Toyota Picnic is low on power, how do I solve this problem?

Hello Gavin, 

I have a Toyota Picnic 2000cc 1AZ-FE engine but it has low power when going uphill and also hesitates when accelerating. I have tried cleaning the throttle body, MAF and oxygen sensors without improvement. Also, fuel consumption is high, averaging 8kpl. I'm thinking of finding a suitable modern engine that can swap directly with the 1AZ-FE. Please let me know your thoughts and advise what to do. 


There are more than a dozen likely causes of power loss, hesitation and higher fuel consumption in any car. The underlying reason is that the engine is not getting the right quantity of fuel and/or air in the right proportions, and/or is unable to compress and ignite that mixture efficiently. That means a defect in at least one component in any part of the entire fuel, induction and ignition systems…or there is a defect in one or more of the sensors or mechanical or electrical controls which operate those components.

The theoretical list of potential culprits is long, and includes filters, pumps, pipes, tubes, injectors, plugs, piston rings, valves, the timing belt, gearbox clutches, the throttle body, the catalyser, the exhaust assembly, cables and linkages,  and, in most 21st Century cars, myriad microchip sensors. In many cases (and thus the first suspect) is that their problem is a build-up of carbon or other dirt in the wrong place.

The first thing you need is a diagnosis using a specialist computer to interrogate the sensors, and/or an expert mechanic on site who might be able to pick up more clues than your description offers, and use preliminary tests as a process of elimination. 

The first thing mechanics are likely to check (subconsciously, at least) is the age and mileage of the vehicle, so they know whether they are dealing with a troubled youth, a mid-life crisis or plain old-age. Then they’ll start looking and listening and smelling and touching. There is a good chance that a hands-on check will find the problem quickly and the remedy will be straightforward, or they will at least establish which system (fuel, electrics of mechanical) is the problem area.   If that “external” inspection does not reveal a compelling answer, they’ll have to start piece-by-piece testing and possible dismantling of the remaining (shorter) list of possibilities.   

With luck, you could quickly restore the car to full power and a fuel economy of 10kpl or more, at relatively low cost. After all, the symptoms you describe could be caused by nothing more serious than a blocked air or fuel filter or a squashed exhaust pipe. If, instead, the prospective remedy involves major expenditure, assess the merits and condition of the rest of the car. Is it time to change more than just the engine?

On engine overhauls, I would lean towards overhauling a known engine, or buying a brand new one, rather than taking a further risk on an unknown used engine. I say again, if the used engine is in perfect working order, why is it not still in the car it was born with? There is sometimes a good answer to that, but not often.

Do you have a motoring question? Email [email protected]