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Diesel Electronic Water Traps: The Good and the Rest

Hi; just a note for anyone looking to protect their diesel vehicle with an electronic water trap.

We bought a second-hand desel ute in 2015. About 6 months after purchase, we had a common rail failure due to water, so we repaired it and fitted an (expensive) diesel water trap with electronic alarm, recommended by the repairer. Overall, cost a bomb.

About a year later, there was a repeat. The electronic alarm did not sound. Expensive repairs again; the manufacturer stumped up for a replacement alarm, but that was all.

Rinse and repeat: another year later and the alarm did sound while I was going around a corner, but by the time I had completed the turn and pulled off the road, the fuel system was stuffed again. However this time there was a difference: we had become dissatisfied with the original repairer due to unrelated poor workmanship, and had moved to a new one in whom I have a great deal of confidence. He advised fitting a new water trap of a different brand: Water Watch. This worked perfectly: the next time there was a problem, a further year on, it gave an alarm essentially instantly, in time to connect the alarm to an action of mine, and thus to diagnose and fix the underlying fuel system fault.

Bottom line: please investigate these devices carefully before fitting - especially, read the accompanying literature carefully. The biggest warning sign I can see is that the first device has disclaimers in the fine print: even the manufacturers don’t seem to trust it to quickly and reliably detect water. But you might prefer an alarm that does reliably detect water. Water Watch appear to stand behind their device, and in my experience, with good reason. There are probably other brands in the market; if there are, I don’t have any view on their reliability.

It would make a great topic for a Choice report some day…

What does that mean? Take it step by step.
What is a common rail and what does it do?
Why is it susceptible to water failure?
How does that happen and how can it be prevented?
What does the electronic alarm do?

Uh-oh, I’ve been pinged for more information, so here goes. I’ll do my best, but looking it up in wikipedia would probably be more reliable…

What is a common rail and what does it do?
It’s part of the fuel system of many modern diesel cars; if your car has one (i.e. if it affects you), you’ll probably know - the salesman probably made a big fuss about it. In a traditional diesel, the injector pump pumps diesel directly to the cylinders, with a mechanical distributor getting the high pressure fuel to the right cylinder at the right time; in a common rail diesel, the common rail acts as a store for the pressurised fuel, and the injector itself controls when the fuel flows to the particular cylinder. This allows higher pressures and much more finely controlled fuel injection, which increases power and gives better fuel economy, at the cost of substantially increased complexity and fragility.

Why is it susceptible to water failure?
The common rail operates at extremely high temperatures and pressures, at which the water in the fuel becomes corrosive and very rapidly attacks the metal of the common rail (if it gets through to the diesel engine, that’s not good either, and can be a serious problem with non-common-rail diesles, but the common rail generally fails and the car stops before serious engine damage occurs). It probably also takes out the - expensive - injectors. If there’s enough water to damage the common rail, it may also take out the high-pressure pump (which operates at much higher pressures than a traditional diesel injector pump). A traditional diesel engine can generally cope with the tiny amounts of water that get through the catch can (see below) before it fills up; a common rail often won’t.

How does that happen and how can it be prevented?
Any way that water can get into the fuel. Finding out how it’s getting there (whether in the purchased fuel or some other way) is the crucial question. That’s what the alarm helps with.

What does the electronic alarm do?
The alarm is part of a catch can, a physical device that separates the water from the fuel but can only hold so much water before the water flows on to the common rail and then the engine. That’s why it’s critical that you find out there is water in the catch can as soon as it appears. They were important devices even before common rail diesels, but the margin for error with a common ra.il is much smaller than for a traditional diesel. With traditional diesels, tractors and trucks etc, it was generally enough to check for water in the catch can every few days, an alarm was a nice to have, but generally wasn’t included.

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It might also require some in depth research and professional advice from one of the motoring organisations. Particularly relating to the frequency and nature of the water related fuel problems they come across in breakdowns and servicing. Additional advice might include their assessments of the quality of fuel protection and monitoring in modern vehicles. Should we be concerned?

A similar look at diesel fuel concerns, and some recommended management strategies.

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Thank you for going to the trouble I now understand more about it.

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