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It's a pity we have to have EGR valves - a diesel likes to run hot, where it will give a complete burn and produce hardly any soot. Hot combustion causes nitrous oxides to form, though, and that is thought of as a nasty pollutant, so the EGR valve allows exhaust gasses back to the inlet to richen the mixture and reduce the combustion temperature, but this reduced temperature won't burn off the soot and soon the EGR valve becomes blocked. When the Powers That Be declared that diesels should be fitted with a particulate filter, Ford and other manufacturers used 'Eolys Fluid' in a seperate tank with a device to squirt a measured amount into the fuel tank at each fill-up. The fluid contains something (cerium oxide) which makes diesel soot burn off at a lower temperature, so there's less soot to block the particulate filter, and less soot to block the EGR valve. This makes it tempting, but more expensive on each fill-up, to add something similar to Eolys Fluid (Millers Diesel Eco-Max, Diesel Rhino, etc.) to the fuel of an older diesel.
My 1.4Tdci tends to give a P0404 'EGR performance' fault on a hot day, but clears a few days later. A local mechanic tells me that other diesel owners report the same thing. A motorway run on a rainy day clears it out for months, showing that wet air can have a cleaning effect. Reading the diesel forums, it seems that American owners of big-capacity diesels like to use a water & alcohol injection system to boost power, but I have read comments that this keeps the turbo vanes and EGR valves sparkling clean.. Perhaps if diesel car manufacturers fitted a water-injection system, which, they say, reduces the combustion temperature, we wouldn't need to have an EGR valve to worry about!
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