The exhaust gas recirculation valve on a modern diesel engine does exactly this. It diverts a portion of the exhaust product back into the intake manifold of th

The exhaust gas recirculation valve on a modern diesel engine does exactly this. It diverts a portion of the exhaust product back into the intake manifold of the engine to achieve more complete combustion and lower emissions. Yes, it does result in a buildup of goo in the intake. No, the engine does not run well in that state. That’s what maintenance is for. Clean out the goo every 50,000km or so and it’s fine. Modern diesels are very reliable machines, just like modern petrol engines. They’re certainly not the robust rocks they used to be, though. High pressure common rail fuel systems, precision injectors and multi stage turbos are standard issue these days and they all require regular maintenance, clean air and clean fuel to operate properly.