You’ll almost certainly cause the dashboard to present you with a `check engine’ warning light, Brett, as blocking off the Exhaust Gas Recirculation (EGR) valve is an intrinsic part of the vehicle’s emission controls. Having the vehicle go into limp-home mode is a very real possibility as well.
But perhaps more financially damaging would be the consequences of being caught driving the vehicle with a blocked-off EGR. The government takes a very dim view of anybody disabling any part of a car’s emission controls and the fines for doing so can be huge.
However, I know this is a real issue for many owners of modern turbo-diesels. The problem is that soot from the exhaust gas is mixed with oily fumes from the crankcase and then ingested by the engine as part of keeping emissions down. But the mixture of oil and soot can form a black gunge that can block the intake system. Really, though, blocking the EGR is a fairly clumsy way of getting around this. My advice would be to fit an oil separator which should keep the oily mist from the crankcase ventilation out of the equation.