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The engine light in my 2011 Mitsubishi Challenger comes on, The workshop has replaced the EGR valve but the engine light still comes on, Now I'm told they need to remove the inlet manifold to clean out the carbon, and when you switch the engine off it runs for a few seconds.
This definitely does sound like a build-up of carbon and soot inside the intake system of the engine. Believe it or not, this is not an uncommon problem and occurs partly because of the emissions controls forced upon engine manufacturers.
The diesel engine in your car features an EGR (Exhaust Gas Recirculation) system which means the engine inhales a portion of its exhaust so that it goes through the engine twice and is burned more completely. That's good for emissions, but it means that exhaust soot is being consumed by the engine via the intake system.
Combine that with the oily fumes from the crankcase ventilation system (which also feeds into the intake) and you can soon wind up with a black, sticky, sooty glug that blocks the internal airways much as cholesterol blocks human blood vessels. In some cases, the only way to fix this is to remove the intake system and manually clean it out. This can be time consuming and expensive, but could easily be why your check-engine light is being triggered.
The other bads news is that this could happen again over time. The solution to that is to fit a quality air-oil separator (also known as a catch-can) which keeps the oil fumes out of the equation.
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