Angus242164
EGR on common rail diesel engines is essential to assist with fuel economy and keep the emissions down, but it's problematic on a lot of cars.
There is a lot of soot in the exhaust gases, which coats the valves and manifolds with black powder, it also mixes with crankcase vapours and forms a sticky black gunk that coats everything. It causes varying problems on different engines, from blocking MAP sensors to jamming swirl flaps to jamming EGR valves. On non common rail Nissan ZD30 engines it blocks up one section of the manifold more than the other, and causes some cylinders to run richer than others, sometimes causing cracked pistons. That is one engine where blocking off the EGR is a good idea.
Personally I believe it shouldn't be done unless you're having persistent issues, as it reduces fuel economy and increases emissions, and contrary to what some people will say, it doesn't increase power as the ECU keeps the EGR valve closed under high load situations anyway.
On my diesel Astra I remove the EGR valve and clean it out every third oil change, it takes about 20 mins and costs nothing, so it seems like cheap insurance to keep the valve moving freely. It's due for a timing belt in 30k so I might pull apart the intake manifold and clean it while it's apart.
EGR valves were often used on petrol engines from the late '70's to the early '00's but that has mostly gone out of fashion, these days exhaust gas is normally recirculated by having cams with more overlap, and variable cam timing, making external valves redundant on most petrol engines.
Diesels don't have variable cam timing so they still use EGR valves.
