They say now it could be the timing chain does any one have any idea how much a new will cost fitted

went to work fine apart from when sitting at the lights the revs went up an down between 0 an 2000 revs it wouldnt just sit on 1000 revs it was ok once i went then when i left work it made aloud sound like fans were about to jump out then it stop im getting mitsubishi looking at it

looks like the seals are gone no compression at all and the modded is a 2009 sportback

can anyone help me my pistons went on my ralliart(2009) plus it looks like mitsubishi heidelberg is trying to find a way to get out of the warranty i just would like to know a rough cost to get it fix an is it worth it to get it done i miss my ralliart