check fuel trims numbers generally over 11 means lean condition .Numbers in the -negative numbers indicates indicates rich running. Check codes  and See if the EGR valve might be stuck partially open, or the PCV elbow collapsed or leaking vacuum beneath the plenum.Power balance test on a DS 708 or maxi sys should  point out a non contributing cylinder, you could test compression and check the timing belt for correct valve timing (think it might have a chain though)

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