Hi to all,
I want to share with you my experience as Mechanic. I had same problem with my car. Renew some parts but the problem still the same. The spark plugs were black increasing fuel consumptions. So, I go further investigations and I found that the catalytic in the exhaust manifold is obstructed by carbon resulting to unstable idling.
I just took it out and let the manifold free. Its week now, my car run perfectly without hassle, good accelerations, idling stable and the spark plugs burning grey colour. Which indicate your engine run perfectly.
Hope this comment can solve some problems.
Jesh