If the hydrocarbons were high then you can imagine the carbon build up not only in the engine but everywhere through out the engine and exhaust. Tbh I don't put much faith in fuel additive cleaners and having had the terraclean services done on two of my vehicles, I can say hand on heart it's worth the money. My Zed is decatted also, and after the terraclean services I got it exhaust gas analysed by my mechanic the day after. Very nearly zero hydrocarbons was the result. Many people claim it's placebo services, but they've never had it done to their car, I have, both a petrol and diesel and the difference is night and day. Well worth the £100 bucks.