Thanks for the info Madsceintist. I will have to take a look under the car. I doubt that there is anything at the back though -but I'll still check, just to be sure. Some purple blaster should clean things up nicely so I can observe the undercarriage from then on.
Somehow, I feel that my AFR is set to lean at throttle though, since HHO feels like it 'completes' the car under pressuring situations (like up steep hills). The engine also vibrates a good amount when I start it in the mornings without HHO. I checked a plug chart and found that my plug strap and center were a too clean or pure grey when I drive in traffic. If I drive more at say 40mph more often, it gets a much better colour -like in the diagrams which indicat very good burn and timing. I guess I'll have to investigate it further.
As always, the floor is open for ideas/suggestions.
Can't get enough knowledge.