De Graaf Under-Car

This is for the Branch in Moreletapark Pretoria - seeing that there is no real online presence I am using all available channels.I booked my car in (mazda 323). The car is old but nothing wrong with the car. It went for its default service with no problems which was to be expected. But I immediately picked up that the car feels like it was going to die when idling, irregular sound. Also when I drive very slow the car jerks. I explained this to them and the one person quickly made an adjustment a week later but this did not fix the problem. I then removed one replaced" spark plugs and put the "old" spark plug in its place and it fixed my problem. Funny thing is I explained this to them and thy wanted to book my car in again. I fixed it in the dark in 5 minutes