Bosch Poor Workmanship

I took my car tho this company for an engine overhaul and paid them R15k for the job. i go t he car back and it is now worse than when i took in. I was given a six months guarantee on the workmanship. 1. there is a knock in the engine (car sounds more like a tractor than a car); 2. the temperature gauge moves up and down between 0 and 90 (this makes it difficult as i not sure if the car is overheating or not). 3. I have left countless messages for the technician who refuses to return my calls.

I just want Bosch to honour the promises they make on their adverti*****ts. If they cant sort out the problem to refund me so i can have the car repaired some place else.