In March I was towing my horsebox with one large horse and a very small pony when the car all of a sudden blew out black smoke and accelerated to 4,000 revs, the car was covered by warranty so went back to the garage, the garage said they did not have a clue what the problem was but put a fuel additive in the car.

On Sun 17th April when I started the car, the rev's were slightly higher than usual at 1,500 when usually tick over is just below 1,000.

When we were getting on the dual carriage way in third and fourth gear the car would have black smoke coming out of the exhaust, we were again towing a horsebox with a small pony in so well below its towing capacity.

We have been towing since we bought the car in late December and its happened twice in that time.

Do you have any idea what could be causing the problem?