Blair, vol.1, p. 423 has a short paragraph on this.

As I understand it, the problem was not the design of the diesel engines, but the way the type VII was used. It was designed for short patrols around the british Isles, but quickly was forced to carry out longer patrols in the middle/western Atlantic and engage in lengthy convoy chases. The Type VIIs only had two (2) diesel engines and they wore them out by pushing them to the limit.

By contrast, the U.S. Fleet Boats which were designed for long range patrols had four (4) diesel engines.

Even large, ocean going type IXs had only two diesel engines, albeit much larger. The sole exception was type IXD2 which had an additional pair of engines (500hp each) paired to electric generators.

Type VIIs had engines made both by MAN and GW. During the early stages of the conflict MAN engines mounted on type VIIA caused some troubles but this was resolved later and both engine manufacturers continued to produce diesel engines for this submarine family. The only other recorded source of trouble was, as I said in my other post, the turbocharger.

That's amazing. In all the accounts of US Fleet Boat repairs, the crew were usually insistent that the repairs to critical areas were only ever carried out or supervised by someone who was going on patrol with them.