As medium-duty truck engine technology advances, more complicated repairs lead to more unscheduled downtime, according to the J.D. Power and Associates 2012 U.S. Medium-Duty Truck Engine and Transmission Study.

The study, now in its fifth year, measures customer perceptions of 2011 model-year Class 5, 6 and 7 gasoline- and diesel-powered engines and provides manufacturers with a comprehensive and objective measure of customer satisfaction and product quality.

Eight attributes are measured to determine overall engine satisfaction: engine reliability and dependability; ease of access for service or maintenance; maintaining speeds on grades; acceleration when fully loaded; control module (ECM); vibration at idle; engine warranty; and average fuel economy.

Overall, the number of engine and fuel problems has decreased to 40 problems per 100 trucks (PP100), down by 11 PP100 from 2011. While the number of problems has decreased, the average length of unscheduled downtime due to these problems is 13.4 days, an increase of 2.7 days from 2011.

"As engine manufacturers continue to make advances in technology, there will likely be fewer problems," said Brent Gruber, director of the commercial vehicle practice at J.D. Power and Associates. "However, the trade-off is that when problems occur, they will likely be more complicated and require more downtime to fix."

The study finds that overall satisfaction with medium-duty truck engines is 760 (on a 1,000-point scale), and increases to 778 when selective catalytic reduction technologies are used to meet emission requirements.

Satisfaction varies among the different classes of trucks, with an 11-point gap in satisfaction between the highest- and lowest-scoring classes. The greatest difference among the three truck classes is the number of reported PP100, with considerably fewer engine problems reported for Class 5 trucks, compared with Class 6 and Class 7 trucks.

Customers' perceptions of their truck engine's performance are the most important factor of overall truck satisfaction. The 2012 study finds that owners who rate their overall engine satisfaction 9 or 10 on a 10-point scale, compared with those who rate their overall engine satisfaction 8 or lower, are more than twice as likely to say they "definitely will" specify the same engine brand again when they're in the market for a new truck (50% vs. 19%, respectively) and repurchase the same vehicle brand (50% vs. 21%, respectively).

"The level of satisfaction directly influences customers' future buying behaviors. By exceeding customers' expectations, engine manufacturers are generating customer loyalty for both the engine and truck brands," said Gruber.

Hino engines rank highest in customer satisfaction for a fifth consecutive year with a score of 801 and performs particularly well in control module; ease of access for service or maintenance; engine reliability and dependability; average fuel economy; and engine warranty. Cummins (793) and Paccar (789) follow Hino in the rankings.

The 2012 U.S. Medium-Duty Truck Engine and Transmission Study is based on responses from 1,272 primary maintainers of one-year-old conventional cab medium-duty trucks. The study was fielded between June and July 2012.