A driver of a 2020 truck with a 3.0L Duramax engine reported spending $7,150 on repairs, only to encounter a new issue the following day: highway fuel economy that wouldn’t exceed 20 MPG. The driver, posting as TomFritz on DuramaxForum, had been considering trading in the vehicle.
TomFritz described his truck, which had 123,000 miles, as running well after the repairs. He recalled achieving around 28 MPG before the work and reported approximately 15 MPG in city driving afterward. Having driven about 200 miles post-repair, it was too early to determine if this was a lasting change.
GM’s Bulletin and the Camshaft-Housing Problem
GM’s Bulletin 22-NA-027 identifies a camshaft-housing issue linked to oil consumption in certain trucks and SUVs equipped with the LM2 engine. Additionally, GM’s 2020 3.0-liter diesel supplement mentions reduced fuel economy and a different exhaust sound during normal particulate-filter cleaning.
TomFritz’s troubles began with a check-engine warning at around 84,000 miles, indicating a low input from a nitrogen-oxide (NOx) sensor. The sensor was replaced under warranty, but the issue resurfaced, which he attributed to oil fouling caused by faulty baffling in the camshaft housing.
The Costly Repair and Its Aftermath
The $7,150 repair included a replacement housing cover, NOx sensor, intercooler, PCV valve, and other emissions-related components. GM declined coverage as the truck had surpassed 100,000 miles. Despite the repairs, TomFritz noticed a concerning drop in fuel economy, prompting him to return to the shop and consider another visit.
GM’s bulletin details an inadequate weld on a baffle plate inside the camshaft-housing cover, affecting 2020 Chevrolet Silverado 1500 and GMC Sierra 1500 trucks with the LM2 engine. The recommended procedure involves replacing the cover and the No. 2 glow plug, followed by an oil-consumption evaluation.
TomFritz suspected that his truck was stuck in regeneration mode due to a deeper exhaust note and believed it wasn’t consuming diesel exhaust fluid (DEF). A diesel particulate filter’s regeneration process can affect fuel economy and exhaust sound, but a deeper note alone doesn’t indicate the duration of the cleaning cycle.
The impact of the fuel economy drop is significant. At 28 MPG, traveling 1,000 miles requires approximately 35.7 gallons of fuel, while at 20 MPG, the same distance consumes 50 gallons—a 40% increase. Using $4 per gallon as an example, this difference adds about $57 per 1,000 miles.
Further Diagnosis and Considerations
To confirm if the fuel economy issue persisted, TomFritz needed to track fuel consumption over several fill-ups, accounting for variables like speed, load, and weather. Comparing similar trips would help establish the authenticity of the decline.
Additional Factors in Fuel Economy and Repair Decisions
Diesel exhaust fluid (DEF) supports the selective catalytic reduction system, reducing nitrogen-oxide emissions by converting them into nitrogen and water vapor. It operates separately from the diesel fuel supply. GM’s manual describes the DEF display as a segmented bar graph, which may remain on the same segment as fluid is used. TomFritz did not specify how he assessed DEF consumption or whether regeneration was active during his fuel-economy complaint.
A newer Sierra 3.0-liter driver with about 12,000 miles towing a Winnebago found the truck suited his needs, maintaining detailed records of towing weights, fuel use, and regeneration data across extended trips.
