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  • Long-Term Report 2: 2018 GMC Sierra 2500HD Denali

Long-Term Report 2: 2018 GMC Sierra 2500HD Denali

Long Term Report 2 of 4

Jan 23, 2019
Photographers: Monica Gonderman, Brett T. Evans
We have thoroughly enjoyed having the 2018 Pickup Truck of the Year award–winning GMC Sierra 2500HD Denali as part of our test fleet for the past half year. Thus far, we’ve logged 15,000 miles, taken the truck on two cross-country road trips, and towed and hauled a wide variety of trailers and junk. There’s not been a task yet that the Sierra hasn’t been up to. It’s rare that we struggle to find at least a small thing that irritates us about a vehicle, but when it comes to the Sierra, we’re struggling to find anything to dislike.
Highway ride is, as one would expect from a ¾-ton pickup, firm but comfortable in most situations. Excessive highway potholes and large upheavals will unsettle the suspension; however, we’ve found that the Sierra handles these better than others in the ¾-ton class. Power is more than adequate, and more often than not, the Allison transmission will remain in Sixth gear when accelerating up a grade or to pass, thanks to the Duramax engine’s copious 910 lb-ft of torque. Sedan drivers often underestimate the big truck’s power and get schooled when trying to race us when merging into traffic. Without traction control, the truck would roast the rear tires when leaving a stop with anything above half throttle. It’s a good problem to have.
Fuel economy has been as expected from a ¾-ton diesel; for this report period, we averaged just over 16 mpg, thanks in part to a long highway road trip from our base of operations in Los Angeles to Minot, North Dakota, and back. On an uneventful jaunt from Las Vegas to Los Angeles, with cruise set at 70 mph (the posted speed limit), we managed 21.09 mph, even with a stiff headwind. We’re seeing around 13 mpg average in daily commuting duties.
Photo 2/18   |   We took a cold December road trip to North Dakota and despite temperatures averaging well below freezing the L5P Duramax never missed a beat and fired up quickly every time. We didn’t even plug in the engine block heater.
Living with diesel exhaust fluid (DEF) has been pretty uneventful. To see what happens when the Sierra’s 7.5-gallon tank runs dry, we proceeded to do just that. Since the government mandates use of the fluid, when the tank runs dry, manufacturers are required to disable the vehicle. The first indication of low fluid came at 4,863 miles and displayed a message saying “1,000 Miles Until Empty.” At 5,200 miles, 337 miles later, we received a warning that in “500 Miles Speed Will Be Limited.” At 5,720 miles, 857 miles from the first warning, we were limited to a top speed of 65 mph and given a new warning of “75 Miles Until Speed Limited to 55 MPH.” Then 78 miles later, the truck limited top speed to 55 mph and displayed a new warning of “75 Miles Until Speed Limited to 5 MPH.” At 5,877, speed was finally limited to just 5 mph, exactly 1,014 miles after the first low-fluid warning.
After being limited to just 5 mph, we filled the DEF tank with one 2.5-gallon tote and started the truck. The display showed “OK” again, despite the tank only being 33 percent full, and the speed limit was removed. After verifying this, we proceeded to completely fill the tank. We drove another 7,500 miles before receiving the low-fluid warning again.
Photo 3/18   |   Don’t worry, when we ran the Sierra out of exhaust fluid we strategically planned to be in a large parking lot and not on public roads. Being limited to 55 or 65 mph wasn’t much of an issue, but 5 mph wouldn’t be safe on the road. We highly recommend refilling DEF as soon as the first warning chimes.
The truck also required its first oil change and tire rotation at 7,792 miles (following the oil life meter’s guidance) at a cost of just $161 at our local GMC dealer. The truck is due for its second service soon, and we’re hopeful for an equally painless experience.
Thus far, the first half of our yearlong test has been fantastic, with the GMC Sierra 2500HD Denali impressing at every turn. We’re excited for the second half and loathe the day it’s going to leave us.
Photo 4/18   |   Anyone that has ever filled a GM DEF tank knows the struggles that come with the under-hood fill port. We were careful to not spill, however a system burp near full still gunked up the engine bay. Thankfully the next generation moves this to the more traditional spot near the fuel filler.

Report: 2 OF 4

Previous report(s): January/February 2019
Base price: $59,600
Price as tested: $73,730
Long-Term Numbers
Miles to date: 15,033
Miles since last report: 7,634
Average mpg (this report): 16.15
Test best tank (mpg): 21.09 (open highway, unloaded)
Test worst tank (mpg): 11.12 (towing 8,000 pounds up a steep grade)
Test Maintenance
Exhaust fluid refill at 6,000 miles.
Oil change at 7,792 miles.
Exhaust fluid refill at 14,000 miles.
Test Problem Areas
None
Logbook Quotes
“This truck is a road-trip machine! I love the 36-gallon fuel tank at nearly 600-mile range.”
“Remote start and the myGMC app are a blessing on cold mornings. A warm seat and steering wheel are definitely welcome.”
“Still getting spam calls to OnStar every time I drive the truck, and there’s no way to turn off incoming calls.”
“I wore out the 4GLTE Wi-Fi service since I love and use it so much. Thankfully, it’s only $20/month for more data.”
“DPF regen cycles seem to neuter the truck’s power pretty severely. It seemed to misfire when accelerating onto a freeway onramp. Thankfully, the cycles only last a few minutes.”

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