FULL-SERVICE: 125,000-Mile GM Diesel Pickup Maintenance

Mobil Delvac helps keep maintenance costs down while extending drivetrain life; helpful service tips for all GM 2500/3500HD 4x4s used in severe-duty environments



By Bruce W. Smith

Preventive maintenance may not be high on the to-do list for the typical consumer or at the top of the annual business plan for companies that seldom use their pickups for anything more than to run errands.

Prosumers and companies where pickups are an integral part of the day-to-day work operation, however, look at PM as one of those key elements with a direct effect on the corporate bottom line.

Being proactive on the service side reduces out-of-service down time and minimizes or eliminates major drivetrain repairs.

In short, PM shortens the ROI for the pickups within the fleet.

A stringent service program also brings monetary benefits when it comes time to sell or trade-in those vehicles. That vehicle’s maintenance record is a business’ version of a CarFax.

We don’t have a detailed service record to show what maintenance our ’08 Mobil Delvac-sponsored GMC 3500HD has had during the 130,000 miles before we purchased it earlier this year.

To be safe, and to give next year’s Big Red sweepstakes winner the best shot at another 200,000 miles of trouble-free service, we are employing the basic philosophy of preventive maintenance: Spend a little now, save a lot later.


We pulled the Mobil Delvac Big Red into Barkley GMC, in Tuscaloosa, Alabama, for a full service from radiator to rear differential using a variety of Mobil Delvac products designed specifically for heavy-duty diesel pickup applications.

Barkley’s service techs are some of the best in the country. Rod Halsten, the master tech handling Big Red, has decades of diesel experience and knows the GMC Sierra pickups and Duramax diesels inside out.

Halsten’s corporate customers include coal mine, DOT, utility, railroad, landscaping and heavy construction fleets operating in the area in and around Tuscaloosa. So he’s seen it all when it comes to maintenance and repairs of pickups used in severe-service applications.

The tips and personal preferences he shared during Big Red’s rack time should be invaluable for keeping your own GM HDs in primo mechanical shape for years to come.


Mobil Delvac:


Barkley GMC:

barkleygmc.com; 877-827-1716


As innocuous as an air filter seems, it plays a critical role in LMM Duramax performance. Halsten says to stick with GM’s filter because the Mass Air Flow (MAF) sensor, which is a key to fuel delivery parameters, is extremely sensitive to changes in air flow – higher or lower. GM designed the honeycomb-like filter specifically for this application.

GM “filter minder” is a good tool for monitoring air filter flow.

Some aftermarket air filters (and cold-air intake systems) flow more air, but it’s one of those instances where more air flow isn’t always better.

“It takes an extremely small change in air mass to make big changes in the Duramax’s fuel delivery,” says Halsten as he shows me the sensor.

“I’ve seen some aftermarket filters and cold-air intakes trip P0101 codes because they create vortices in the intake tube,” says Halsten. “Those variances cause the MAF sensor to signal the ECM (Engine Control Module) there’s an airflow problem, putting the Duramax into the ‘limp home’ mode.”

Halsten advises fleet mechanics to let the “filter minder” attached to the side of the air intake tube be their filter-change guide. “If your goal is to maximize engine performance, replace the filter if you see any red showing on the gauge,” says Halsten.

“If you just want to optimize filter life, wait until the red indicator gets into that 80-percent area.”


Have your service tech run a flow test on the DPF filter if you have a newer Duramax. Test will show if the filter needs cleaning/replacing.

Contrary to what most performance-minded Duramax owners believe, the EGR doesn’t affect horsepower, and it plays an integral role in maximizing both engine life and fuel economy.

(Blocking and/or removing the EGR is also a federal violation most companies can’t afford to get caught breaking.)

Halsten says the Duramax EGR only operates at idle and cruise where it allows “inert gases” into the cylinders so the engine can run leaner for optimum mpg while keeping the combustion chamber cooler and reducing NOx.

“The EGR is automatically turned off under heavier throttle load and WOT to maximize power. “It’s during long idle periods the EGR manifold will collect oily soot,” explains Halsten.

“Extreme soot buildup can cause the EGR valve to stick and affect idle quality. But the EGR system doesn’t affect engine power because that’s all bypassed when the ECM senses 8 to 10 pounds boost or WOT.”

Halsten says to replace the EGR valve when it throws a P0401 (EGR Flow Insufficient) code, which usually doesn’t happen until after 100,000 miles. If the EGR valve needs to replaced, it’s probably time to clean the EGR cooler.

“A clogged EGR cooler will trip both EGR and MAF sensor codes, affecting performance. So you should remove the EGR cooler when the valve is replaced and give it a thorough cleaning,” advises Halsten.”The process takes a couple hours to do it right.”

Downstream is the DPF filter, which Halsten advises should be checked for flow restriction when the EGR is replaced and the cooler cleaned. “Most GM service techs can use their diagnostic equipment to check flow in and out of the DPF without touching it.”

If flow is below acceptable levels, cut the DPF out and have it cleaned by a specialty service such as FSX (fsxinc.com) or DPF Regeneration (dpfregeneration.com), which is 3 to 4 times cheaper than having it replaced with a new one.

Engine Oil

Halsten follows GM protocol on oil service intervals at Barkley. But his personal preference is oil/filter changes be dictated by the pickup’s use or from oil analysis that Mobil offers fleets.

Pouring in the good stuff. Our Duramax gets a fill of Mobil Delvac Super1300 synthetic.

He suggests the Mobil Delvac Super 1300 10W-40 synthetic engine oil used in our Duramax should be changed around 4,500 or 5,000 miles in lighter duty use.

That interval lowers to between 3,500 and 4,000 miles if we use Big Red in severe-service such as towing, parked idling for long periods, or if it sees a lot of off-road or stop-and-go driving.

“The biggest common problem I see related to oil/filter changes is over-tightening the oil filter,” says Halsten. “You can do more damage by over-tightening the filter than stretching the oil-change interval; it only needs to be tightened 1 to 1 1/4 turns after gasket contact.”

Here’s another tip: Always fill the new filter with engine oil before putting it on so the critical areas of the engine have oil immediately.

Don’t over-tighten the oil filter. It took a BIG pair of channel locks and all Rod’s weight to break Big Red’s filter free.

“Don’t worry about oil spilling out when you tip the filter sideways to put on – there’s a check valve that prevents that from happening,” assures the seasoned GM tech.


GM recommends the trans fluid and external filter be changed when the HD’s oil monitor says – or every 25,000 miles for HDs used in severe service (towing, driven in hilly or mountainous terrain, for delivery service or in heavy city traffic where the temps regularly reach 90 degrees.)

Light-duty use the service interval is 50K.

Make sure the magnet on top of the remote trans filter is cleaned and moved to the new filter. It’s easy to overlook.

Halsten’s personal preference is contractors and those in the construction trades should use the fluid color as a good change indicator – not the odometer or hour meter:

“The trans fluid should be reddish and not have any hint of a burned smell,” says Halsten.

“If it’s reddish and smells like ATF fluid, it’s probably good to the next scheduled filter change interval. But if it’s off-color or smells burnt, flush the transmission system, change the internal filter and spin-on filter, and replace the fluid.”

“Make sure the magnet on top of the filter is cleaned and replaced on the new one,” Halsten says.

”I’ve seen some owners miss it.” (Any obvious bits or chunks of metal on the magnet would signal some serious internal problems brewing.) And don’t over-tighten the filter.

Our fluid looks and smells ok and there’s no metal on the magnet, so we’ll wait until our BD Diesel high-capacity trans pan arrives in a few weeks before having Halsten change the internal filter and re-fill Big Red’s transmission with Mobil Delvac Synthetic ATF.

Service tip: Use brake cleaner to clean any oil drips or spills during the service. This way it’s easy to spot new leaks – and shows a professional did the service work.

Transfer Case

GM recommends the transfer case fluid be changed out every 50,000 miles for severe service applications.

Halsten agrees, but notes GM requires the use of ACDelco Auto-Trak II fluid (bluish color) in the transfer case instead of Dexron VI ATF, which is reddish in color, or low-viscosity gear oil as some HD owners have tried.

So Halsten refilled Big Red’s transfer case accordingly and moved on down his 125,000-mile check list.


The white, step-shoulder filter tucked under the plumbing on the rear passenger side of the Duramax is probably the most important filter under the hood.

The water separation/fuel filter is a pain to change, but vital to saving you thousands in injector repairs.

The water separator/fuel filter is the only item keeping water from reaching the injectors, which would mean a costly repair bill if that should ever happen.

“The fuel filter should be replaced every other oil change (or if there’s a water-in-fuel warning on the DIC),” says Halsten, who had just finished a $4,500 injector replacement job for a customer who failed to heed the “water-in-fuel” warning light on instrument panel.

“When the Water-In-Fuel (WIF) sensor, located at the bottom of the fuel filter, sends a signal to the computer warning there’s water in the fuel, turn the engine off immediately. Don’t drive it another 50 miles thinking the warning isn’t that serious. It is.”

Halsten says it’s also important to handle the plastic WIF sensor with care – both unscrewing it from the OEM filter and replacing it on the new filter.

“Owners and fleet mechanics have a tendency to over-tighten the sensor, which leads to it cracking. When that happens, air gets in the system and the truck will start and die or crank and not start at all. Finger-tight and a 1/4-turn more is all you need.”

Another tip: Stretch the larger of the two o-rings that comes with the GM filter a couple times and it will seat in the filter’s retaining groove perfectly every time.

By the way, Halsten warns against using cheap water/fuel filters because they don’t have the extra water-trapping space afforded by the GM factory filter’s step-shoulder design. “That little bit of extra space can mean the difference between saving the injectors or not. So being cheap here can cost you a lot down the road.”

Front / Rear Differentials

Gear lube tends to break down and absorb moisture on its own accord, which is why Halsten likes to check the diffs every 30,000 miles to be sure the lube is doing its job.

Sure enough, when he pulled the front diff plug, out streamed a yellow-tinged, milkfish fluid instead of clean fluid.

Even though our project truck hadn’t been “off-road” fording creeks, the gear lube was compromised. It was refilled with Mobil 1 Synthetic Gear Lube LS 75W-90 to maximize lubrication and minimize fuel-robbing friction.

OEM front diff fluid hadn’t been changed in a while as the milky lube stream indicated. Diff lube will break down on its own over time and attract moisture.

He also checked the rear diff (it was ok) and topped it off with Mobil’s gear lube. (We’ll drain the rear diff and re-fill with fresh lube when the aFe rear diff cover arrives.)

“I like to fill the rear ­differential a little higher on the HDs than GM recommends,” notes Halsten pumping the handle on the fill bottle,” bringing the lube up just below the fill plug lip. That way I know lube is getting into the axle shafts and out to the bearings.”

Now Big Red is set to go. The next service will be in about 4,500 miles.

By that time our sweepstakes GMC Dually will look a lot different when it pulls back into the Barkley service center.


“BIG RED” Presented by Mobil Delvac

Vanair: vanair.com; 800-526-8817

Fleetwest: fleetwest.net; 866-497-7200

Hypertech: hypertech.com; 901-382-8888

American Force Wheelsamericanforcewheels.com; 800-620-6259

Mile Marker: milemarker.com; 800-886-8647

Hellwig Products: hellwigproducts.com; 800-435-5944

BD Diesel Performance: dieselperformance.com; 800-887-5030

Barkley GMC: barkleygmc.com; 205-556-6600

Marathon Seat Covers: marathonseatcovers.com; 800-735-2769

B&W Trailer Hitches: turnoverball.com; 800-248-6564

Fab Fours Bumpers: fabfours.com; 866-385-1905

Buyers Products Company; 440-974-7766; www.buyersproducts.com