Pattern Problems: 6.7L Power Stroke vs. LML Duramax (2023)

  • Story By
    Mike McGlothlin

Pattern problems. Every engine ever assembled has them. Some experience fewer trivial failures than others, but in the end all internal combustion power plants have their quirks. When it comes to diesel pickups, down time often means lost income, so it definitely pays to know what you’re getting into when you invest upward of $40,000 in a late-model (new or used) ¾-ton or larger diesel truck. In the case of the 6.7L Power Stroke and the LML Duramax—both having been out since 2011—most of these engines’ mild to moderate flaws have been exposed and well-documented by now.

On a recent visit to Flynn’s Shop in Alexander, Illinois, a shop that specializes in each of the Big Three, we were clued in as to which repeat issues occur most with each brand’s highly-esteemed V8. Though Ford seems to have ironed out the majority of the issues associated with the Navistar-derived Power Strokes of yesteryear thanks to building its own Power Stroke in-house, many 6.7L-powered Super Dutys drive into Flynn’s service bays with coolant and oil leaks. As for the LML Duramax, emissions system-related failures run rampant and the age-old clogged cooling stack scenario continues to play out regularly on trucks that work out in the field.

For an in-depth look at the top four pattern problems for each engine, keep scrolling.

6.7L Power Stroke

Oil Leak From the Vacuum Pump Gasket

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (1)

Arguably the most common issue on the 6.7L Power Stroke is associated with the vacuum pump. Over time, the pump’s mounting bolts loosen and can even back out completely. When they loosen their grip, oil is allowed to escape past the gasket sandwiched between the vacuum pump and the block, with an oil drip or small puddle inevitably developing under the oil pan. For best results, it’s ideal to pull the vacuum pump completely, replace the gasket and hit the mounting bolts with Loctite before reinstalling them. However, in a pinch you can tighten three of the four bolts by removing the factory air intake (to gain access) and using an 8mm open-end wrench.

(Video) 6.7 Powerstroke Best Year

Coolant Leak at the Turbo

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (2)

A close second to the vacuum pump gasket issue is the tendency for the turbo coolant feed line to leak on ’11-’14 engines. Due to vibration, the seal inside the quick-connect type fitting at the turbo fails, causing a small yet noticeable coolant leak. Replacing the coolant supply line is fairly straightforward, but you have to remove the upper intake plenum to gain proper access to it. Both the OEM hard line and fitting can be purchased for less than $40.

Questionable Water Pump Reliability

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (3)

Premature water pump failure is fairly common. Just to clarify, the 6.7L Power Stroke makes use of two separate cooling systems (a high-temp primary and a low-temp secondary system), so in this instance we’re referring to the engine’s primary water pump. For whatever reason (casting sand, supplemental coolant additives, etc.), a lot of water pumps don’t make it 100,000 miles before failing—and a fair share of pumps even kick the bucket before hitting 40,000 miles. So far it seems to be the luck of the draw on getting an engine with a good primary water pump, as some die early while others last well beyond 150,000 miles. To keep your water pump, water neck, radiator hoses and even radiator in optimum health, a coolant filtration system (such as the one offered by DieselSite) is never a bad idea.

(Video) Everything Wrong with the 6.7L Powerstroke

Leaking Radiators

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (4)

This one has been a problem for Ford since the 6.4L Power Stroke debuted in ’08 and it’s still a semi-frequent failure today. Once again, we’re referring to the 6.7L Power Stroke’s primary cooling system when we talk about radiator failure. Similar to the problem found in the 6.4L application, the leaks originate where the plastic end tanks crimp onto the metal core. While leaking radiators were most common on early ’11 model 6.7L Power Strokes, the folks at Flynn’s still see plenty of ’12-’16 Super Dutys experiencing this issue. The verdict is still out on the ’17-’19 trucks, as they’re still pretty new.

LML Duramax

Cracked EGR Coolers

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (5)

As the 6.6L LML Duramax begins to age, more ’11-’16 Chevy and GMC HD trucks are being diagnosed with cracked exhaust gas recirculation (EGR) coolers. But this emissions-related, coolant-burning failure isn’t reserved solely for high-mileage candidates. Some EGR coolers rupture even before the 50,000-mile mark. Telltale signs of a cracked EGR cooler include disappearing coolant (that’s not making it into the crankcase) and white smoke out the tailpipe. We’ll note that some shops misdiagnose failed EGR coolers for blown head gaskets—the difference between the labor and parts involved in both jobs being tremendously different. Always make sure the EGR cooler is pressure tested for leaks before committing to a head gasket job on your Duramax.


DEF Tank Heater Failure

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (6)

Problems with the emissions control systems on modern diesel trucks are highly common. With so many new technologies and components being employed to curb NOx, particulate matter emissions and CO2 emissions, it’s no wonder so many trucks barely make it out of warranty before experiencing a failure. Fortunately for LML owners, GM is well aware of its chronic diesel exhaust fluid (DEF) reservoir heater failures. If your truck’s VIN falls into the covered range, a 10-year/120,000-mile warranty applies to your DEF heater. When this baby goes out it’s usually associated with a CEL and a P20B9 code being stored, along with a “Service Exhaust Fluid System” message on the dash. But don’t put off dealing with the problem. If you continue to drive with a bad DEF heater, eventually the ECM will place itself in power reduction mode or limit the truck’s top speed until it’s fixed. We’ll note that this failure is much more common in colder climates (i.e. the upper Midwest and Canada).

Airflow Blockade

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (7)

If you start noticing higher coolant temperatures than you’re used to seeing, a common problem among all Duramax-powered GM HDs may be at work: a plugged cooling stack (all the heat exchangers mounted in front of the engine). Especially on trucks that are worked hard and rarely cleaned, see frequent field use or are full-on service trucks, the cooling stack can become chock full of debris over time, which blocks airflow across the radiator. For optimum results, it’s best to take the time to disassemble and remove the cooling stack, cleaning each heat exchanger individually. However, in a time crunch a power washer can work wonders for opening up airflow through the cooling stack.

(Video) Ford's 6.7 PowerStroke | Reliability and Common Problems

The Duramax's End-Game: Broken Crankshaft

Pattern Problems: 6.7L Power Stroke vs. LML Duramax (8)

The broken crankshaft phenomenon extends across all model years of the 6.6L Duramax. The fact that it occurs just as much in bone-stock to moderately modified engines as it does in excessive rpm, high horsepower mills leaves a lot of enthusiasts scratching their heads. While a broken crankshaft is nowhere near as common as an EGR cooler or DEF heater failure, the folks at Flynn’s still see them more often than they’d like to. To date, poor external balancing from the factory, a lack of meat in key areas and the engine’s firing order have all been blamed for the crankshaft’s relatively high (as compared to Cummins and Power Stroke mills) failure rate, with no definite causes(s) yet known.

Turbo failure is also a fairly common occurrence on the ’11-’14 6.7L Power Stroke. You can read up on the problem (along with the long-term solution) here.


1. Is a 6.7 Power Stroke Worth it!?
(Custom Offsets)
2. CP4 to DCR Pump Conversion: Ford 6.7L Power Stroke #fordtrucks #powerstroke #diesel #ssfueled #short
(Thoroughbred Diesel)
3. L5P & LML Comparison with Gale Banks
(Diesel Performance Podcast)
4. Do YOU need an EGT sensor replaced?? **CHECK THIS OUT** 11-16 Ford 6.7 Trucks | BONUS LIMP MODE EXIT
(PowerStroke Tech Talk w/ARod)
5. 6.7 - Exhaust Fluid Fault Warnings
(PowerStroke Tech Talk w/ARod)
6. How Navistar Almost Killed Ford's Powerstroke Lineup
(Dust Runners Automotive Journal)


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