Your truck is dying on the grades. Is it the load, or is something failing?

What normal looks like on a grade

Climbing takes power, and a heavy truck has only so much of it for the weight behind it. A loaded tractor-trailer will slow on a long grade and need lower gears, and a heavier load or a steeper, longer climb makes that more noticeable. That is weight and gearing doing what they do, not a fault.

The comparison that matters is the truck against itself. If it used to hold a gear on a hill with a similar load and now drops two, if it will not rev the way it used to under load, or if it loses power on climbs it once took comfortably, something has changed. A warning lamp, smoke or a climbing temperature gauge on the same climb confirms it.

Signs it is more than the load

A few clues separate a truck that is simply working hard from one with a fault worth diagnosing.

• It feels like hitting a wall. Power fades suddenly at the same point under load, or surges and fades, instead of tapering off gradually as the grade steepens.
• A warning lamp comes on. Check engine, DPF, stop engine, or a fuel filter lamp on trucks that have one. Our post on the check engine light explains what each color means.
• Black smoke under load. Turbocharger makers' troubleshooting charts list the same causes, such as a dirty air cleaner or an air leak between the turbo and the intake manifold, for both black smoke and an engine that lacks power.
• A new noise under load. A sound from the engine bay that was not there before can point to an air leak, which the same charts link to a noisy turbocharger as well as lost power.
• The temperature gauge climbs on the hill. Heat can be the reason for the power loss, not just a side effect of it.
• It pulls normally on the flat. A restriction the engine can live with at cruise may only show when it asks for full fuel and air on a climb.

Fuel supply: a filter that cannot keep up

Under load the engine needs the most fuel, and a filter that is starting to clog may cope at cruise and fall short on a grade. Filter makers list a loss of power that shows most under load as a warning sign of a clogging fuel filter, along with harder starting and, when the restriction is severe, stalling. Detroit's instrument panel lamps include a fuel filter restriction lamp, which Detroit describes as a restricted filter that needs service soon.

Water in the fuel, and the sludge that microbes form where water sits in a tank, both shorten filter life. If the truck has also shown a water-in-fuel lamp, read our post on water in diesel and clogged filters. Fuel filters and water separators are handled at the roadside, with the system primed afterward so the engine pulls clean fuel.

Air in, boost out: intake restrictions and charge-air leaks

A turbocharged diesel needs air in proportion to the fuel it burns. The turbo draws air through the air cleaner, compresses it, and pushes it through the charge-air cooler and its hoses and clamps into the intake manifold. A restriction on the way in or a leak on the way through leaves the engine short of air, and the troubleshooting charts from Holset and Garrett both list a dirty air cleaner and air leaks between the turbo and the intake manifold as causes of an engine that lacks power.

Many trucks have an air intake restriction indicator on the air cleaner or the intake duct. On Freightliner's standard go/no-go type, a colored bar showing in the clear window means the filter has reached its restriction limit; check it with the engine off. Freightliner also notes that rain or snow can wet the filter and cause a temporary high reading.

Charge-air leaks are a recognized cause of low power complaints. Navistar tells its dealers that when a low engine power complaint comes in and a charge-air cooler leak is suspected, the cooler must be leak tested, and it lists frequent regens alongside low power among the complaints. A Ford service bulletin traced a lack of power on one of its diesel engines to a charge-air cooler outlet tube coming off or separating at its crimp clamp. With the truck parked somewhere safe and the engine off and cool, a look along the large charge-air hoses for one that has come off, split or lost its clamp is a reasonable driver check.

Aftertreatment restriction and derates

The diesel particulate filter traps soot, and a regeneration burns it off. When soot builds faster than the truck can clear it, the engine protects the filter by cutting power. Detroit's lamp guide says a flashing DPF lamp means a regeneration is required immediately, and that a derate or shutdown could follow if it is ignored. On a climb, that power cut feels exactly like a truck that will not pull.

A derate can also come from DEF level or quality, or from the engine protection system responding to another reading. Our posts on a DEF light or a regen that will not complete and on derates and limp mode explain that warning ladder. At the roadside we read the codes, check whether the engine will allow a parked regen, and repair sensors and connectors. DPF cleaning or replacement is shop work.

Sensors and heat protection

The engine computer decides how much power to allow based on what its sensors report. A sensor or its wiring reading out of range can cut power on an engine that is mechanically healthy, and the fault code is how a sensor fault is told apart from a real condition. Diagnosing and repairing sensor problems where the truck sits is part of our mobile truck repair service.

Heat is the other quiet cause of power loss on hills. Freightliner's driver's manuals list coolant temperature, and on some engines oil temperature and intake air temperature, among the readings that start a warning and a derate or shutdown when they reach preset levels. A long loaded climb is when a weak cooling system, a slipping belt or a fan that will not engage shows itself, and the power cut is the engine protecting itself. When the temperature gauge is climbing, treat it as an overheating problem first and stop somewhere safe rather than pushing to the top.

What stays roadside and what goes to a shop

Diagnosis comes first and is roadside work: reading the codes and checking the fuel filters and separator, the intake restriction indicator, the charge-air plumbing and the sensors. Fuel filters and separator elements are replaced on the spot, and sensor and wiring faults are repaired where the parts allow. When the problem is a charge-air leak, we find it and tell you straight whether the part involved can be dealt with where the truck sits or needs a shop.

Turbocharger repairs, EGR valve and cooler work, DPF cleaning or replacement, injector work and anything inside the engine are shop jobs. A charge-air cooler that is leaking beyond its limit is replaced, not patched; Navistar tells its dealers not to attempt field repairs on one. If the truck is derated to a crawl or cannot safely get off the grade, the answer is a tow to the shop of your choice.

Notes that speed up the diagnosis

• The load. Roughly how heavy, and how that compares with loads the truck has pulled up similar hills without trouble.
• When it happens. Only on climbs, at a particular speed or gear, after a long pull, or all the time.
• Lamps and codes. Every lamp that came on, whether it was steady or flashing, and any message on the display.
• Smoke and noise. Black smoke under load, or a new noise from the engine bay.
• Gauges. Coolant temperature, and anything else that moved on the climb.
• Recent work. A recent fuel stop, a filter change, or any work on the air intake or charge-air plumbing.
• Where you are. Highway, direction and mile marker, and whether you are safely off the road. If you had to stop on a shoulder, flashers and triangles come first; our post on where the warning triangles go covers placement.

Related reading

Fifth Wheel Won't Release the Kingpin: Why and What to Do — A fifth wheel that will not release is usually binding because the tractor is pulling or pushing against the kingpin, which loads the locking jaws.

Check Engine Light On a Semi: Can You Keep Driving? — A steady amber check engine lamp on a truck that is driving normally generally means you can carry on carefully and get it diagnosed soon.

Serpentine Belt Broke on a Semi? What to Do Next — Get off the road at the first safe place and shut down.