Keep Your Fleet Running Smoothly: Schedule Regular Valve Adjustments

Valve Adjustments: The Unsung Hero of Engine Longevity

In the world of diesel fleet maintenance, oil changes, tire rotations, and filter replacements get all the attention. But lurking deeper inside your engine is a lesser—known but equally vital maintenance item: the valve adjustment.

Also called a “valve lash” adjustment, this simple yet critical procedure ensures that the intake and exhaust valves in your engine open and close at precisely the right intervals. Over time, even microscopic shifts in clearance can affect power, efficiency, and durability.

At General Diesel LLC, we’ve seen it firsthand—fleets running rough, losing MPG, or throwing codes, all because their valve lash was overdue. The good news? You can prevent those problems by staying on top of this one key procedure.

Let’s break down what valve adjustments are, why they matter, and how they keep your fleet running at peak performance.

What Is a Valve Adjustment?

Understanding Valve Lash

Inside every diesel engine, intake and exhaust valves open and close in precise sequence, letting in air and fuel and releasing exhaust gases. Valve lash is the tiny clearance between the valve tip and the rocker arm or cam follower.

This space is necessary to prevent components from overheating and to compensate for expansion. However, if that clearance becomes too large or too tight, it affects valve timing—and your engine’s performance suffers.

Why It Changes Over Time

As your engine racks up hours and mileage, parts wear and metal expands. These small changes throw off your factory-set lash clearance. A tight valve may never close fully, while a loose valve could delay opening—either situation reduces efficiency and can lead to damage.

What Happens When Lash Goes Unchecked?

Reduced Power and Efficiency

When valve lash is off-spec, your engine’s combustion process becomes less efficient. Power output drops, and fuel economy suffers. If your trucks are suddenly struggling with acceleration or pulling power, lash issues could be part of the problem.

Engine Ticking and Noise

One of the earliest signs of excessive lash is a ticking or tapping noise from the top of the engine. This sound indicates increased mechanical movement—and it only gets louder as lash worsens.

Increased Emissions and Regens

Improper valve timing throws off the air-fuel mixture, affecting exhaust gas temperatures and aftertreatment performance. That means more DPF regens, increased soot, and potentially a failed emissions test.

Valve and Camshaft Damage

Unchecked lash puts extra stress on valve stems, pushrods, and camshaft lobes. Over time, this causes excessive wear, misfires, and even burnt valves—turning a simple adjustment into a full-blown top-end rebuild.

How Often Should Valve Lash Be Checked?

Manufacturer Recommendations

Most diesel OEMs recommend checking and adjusting valves between 100,000 to 150,000 miles, or after a set number of engine hours. Cummins, Detroit, and PACCAR all specify lash intervals based on load and duty cycle.

Heavy-haul, idle-heavy, or stop-and-go fleets may require more frequent checks. Check your engine’s maintenance manual—but also listen to your truck. If performance drops or emissions increase, it’s time to check lash.

Ideal Service Intervals

For fleets like those serviced at General Diesel LLC, we recommend checking valve lash every:

• 100,000 miles for heavy-duty OTR trucks
  
  
• 50,000–75,000 miles for urban delivery or idle-heavy trucks
  
  
• Annually for low-mileage units or specialty equipment
  
  

How a Valve Adjustment Is Performed

Step 1 – Engine Cooldown

Valve lash must be set with the engine cold. The technician allows full engine cool-down to ensure accurate measurement and prevent thermal expansion from skewing results.

Step 2 – Valve Cover Removal

The technician removes the valve cover(s) to access the rocker arms and valve train. From here, each cylinder is brought to top dead center (TDC), one at a time.

Step 3 – Clearance Measurement and Adjustment

Using feeler gauges and OEM specs, the lash is measured and adjusted with a micrometer screw or adjustment nut. Intake and exhaust valves often require different clearance values.

Step 4 – Reassembly and Inspection

After adjustment, the valve cover is reinstalled with a new gasket, and the engine is run to check idle smoothness and noise. Often, fuel efficiency and throttle response improve immediately.

Benefits of Staying on Top of Valve Lash

Smoother Operation

Engines with properly set valve lash run smoother, idle better, and accelerate more consistently. It also reduces vibration—protecting mounts, brackets, and other engine components from fatigue.

Improved Fuel Economy

Even a few thousandths of an inch off in valve clearance can drop MPG. When optimized, your engine breathes more efficiently—saving thousands in fuel over the year.

Extended Engine Life

Valve adjustment prevents burnt valves, cam lobe wear, and injector timing problems. That translates into fewer breakdowns, longer engine life, and fewer surprise repairs.

Lower Emissions and Fewer Regens

Proper combustion keeps EGTs within range, reducing DPF loading and regeneration cycles. That means less downtime and longer DPF life.

Common Myths About Valve Adjustments

“If It Ain’t Broke, Don’t Fix It”

Valve lash can drift silently, without any codes or warnings. By the time symptoms show up, you’re already seeing power loss or damage. Regular checks prevent expensive surprises.

“We Did That Last Year”

A single valve adjustment doesn’t last forever. Heavy loads, vibration, and thermal cycling wear parts over time. Set a schedule based on real-world usage, not just mileage.

“It’s Too Expensive”

Compared to a head gasket job, valve replacement, or cracked rocker arm, a valve lash adjustment is one of the most affordable services you can perform. It’s an investment, not a cost.

Partnering with a Diesel Shop That Gets It

At General Diesel LLC, we treat valve adjustments with the seriousness they deserve. Our technicians follow OEM specs to the letter, use calibrated tools, and document every setting. Whether you run PACCAR, Cummins, Detroit, or CAT engines, we’ve got the experience and data to keep your lash perfect and your trucks on the road.

We include lash checks as part of our comprehensive PM packages, and we track valve trends over time—so we can spot wear patterns before they become problems.

Final Thoughts

Valve adjustments might not be flashy, but they’re one of the most important maintenance steps for a diesel fleet. Ignoring them invites inefficiency, damage, and downtime.

Don’t wait for ticking, smoke, or regen problems to schedule service. Make valve lash checks a regular part of your preventive maintenance plan—and your trucks will thank you for it.

Want to extend the life of your engines and reduce costly repairs? Book your fleet’s next valve lash service at General Diesel LLC. Our team keeps lash in line and your trucks on time.

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