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If your diesel truck suddenly seems to be spending a lot more time at the fuel pump, there is usually a reason.
Sometimes the problem is simple. Low tire pressure, extra weight, driving habits, or a dirty filter can all affect diesel fuel economy. Other times, poor MPG can point toward a fuel system problem, emissions issue, worn component, or a truck setup that simply is not working as efficiently as it should.
The important thing is not to immediately start throwing parts at the truck.
Start with the basics, look for anything that has changed, and work your way through the system.
Here are some of the most common reasons a diesel truck starts getting bad fuel mileage.
Let's start with the part nobody wants to hear.
Your right foot has a lot to do with your fuel economy.
Hard acceleration, higher highway speeds, frequent braking, and constantly getting back into the throttle can make a noticeable difference in how much diesel your truck uses.
This becomes even more noticeable on a modified truck where additional horsepower makes it very easy to use more fuel.
If your MPG has dropped, ask yourself:
Sometimes the truck is fine. The operating conditions have simply changed.
Tire pressure is one of the easiest things to check, and it is also one of the easiest things to ignore.
An underinflated tire creates additional rolling resistance.
That means the engine has to work harder just to move the truck down the road.
Check all four tires and make sure you are running an appropriate pressure for your truck, tire size, and load.
If the truck is still on the factory tire size and configuration, the tire-pressure information inside the driver's door jamb is a good starting point.
If one tire keeps losing pressure, fix the problem instead of topping it off every few weeks.
Larger tires look good, but they can work against fuel economy in several ways.
A larger tire can:
This is especially important if your fuel mileage dropped shortly after installing larger tires.
You may not have an engine problem at all.
The truck may simply be working harder because of the new tire size.
And if your speedometer has not been corrected for the new tires, the MPG number you are seeing may not even be accurate.
Weight matters.
Toolboxes, equipment, fuel tanks, campers, parts, construction supplies, and everything else we tend to leave in our trucks adds up.
If you are carrying around several hundred pounds that you do not actually need every day, the truck has to move that weight every time you accelerate.
The same thing applies to mud and debris packed underneath a work truck.
Take a look at what you are hauling around.
You might be surprised how much unnecessary weight you can remove.
An alignment problem does more than wear out tires.
If the wheels are fighting each other while the truck travels down the road, you are creating additional resistance.
That hurts efficiency.
Alignment problems are especially worth checking if you recently:
If the truck does not roll efficiently, the engine has to make up the difference.
A worn component can sometimes create more resistance than you realize.
Ball joints, tie rods, wheel bearings, U-joints, and other steering or driveline components should all move the way they were designed to move.
If something is binding, worn, vibrating, or beginning to fail, it may be creating additional drag.
Pay attention to symptoms such as:
Poor fuel economy may not be the biggest problem in this situation. A worn component can eventually turn into a much more expensive repair.
Your diesel fuel system needs a consistent supply of clean fuel.
A restricted fuel filter can make it harder for the fuel system to maintain the pressures and flow the engine is asking for.
If you do not remember the last time the fuel filter was changed, start there.
Fuel-filter maintenance is especially important on modern common-rail diesel engines where the injection system operates at extremely high pressure.
Staying on top of fuel-filter changes also helps protect expensive fuel-system components.
Diesel engines need air.
If the air filter is heavily restricted, the engine may not be able to breathe as efficiently as it should.
You do not need to replace an air filter every time it gets a little dusty, but you should inspect it according to the manufacturer's recommended service schedule and your driving conditions.
A truck that spends its life on dusty roads, farms, jobsites, or off-road is going to need more attention than one that spends its life on the highway.
Diesel injectors have an enormous effect on how efficiently the engine burns fuel.
The injector has to deliver the correct amount of fuel at the correct time with the correct spray pattern.
As injectors wear or develop problems, fuel atomization and combustion can suffer.
Depending on the truck and the type of injector problem, you may also notice:
Do not assume bad MPG automatically means you need injectors.
Diesel injectors are expensive.
Proper diagnosis matters.
The high-pressure injection system still needs a reliable supply of fuel coming into it.
If your truck uses a lift pump and that pump is weak, worn, or unable to keep up with the truck's needs, the rest of the fuel system may not operate as efficiently as it should.
This becomes even more important on modified trucks with additional fuel demand.
Fuel pressure should be tested and diagnosed rather than guessing.
Depending on the application, replacing a worn factory pump or upgrading the low-pressure fuel supply system may be worth considering.
If you own a newer diesel with a diesel particulate filter, pay attention to how often the truck is going through regeneration.
During an active DPF regeneration, the truck uses additional fuel to raise exhaust temperatures and burn accumulated soot from the filter.
An occasional regeneration is normal.
A truck that is regenerating much more often than it used to deserves a closer look.
Frequent regeneration can be related to things such as:
If your truck's fuel mileage suddenly dropped and you are noticing more frequent regens, the two may be related.
The emissions system is part of the overall operating system of a modern diesel truck.
If the EGR, DPF, sensors, or related components are not operating properly, engine efficiency can suffer.
A truck that spends most of its life making short trips around town may have a harder time maintaining proper DPF operation than a truck that regularly reaches full operating temperature and spends time on the highway.
If you are seeing warning lights, increased regeneration frequency, high soot levels, reduced power, or other emissions-related symptoms, diagnose the problem rather than ignoring it.
Some DPFs can also be professionally cleaned depending on their condition and application.
Diesel engines rely heavily on airflow and boost.
A boost leak, damaged intercooler boot, leaking charge-air connection, turbo problem, or other airflow restriction can reduce engine efficiency.
You may notice:
Inspect the charge-air system carefully.
A loose clamp or damaged intercooler boot can create a surprisingly large problem.
This is especially important on modified diesels.
Maybe the truck was originally built around making horsepower.
Now you daily-drive it.
Maybe you tow with it.
Maybe fuel economy suddenly matters more than it did five years ago.
Oversized injectors, an incorrectly sized turbo, huge tires, aggressive gearing choices, and other performance modifications can make the truck less efficient if they do not match how the truck is actually being used.
The biggest part is not always the best part.
A properly matched combination usually works better than a collection of parts chosen only because they support a bigger horsepower number.
Tuning can potentially improve the way a diesel truck operates, but the tune has to work with the truck.
If the truck has been modified since it was tuned, or if the calibration does not match the current combination, it may not operate as efficiently as it could.
You also have to consider how you are using the tune.
More available horsepower does not automatically mean poor fuel economy.
Using that horsepower constantly probably will.
A monitor can be extremely useful here because it allows you to watch real-time MPG and other engine parameters while you drive.
Changing tire diameter changes the effective gearing of the truck.
If you installed substantially larger tires without changing gearing, you may have moved the engine outside the operating range where it performs best for your particular driving conditions.
That can affect:
Regearing is not automatically the answer for every truck with larger tires, but gearing should absolutely be part of the conversation when building a truck around a specific tire size and use.
If your MPG drops every winter and comes back in the spring, the truck may not have a mechanical problem.
Cold weather changes a lot of things.
The engine takes longer to warm up, tire pressure drops as temperatures fall, owners idle trucks longer, winter fuel formulations can differ, and cold fluids create additional resistance until the truck reaches operating temperature.
Short winter trips make the effect even more noticeable.
So when diagnosing a seasonal MPG drop, compare similar driving conditions before assuming something is wrong.
If your diesel truck is getting bad fuel mileage, do not begin with the most expensive possible repair.
Start simple.
Check your tire pressure.
Look at your driving habits.
Think about whether your route or towing habits have changed.
Check your maintenance schedule.
Look at your filters.
Inspect your tires.
Pay attention to regeneration frequency.
Listen for driveline problems.
Look for boost leaks.
Then move deeper into the fuel system, turbocharger, emissions system, injectors, tuning, or performance setup if the basic checks do not reveal the problem.
The goal is not just to chase a bigger MPG number.
The goal is to make the truck operate as efficiently as possible.
And that is something we talk about all the time at Thoroughbred Diesel.
If your diesel fuel economy has dropped and you are trying to figure out whether the problem is maintenance, fuel delivery, tuning, airflow, injectors, turbo sizing, or the overall setup of the truck, Call for Free Advice. We can help you work through what you have and what makes sense for your truck.