Diesel Tuning

Reprogramming diesel engines for better fuel efficiency

As for any type of engine reprogramming via the engine ECU, "very, very little is allowed and even then under strictly controlled circumstances," McKenna says. However, there are a great many customer programmable features in the vehicle ECU that are easily enabled/disabled or reprogrammed.

There are dozens of programmable engine parameters can be set using software approved by the engine manufacturer to help maximize efficiency, adds Mark Thomas, director electrical and electronic engineering, Daimler Trucks North America (DTNA). These include road speed governing, maximum vehicle speed, idle shut-down, gear-down protection and cruise control settings and limits.

The primary function of programmable parameters is to enable the vehicle to be customized to more closely meet the needs of the customer, Freightliner Trucks' Bryant says. For instance, a customer could have ordered a truck for a utility bucket application but then decided to upfit it with a digger/derrick. The PTO set speeds and demands are different for these applications and a certified dealer would be able to make these modifications.

"Customers are always looking for ways in which to improve performance and lower operating costs, observes McKenna of Mack Trucks. Engine manufacturers "offer the ability for this to be electronically accomplished to fine tune the truck to a specific application."

Reprogramming does not affect the engine's warranty provided it is done following the manufacturer's published protocols and policies, Navistar's Ehlers says. Warranty is also unaffected if reprogramming the engine parameters is done by a certified technician using software approved by the engine manufacturer and the appropriate tools.

"Most of the major fleets, both highway and vocational, have the ability to reprogram a predetermined level of features that do not effect either warranty or emission compliance," says McKenna.

When changing an engine rating, it has to be is approved by the engine manufacturer for the engine platform and components for the warranty to be unaffected, adds Caterpillar's Coe.

Engine rating changes are more commonly done by the engine manufacturer, Bryant says, and typically have a cost requirement which ensures the warranty is updated for the new rating.


Replacing the ECM with a new or remanufactured unit is not an alternative method for improving engine performance. "There would be no difference whatsoever," explains Mack Trucks' McKenna. "It is the programming data within the ECU that will affect performance."

"Think of the ECU as the data bank - meaning that all pertinent information and control algorithms are stored here. A corrupted file can cause problems such as ghost codes - fault codes that are read but did not exist. Tampering is a common cause of this."

Engines are developed with software and fuel map calibration relative to an engine and how it will be used, and for emissions certification, says DTNA's Thomas. If ECUs have to be replaced, it should be with an OEM unit only. ECUs should not be tampered with by "unauthorized" people.

"OEM engineers spend thousands of hours validating engine calibrations in a wide range of operating conditions to ensure long-term performance, durability and emission compliance," adds Bryant of Freightliner Trucks. "Replacing the ECM with a non-OEM part would not be a recommended method to improve performance of the engine and most likely would void both the warranty and emission certification of the engine."


There are a number of aftermarket products and add-ons for reprogramming the engine management system to improve diesel engine performance and lower fuel costs. The engine manufacturers stress that before using such devices, vehicle owners should make certain that these devices do not interfere with the emissions compliance and performance of their engines.

"Engine manufacturers spend hundreds of millions of dollars to ensure the products they offer meet all of the federally mandated emissions requirements and provide the best possible performance," says Coe of Caterpillar. "In most instances," McKenna adds, "something that might improve performance in one area generally will compromise performance or legal compliance in another."

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