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Why Diesel Fuel Injectors Fail

How to Avoid it

Text By Jason Thompson, Photography by Exergy Engineering

Diesel injectors fail because of two main reasons. The first has to do with the mechanical soundness of the injector structure, and the second has to do with the quality of the fuel running through the injector. In order to get an understanding of the workings of an injector (and what actually makes them fail), we contacted Exergy Engineering. The company supplied us with many of the images of failed injectors you see here, taken with a microscope, to help you to keep this damage from happening to your diesel.

To find out as much as we could about the fuel side of the equation, we contacted Afton Chemical, a company that specializes in fuel additives. Afton has 85 years worth of experience working with OEMs and fuel companies. By leveraging both of these companies’ expertise, we hope to make fuel injector problems a thing of the past.

Is There a Fuel Injector Problem?
If proper maintenance is performed and problematic practices are avoided, the vast majority of diesel owners will go thousands of trouble-free miles without a problem. If you’re a diesel owner with an older engine (pre-common-rail), most of this article (besides the general maintenance advice, like changing your fuel filter regularly) doesn’t apply to you. This is because older diesel injection systems only use about 1⁄2 the fuel pressure modern engines do, and older injectors send the fuel through much larger passages.

Why is there such a difference with common-rail injectors? Modern common-rail diesel injectors can fire two or three times per engine cycle—this doubles the wear on the injector compared to diesels of the past. To tell if there is a problem with common-rail injection systems fueled with ultra-low-sulfur diesel (ULSD) fuel, we need to know how many injectors have failed since its introduction. The feedback we get from our readers and online reports of failures suggests there’s always room for improvement in our fuel, too.

Mechanical Failures
According to Exergy Engineering, injectors fail because of five major reasons. We listed them here, along with the indicators of the problem, the causes, and how it’s prevented.

Failure: High Internal Leakage or Return Flow
Indicators:
1. Engine is hard to start (requires increased cranking time in order to start)

2. Low common-rail pressure codes

Causes:
Worn injector ball seat

Leaking cross feed tubes (Cummins)

Blown internal high-pressure seal

Incorrect nozzle needle clearance

Cracked nozzle body

Cracked injector body

Prevention:
Keep fuel system clean, change fuel filters, purchase fuel from reliable sources, avoid filling from portable construction fuel tanks

Avoid overly aggressive tuning that increases rail-pressure and injector pulse widths and do not remove pressure-limiting devices from the system

Do not use remanufactured or aftermarket injection components that are not properly designed or manufactured

Reject all fuel system replacement parts that have metallic burrs

Use only Bosch nozzles, as they are reported to have superior crack resistance

Do not mix nozzle needles, because they are matched to the body and moving one from another can result in excessive clearance or improper needle lift

Failure: No Injection
Indicators:
Balance rates are high (positive), indicating fuel is being added to the cylinder because the computer thinks the fuel injector is not flowing enough. The computer makes this decision based on the two things it knows: the rotational speed of the crankshaft and the amount of fuel delivered. If the crankshaft is not spinning as fast as the computer thinks it should (or is spinning faster than it should), fuel (via pulse width) is added or taken away to even out crankshaft acceleration from each injector firing.

Cylinder contribution low (cylinder contribution test is performed by shutting off one injector at a time while taking note of drop in engine RPM)

ECU fault codes

Causes:
Debris or rust in the injector plugging the nozzle

Armature and/or nozzle needle stuck

Bad stator (rare)

Loss of cylinder compression or other mechanical problem

Prevention:
Keep fuel system clean, change filters, purchase fuel from reliable sources, and avoid filling from portable construction fuel tanks or questionable sources

Do not use remanufactured or aftermarket components that are not properly designed or manufactured

Reject all fuel system replacement parts that have metallic burrs

Avoid tying the returns from multiple high-pressure pump kits and injectors to a single return line; excessive return pressure acting on the injector stators can lift them (and in extreme cases blow them off), shutting the injector down

If a long storage time of the vehicle is expected, arrange to have it started on occasion to prevent internal varnishing and corrosion of internal components; aftermarket fuel additives specifically designed for stabilizing diesel fuel should also be added

Failure: Excessive Injection
Indicators:
Excessive smoke at idle, poor running, and banging

Balance rates high (negative), indicating the computer is removing fuel from the injector

Cylinder contribution test is high, meaning as each injector is activated one will increase engine rpm more than normal

Excessive exhaust gas temperature

Engine damage from excessive heat or hydraulic lock from excessive fuel in the cylinder

Causes:
Worn ball seat in injector or poor end of injection cut off

Nozzle needle seat worn or damaged

Debris in control system of injector, which holds it open

Debris in nozzle needle seat holding it open

Cracked nozzle from overpressure, or overheated nozzle from improper installation of injector

Prevention:
Replace worn and high-mileage injectors; do not use these injectors as a foundation for building a high-output injector set

Replace worn nozzles

Keep fuel system clean, change filters, purchase fuel from reliable sources, and avoid filling from portable construction fuel tanks or questionable sources

Reject all fuel system replacement parts that have metallic burrs

Do not use remanufactured or aftermarket components that are not properly designed or manufactured

By Jason Thompson
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