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Engine emissions can be affected—sometimes significantly—as the engine deteriorates due to normal wear and/or lack of proper service. A number of studies carried out before the wide spread application of technologies such as EGR and NOx aftertreatment have shown that diesel engines usually deteriorate to have higher PM, CO and HC emissions and lower NOx emissions.
Fuel injection system faults are the most common reasons for increased PM emissions. For older mechanical fuel injection systems, this includes problems with the fuel injection pump such as transient air/fuel ratio control and maximum fuel stop settings. Normal engine wear can result in decreased injection pressures and delayed fuel injection timing . A number of different engine malfunctions can cause retarded or delayed injection timing, which would increase PM emissions while decreasing NOx . Increased intake air restriction, intake air leaks, improperly set injector lash, disabling of throttle delay, and injection timing retard can have the same result . Problems with fuel injectors are also common.
Faults resulting in increased PM/CO/HC emissions are often detected through smoke opacity testing under heavy-duty diesel inspection and maintenance (I/M) programs. Engine failures resulting in increased NOx, while less common, appear to be more difficult to detect and correct through mechanical repairs. The introduction of such technologies as EGR and NOx aftertreatment opens the possibility that deterioration to increase NOx may become more common, but data to support this is not yet available.
The effect of engine faults and maintenance procedures on emissions usually follows the NOx/PM trade-off. Engine faults that increase PM usually reduce NOx emissions and vise versa. This also means that repairs to correct high PM emissions will increase NOx emissions and vise versa. Restoring engine settings and performance to factory settings is the only way to balance the two and to ensure that PM or NOx emissions do not become excessive.
Tampering has been observed in a large proportion of engines with mechanical fuel injection systems that were tested under I/M smoke tests. Tampering, in the form of modified engine management software, is believed to be common in newer electronic engines, but data quantifying its prevalence and effect are unavailable.
Common diesel engine faults and a qualitative estimate of their frequency of occurrence are listed in Table 1. The table includes information from work done in 2001  as well as additional information to reflect advances in diesel engine technology since that time.
Dirty air filters and leaky turbocharger oil seals are relatively common. More serious turbocharger damage or problems with intercoolers are not. Intake and exhaust valve timing and leaks are also relatively rare. If a valve leak is significant, engine performance will deteriorate and/or noise and vibration will increase significantly.
Governor tampering and tampering with transient air-fuel ratio control devices is common on engines with mechanical fuel injection systems. Over-fueling to increase engine power by advancing the maximum fuel stop on engines with mechanical fuel injection systems is relatively easy for certain engine models and relatively common for those models. Advanced injection timing due to tampering may occur, especially in 1977-1984 engines when retarded injection timing was often used to meet NOx standards.
Fouled or leaking injectors will result in poor fuel penetration and atomization within the combustion chamber. Even though the overall fuel-to-air ratio may be sufficient for complete combustion, local fuel-rich zones resulting from poor fuel vaporization and mixing will also increase HC, CO, and PM. Spray hole erosion may be common in older high mileage engines. Installation of incorrect sized injectors could occur during replacement or rebuild. This may simply raise the maximum fuel delivered to another certified rating level with no increase in emissions. In some cases however, the mismatch between the existing turbocharger/intake system and the larger capacity injector may lead to high PM emissions.
The use of incorrect parts during repair or rebuild can similarly result in higher smoke in some cases, but is believed to be relatively rare. Very worn engines with leaky valve guides, or worn piston rings, are likely to be found near the end of an engine’s useful life.
|Components||Effect on Emissions||Frequency|
|Air filter clogging (dirty)||Increased PM and CO; can increase full throttle PM considerably||Extent of blockage varies, but is relatively common|
|Turbocharger seals worn||Can leak oil and cause increased PM and hydrocarbons||Minor oil leaks are common in older engines|
|Turbocharger damage||Significant damage is catastrophic, but minor damage has little effect on emissions||Minor nicks on turbo are common|
|Intercooler internal leaks||Coolant induction can cause white smoke||Rare|
|Intercooler - restricted coolant flow||High charge temperature will increase PM and NOx||Unknown|
|Valve timing||Incorrect valve timing can have minor emissions effect||Rare|
|Valve leaks||Loss of compression and high PM; engine is hard to start||Relatively rare, self correcting due to poor startability|
|Governor RPM setting||Increased RPM setting can increase hydrocarbons, CO and PM in some trucks||Common among independent trucks (tampering)|
|Maximum fuel stop setting||Increased hydrocarbons, CO and PM at full throttle||Relatively rare but can occur for certain engine models (tampering)|
|Injection timing setting||Advance causes increased NOx, retard increased hydrocarbons, CO and PM||Unknown|
|Air-fuel ratio control||Causes excessive PM during acceleration||Common among independent trucks (tampering)|
|Worn injector spray holes||Increase hydrocarbons, CO and PM||Occurs in older trucks|
|Injector plugging||Asymmetric spray can cause increase hydrocarbons, CO and PM||Occurs in older trucks|
|Injector tip cracking||Excessive PM, but is catastrophic to engine||Unknown|
|Incorrect injector size||Effect can vary, but hydrocarbons, CO and PM increase with increasing injector size||Could be common in replacement of injectors|
|Worn piston rings||High PM from low compression/oil leak||Relatively rare, as vehicle is hard to start|
|Leaking valve seals||Blue smoke from oil consumption, hydrocarbons increased||Unknown|
|Wrong part numbers||Minor effects if mismatch is not severe||Unknown, but could be a problem with aftermarket parts|
|EGR valve - low EGR flow||Increased NOx emissions||Unknown|
|EGR valve - excessive EGR flow||Increased particulate matter and CO emissions||Unknown|
|Diesel particulate filter damaged||Increased CO and PM emissions||Unknown|
|Diesel particulate filter blockage||May have little noticeable effect on emissions||Severe blockage would be self correcting as the engine may loose power|
|NOx aftertreatment damage or malfunction||Increased NOx emissions||Unknown|