Common Causes of Engine Misfires
How to Diagnose Engine Misfires and Tips For Fixing the Problem:
Question from Randy in Alabama: What are some possible causes for diesel engines misfiring and how do you go about fixing it?
Engine misfires are a common occurrence with diesel engines but the good news is that they are typically an easy issue to diagnosis.
Automotive misfires are a whole different animal than with diesel engines. Diesel engines ignite the fuel through compression where as automotive gas engines use a spark from an ignition system. With automotive misfires an ignition system is the first place to start looking for the misfire. The next appropriate steps to diagnose the problem include inspection of the ignition cables, spark plugs, distributor cap/rotor components and the ignition coil. The spark generated for ignition oftentimes will not be properly transferred as those components mentioned above wear significantly over time. The inability to ignite the air/fuel mixture in the combustion chambers will result in the engine not being able to turn over.
If you encounter a diesel engine misfire follow these steps:
1. Firstly make sure you are using a quality diesel fuel. Inspect the diesel fuel to verify the source has no contaminants; including water, dirt or oil.
2. Check the fuel system service log. Replace the fuel filters as needed and drain water from the system using a fuel water separator. 
3. Once you've made sure the engine is up to service and the fuel filters are replaced inspect for broken or leakage in the high pressure fuel lines. You can pressure test the fuel line however most leaks are exposed and are easy to get to.
4. Fourthly, check for low fuel supply pressure.
5. Fifthly, check for bends or kinks in the fuel line and transfer pump.
6. Check for clogged suction pipe (in fuel tank) or plugged suction hole.
7. Check for air in the fuel system and check fuel pressure.
8. If pressure is lower than specified replace the fuel filters.
9. Inspect the return valve for free moving poppet, if low replace the transfer pump.
10. If all of the above fail to diagnose the issue there are 4 additional potential causes of the engine misfire:
- Improper valve lash adjustment
- Worn Camshaft Lobe
- Valves not seating properly
- Defective fuel injection nozzles or unit pump
The good news is that engine misfires are a common occurrence and not a symptom of something more serious that is wrong with the diesel engine. Each engine is different and for example Mack E-7 or E-Tech engines have a thicker gauge fuel line than say a Detroit Series 60 or a CAT 3406E. On a Mack engine the problem could be more with the injectors than the fuel line. It is important to check each engine component separately to rule out issues one by one. Start with the easiest fix and work your way up. If you still have engine misfires feel free to contact a diesel parts specialist for further assistance.
