Common Cummins Engine Problems and Troubleshooting Guide

Owning a diesel vehicle can greatly benefit you or your company, and a Cummins diesel engine is a reliable choice. The Cummins name is synonymous with innovation and quality, providing power for everything from trucks to marine vehicles and mining equipment. However, like any other machine, a Cummins diesel engine has to be regularly maintained and even then, wear and aging may introduce some common issues.

Luckily, typical Cummins problems are easy to troubleshoot without too much hassle. You have to be sure you can correctly identify the problem and repair it at the source. This guide will take you through engine troubleshooting for a Cummins diesel engine and create a proper maintenance schedule.

What Can Go Wrong With Cummins Diesel Engines?

All mechanical power sources will experience issues, regardless of how well you take care of them. They can range from a minor nuisance to a major problem that needs immediate attention, some of which can keep you from even running your vehicle. 

The most critical step to engine troubleshooting is identifying the source of your engine complication. You can typically narrow down the possibilities by first documenting precisely what problem you’re facing. By understanding the area that isn’t functioning properly, you can find the most probable diagnosis and use trial and error to solve the issue.

From your observations of what’s happening when you start — or attempt to start — the engine, you can deduce what might be going wrong. Here are some of the most frequent Cummins diesel engine problems and what might be causing them:

1. Engine Not Starting or Hard to Start

In some cases, Cummins diesel engines will turn over but will flat-out refuse to start. When a Cummins NT855Cummins M11, or other engine experiences difficulties starting or delay in some way, it may be due to one of a few typical issues, including:

Low engine compression: If there isn’t sufficient pressure, the engine will not generate enough heat to ignite the fuel. Generally, this issue is more prevalent in engines with higher mileage. A cold engine compression test on your Cummins diesel engine can identify it as the source of the problem.

Low fuel pressure: Most fuel supply issues originate from insufficient pressure. The best way to identify if this is the problem is to make sure the tank has enough fuel. Next, you can check whether or not the pump is delivering fuel to the engine. If those are both correct, check if the pressure is low.

Low cranking speed: The engine needs to turn over quickly for the fuel pump to generate pressure. The fuel pressure activates the injection, and without the proper processes, a Cummins diesel engine will have hard starting issues. Low cranking speed is more common during colder seasons.

Insufficient fuel supply: Whether there isn’t enough fuel in the tank or some of the supply pipes are damaged, an insufficient fuel supply will cause hard starting. Additionally, a blocked breather can result in creating a vacuum, which draws fuel back into the tank.

Fuel quality: Poor fuel quality can cause damage to the engine’s internal components, mainly the injectors. Cummins injector failure can, in turn, create hard starting issues.

2. Running Rough at a Lower RPM

If your Cummins diesel engine is running rough even at a low measure of revolutions per minute (RPM), it often has something to do with the fuel. You’ll need to do engine troubleshooting for:

Low fuel pressure: Fuel pressure is a frequently cited catalyst for engine starting and running issues. The best way to rule it out is to check if the fuel tank is sufficiently filled and make sure the pump delivers fuel and runs diagnostics on the actual pressure.

Insufficient fuel supply: The engine needs a sufficient supply of fuel to run correctly. Check the supply pipes for damage and the fuel tank breather for blockages.

Fuel quality: Internal components in a Cummins diesel engine need a high fuel quality to function properly. If the fuel quality is poor, it can damage Cummins diesel injectors and cause the engine to run rough.

Faulty fuel injectors: Worn out parts in Cummins diesel fuel injectors will cause return flow and a drop in fuel pressure and delayed injection, resulting in either rough running or a complete failure to start.

Air intake restriction: If the engine isn’t receiving enough air, it could be due to a restricted intake. The most common types of restrictions are dirty air cleaners, blocked pipes and stuck butterfly valves. Your engine may also have problems if there is a faulty air flow sensor on the intake.

While the issue could be a blockage or may be solved with cleaning, you may need to replace some Cummins parts like supply pipes, injector or pump parts and other components. The root of the problem may encompass multiple components.

3. Lack of Power

If you run into issues where your Cummins diesel engine struggles with starting or accelerating, it’s likely experiencing a lack of power. With these types of issues, you can most often link it to a fuel-related component when seeing 5.9 and 6.7, and other Cummins engine problems. Be sure to check for:

Dirty fuel filters: If the fuel filters are dirty enough, they can cause power issues, including creating a vacuum in the fuel supply. You may need to replace them to solve the power problem.

Loose throttle linkage: For the engine to run correctly, the throttle cable needs to maintain the proper tension. A loose connection could be the answer to a lack of sufficient power.

Restricted air intake: A restriction in the air intake system can prevent a Cummins diesel engine from achieving true power. Restrictions include blockages in the pipes and a dirty air cleaner. If this is the issue, you may also notice smoke from the engine.

Faulty fuel injectors: Faulty Cummins injectors can cause return flow, decreased fuel pressure and starting problems. Similarly, faulty pressure pumps, supply pumps and regulator sensors can negatively affect power as well.

If fuel turns out to be your problem, you may need to install new components with Cummins parts to replace anything that’s worn or damaged.

4. Black Smoke

If you see black smoke coming from your Cummins diesel engine, it typically signifies an imbalance in the air-to-fuel ratio. The fuel system is either suffering a lack of clean air or delivering too much fuel to the engine. Black smoke means you need to engine troubleshooting for:

Faulty fuel injectors: If your engine has faulty Cummins injectors, they can cause a return flow or drop in fuel pressure, which alters the amount of fuel reaching the engine.

Faulty injector pump problems: Pumps directly affect the fuel pressure and injectors, and a faulty pump can do a lot of expensive damage. Black smoke may be a sign to check on the Cummins injector and fuel systems, as letting the problem continue will only allow it to worsen.

Dirty air cleaner: When a Cummins diesel engine doesn’t receive enough air to balance with the fuel, it can create black smoke. The imbalance can be caused by restrictions in the air intake system, like blocked pipes and dirty filters and cleaners.

Faulty intercooler or turbocharger: If the turbo wastegate starts sticking, it can cause the engine to smoke or the vehicle to shut down. Black smoke can also come from carbon if the engine has a variable vane turbo.

Cylinder head problems and clogged valves: Build-up is detrimental to engine function, and burning can produce smoke. A faulty exhaust gas recycling (EGR) unit can clog the valves and create black smoke from the engine.

In summary, if you see black smoke coming from your Cummins diesel engine, you need to check on the air and fuel systems. It may be a minor fix that requires cleaning, or you may need to find replacement parts if the problem is persistent.

5. White Smoke

White smoke coming from your Cummins diesel engine signifies that the fuel in the cylinder may be burning incorrectly. You can identify this as the issue if the smoke makes your eyes burn when you’re near it. If this is the case, perform engine troubleshooting for:

Engine and pump timing out: If the pumps endure fuel starvation, the timing can go off and cause them to operate improperly. The pumps affect fuel pressure and can result in improper burning.

Low engine compression: In addition to causing a Cummins diesel engine to run rough or not start, low compression can also produce white smoke. Low compression affects heat generation, and a low temperature causes the fuel to burn incorrectly. You can check the pressure through a cold engine compression test.

Water or oil in the fuel: Clean fuel will burn properly, but if there is anything added in, it can prevent the fuel from burning as it should. If water or oil is in the mix, it may cause white smoke coming from the engine. Water can cause major disruption, so it needs to be taken care of promptly.

If you observe white smoke coming from your Cummins diesel engine, your first reaction should be to check your fuel supply, pumps and pressure strength.

6. Blue Smoke

If you observe smoke coming from your engine with a blue tint, the engine is burning oil rather than just diesel. Blue smoke coming from your Cummins diesel engine may be the result of:

Worn piston rings or cylinders: Wear that occurs naturally over time can damage or put cracks in seals. This can cause the oil to leak and burn, resulting in blue smoke.

Faulty valves or stem seals: The same concept as wear on cylinders and piston rings applies here, but the oil leaks from a different source. To properly diagnose where the oil is leaking from, check all valves, seals, pistons and cylinders. Replace or tighten anything that is causing a leak.

Over-filling the engine oil: If the engine oil reservoir is too full, it can cause oil to burn along with the fuel, creating blue smoke. You may have to empty some of the oil if this is the case.

Faulty lift pump or injector pump: If the injector and lift pumps are faulty, they will allow oil from the engine to mix in with diesel fuel. The result of the engine burning this blend is blue smoke, which means you may need to replace the pumps.

If you’re still unsure how to identify the issue, the most common sources involve fuel quality, pressure and sensors. Troubleshooting Cummins diesel engines takes a thorough knowledge of how they operate and how to identify warning signs. If you want an accurate diagnosis, a professional opinion is your best bet.

Before you need to go through the stress of diagnosing no-starts in your Cummins marine engine, you should look into creating a detailed maintenance schedule.

Cummins Engine Maintenance Tips

One way to avoid needing to fix your Cummins marine diesel engine problems altogether is by properly maintaining your equipment. Maintenance is a preventative practice, so it focuses on keeping your Cummins diesel engine clean, repairing or replacing aging Cummins parts before they give out and making sure it continues to run smoothly overall. Preventative maintenance keeps your engine healthier longer and costs you less than making repairs after the damage has been done. 

The best way to keep track of when your vehicle needs a checkup and what should be replaced or refreshed is by creating a scheduled timeline. Keep a check sheet or calendar of short and long-term maintenance milestones organized by how frequently you should be performing them. We suggest breaking down your schedule into the following sections:

1. Daily

Coolant: Check the coolant level and correct it if necessary.

Water separating fuel filters: Check the filters and drain them.

Oil: Check the engine and marine gear oil levels and correct if necessary.

Strainer: Clean out the seawater strainer.

2. At 125 Hours or 3 Months

Repeat: Complete the listed daily checks first.

Pump belt: Check the seawater pump belt and adjust if necessary.

Electric: Check all electrical connections to see if they’re secure.

Air cleaner: Check for any restrictions and correct if necessary.

Zinc anodes: Check the zinc anodes and replace if necessary.

3. At 250 Hours or 6 Months

Repeat: Complete the daily and 125-hour checks first.

Replace: Change the engine oil and filters, engine-mounted fuel filter and the fuel and water separator element.

Antifreeze: Check the antifreeze concentration percentage and correct it if necessary.

Pump and impeller: Inspect the seawater pump and impeller for wear.

4. At 500 Hours or 1 Year

Repeat: Complete the daily, 125-hour and 250-hour checks first.

Check: Inspect the seawater pump, batteries, air cleaner element, engine mounting bolts and vibration isolators and make repairs or replacements as needed.

Belts: Check on the belt tensioner and drive belts and correct or replace them if necessary.

Flush: Look over and flush out the aftercooler, gear oil cooler and heat exchanger.

With this fully comprehensive list of what and when your mechanic needs to check, you can lengthen your Cummins diesel engine’s lifespan and keep it running with a low risk of encountering any issues. Sticking to a preventative schedule is less of a hassle for you and your mechanics, as the steps are routine in preventative care. All you have to do is be sure to take it to a reliable resource for Cummins engines.

Engine Troubleshooting With Help From Diesel Engine Solutions

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