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While the 5.9 Common Rail Cummins is an extremely dependable engine, there are certain things to look out for.

What Breaks When: 2003-2007 Dodge Cummins

Diesel Performance Parts, Diesel Truck Maintenance, Dodge Cummins, Ram, Tips and Tricks

Written By: Lawrence “LT” Tolman

2003 marked the first year of a new body style pickup for Dodge, and to go along with its new looks, a new engine could also be found under the hood, and the major difference between it and the earlier 5.9 liter Cummins engine (and its Achilles heel) was the addition of common-rail fuel injection. While the 5.9 Cummins is a very robust engine, it does have a few weaknesses that can leave the truck stranded, but nothing that will lead to a catastrophic failure or meltdown. As part of our “What Breaks When” series, this week we’re taking a closer look at the 2003 to 2007 5.9-liter Cummins.

03-04 vs 04½ -07 What’s the Difference?

The 5.9 common-rail is divided into two separate groupings: the earlier 2003-04 models and the later 2004.5 to 2007. While they share most of the same architecture, there are a few key differences which make certain parts not interchangeable, and most of those had to do with fuel delivery. The early 5.9 CR featured an 8-hole injector nozzle with a wide angle spray pattern, and there were two injection events per stroke: pilot (which injects a small amount of fuel to pre-heat the combustion chamber before ignition) and the main injection event, which is where the engine’s power comes from. The pilot injection event is useful since it reduces noise and helps the main combustion event take place, and interestingly enough the addition of pilot injection is why common-rails are so much quieter than the mechanical engines which have only one injection event. The later 04.5 to 07 Cummins 5.9s featured a five-hole nozzle, a narrower spray pattern, and to match that pattern, a different shaped piston bowl needed to be used. In addition, the later 5.9 Cummins also featured a third, post-injection fuel event after the main spray, and the main purpose is to reduce harmful emissions. The third injection event is said to have a slight negative impact on fuel mileage, however with aftermarket tuning, the loss in mileage can be restored.

To ensure your engine lives a long, healthy life, its imperative to keep your injectors in tip-top shape.

On the earlier engine, the wider-angle spray pattern means the piston has to be closer to top dead center when the fuel is introduced, so when tuning there is a limit to how much injection timing and duration you can safely run. On the later trucks, the narrower spray pattern and piston design gives a little more margin of safety and allows for increased injection duration, since the piston will be in the “sweet spot” for more degrees of crankshaft rotation. While there are no inherent strength differences between the two piston designs, you have to be especially careful to monitor EGT on the early design engines.

Fuel System Failures:

Manufacturers found early on if you can inject diesel fuel into an engine at a higher pressure through a small orifice, you can greatly improve how quickly and efficiently the fuel will burn, and also reduce the output of harmful pollutants. In order to operate at pressures approaching 27,000 psi, the pieces inside common-rail injectors need to be machined within a couple microns of each other for proper tolerance. It’s impressive how modern manufacturing can mass-produce such precise and accurate parts, but those tight tolerances and high pressures create an injector which is very sensitive to contamination from water and dirt. If you know anything about diesel fuel, you’ll know it’s not inherently clean, it sometimes contains water, and usually has some dirt in it. If the fuel filters are not regularly changed, there can be a small amount of dirt and moisture that winds up in the injectors, and as a result, early common-rail 5.9’s have a bad rap for faulty fuel injectors, going out somewhere around the 150,000 mile mark. Symptoms can include hard (or no) starting, smoking, knocking, and even fuel in the engine oil. If you are experiencing those issues with your 5.9, chances are you have a failed injector and it’s time to replace them.

If you find yourself in need of new injectors, you have a lot of options. If you are considering turning your truck into a high-performance build, this may be a good time to consider a set of injectors that flows more fuel than stock for some added performance, like a 60hp set from BD. Or, if you just need a stock sized replacement, new Bosch Stock Flow injectors are an affordable and high-quality option to get you back on the road. But either way, be very cautious when ordering sticks for your truck and be absolutely sure of what year engine you have. The injector bodies are the same from ‘03 to ‘07, but remember the nozzles have two different spray patterns. Any 5.9 CR injector will physically install into any head, but if you install an earlier injector into a later engine or vice versa the improper spray angle will cause a rough running engine, excessively high EGT, low power, and likely will cause damage to your engine from the mismatch of components. If you are unsure which model you have, check the tag on the driver’s side of the valve cover for some help. If it’s labeled as a 305hp version, it’s the early, wide spray pattern injector, and if it says 325hp, it’s the later model, narrow pattern injector and piston.

The connector tubes originally found in 6.7 Cummins are finding their way into many 5.9’s due to the fact they completely interchange, last longer, and have better flow characteristics compared to the original 5.9 connector tubes.

And one last note on injectors: any time you replace or remove an injector from the head, it’s a smart move to replace the mating connector tube which comes from outside of the head and delivers high pressure fuel into the injector body. There is a very critical connection between the injector and the tube which is made from two precisely machined surfaces which can easily be disturbed during removal. While I have re-used connector tubes myself, it’s cheap insurance and gives peace of mind to simply replace them as part of an injector change. Plus, you can use the tubes from a 6.7 which have a larger inner diameter to support a little more horsepower if you’re into that sort of thing.

Once you have a brand-new set of injectors, (or if your existing set is still functioning properly) there is a way you can prevent injector failure from contaminated fuel: maintenance. Many new diesel owners are surprised to learn you are supposed to change your fuel filter on a regular basis, and so their truck runs for 30 to 40 thousand miles until the filter clogs and performance suffers. According to Dodge, every 15,000 you should be swapping out the fuel filter, but don’t settle for the cheapest one you can find, since so much is at stake. There are many different options on the market, and each can have a different micron rating, which is a measurement of the size of dirt particle the filter will trap. A great filter for any 5.9 common-rail is the Baldwin 7977. It has a rating of 5 microns, which is the best you can get for a stock filter housing, and it only costs $17 so you can’t really complain about price either. In fact, you should buy two so you can keep a spare on the shelf, since 15,000 miles can go by pretty quickly.

The Rest Of The Fuel System

Once you have the injectors squared away, you can rest easy since the big-ticket item on the fuel system is already taken care of, although there are a few smaller and much less expensive items on the common-rail 5.9’s that will likely fail at some point or another. Thankfully, the Bosch CP3 injection pump is very reliable, however the one user-serviceable item attached to the back of the pump is failure prone. It’s called a Fuel Control Actuator, or FCA for short, but it is also referred to as a regulator. While it ultimately regulates the amount of fuel pressure the pump puts out, it does this by controlling the flow of fuel going into the pump. Whenever there is low fuel demand like at idle and part throttle, a high voltage is applied to the solenoid, and the FCA partially closes, restricting flow, and keeping rail pressure at a lower level. As fuel demand ramps up, less voltage is applied from the ECM, more fuel is allowed to enter the CP3, and as such, fuel pressure raises.

The Fuel Control Actuator takes information from the ECM and essentially tells the CP3 how much fuel to deliver based upon the signals from the ECM.

The FCA is both an electronic and mechanical part, and there are several small moving parts inside which can become stuck or clogged. A faulty FCA can cause a variety of issues, from rough idle, low power, a deviation from desired rail pressure, and even a no-start condition. It’s a pretty common failure, and luckily a new FCA is very easy to install on a Cummins, and it’s also affordable at a hundred and thirty bucks.

One other part on a common-rail 5.9 that’s known to fail would be the Fuel Pressure Relief valve. Whether the valve seat becomes jammed with dirt or the internal spring has failed, the result is a valve that is stuck open and little to no rail pressure, since all the fuel will be returned to the tank. The symptoms are very similar to those of a failed FCA: low power, rough running, or even no-start. Luckily, it’s also very affordable to get a new genuine Bosch FPR valve, and again, the magic number is a hundred thirty bucks.

Unlike GM, Dodge was smart enough to equip their trucks with a lift pump which pulls fuel from the tank and sends it up to the injection pump. As with all best laid plans, the stock lift pump has a reputation for going awry. Early model 5.9’s had an electric lift pump mounted to the side of the engine block, and the later models had the pump in the fuel tank. Since the original block mounted pumps were so unreliable, many early model trucks were adapted by dealerships to run the later style in-tank pump with a retrofit kit, but even the updated in-tank lift pumps weren’t so great. Whenever your stock lift pump fails, I wouldn’t recommend replacing it with another stock pump, especially if you are looking for a bump in power. Instead, you should install higher capacity pump with increased filtration built in, to separate water and dirt, and pull any air out of the fuel. The FASS Titanium Signature Series lift pump will support an elevated power level, but more importantly clean up your fuel, and send nothing but pure, air-free diesel up to your CP3 and injectors. Remember, clean fuel is the most important thing to keeping your injectors living a long and happy life, so a filter and lift pump combo is really an important upgrade for any diesel trick. When ordering a FASS pump for your Cummins, just remember to select the correct option for whether you have a block mounted or in tank stock pump, since the plumbing will be slightly different.

Other Issues

If you don’t consider the fuel system, the 5.9 Cummins is a very reliable engine. The internal parts like the pistons, rods, and crank will last for a zillion miles even with some abuse. The turbos are very responsive and will give a long service life, but they don’t have a tremendous horsepower potential. Other than small wear and tear items like the water pump and belts and hoses, you can expect a lifetime of use from a 5.9 without doing anything more than basic maintenance. But that doesn’t mean there isn’t any room for improvement.If you don’t consider the fuel system, the 5.9 Cummins is a very reliable engine. The internal parts like the pistons, rods, and crank will last for a zillion miles even with some abuse. The turbos are very responsive and will give a long service life, but they don’t have a tremendous horsepower potential. Other than small wear and tear items like the water pump and belts and hoses, you can expect a lifetime of use from a 5.9 without doing anything more than basic maintenance. But that doesn’t mean there isn’t any room for improvement.

One disadvantage of an inline engine is coolant flow is somewhat limited near the rear of the engine. Since number six is at the end of the line, the coolant circulates around the rear sleeve much slower, allowing more heat to be built up, so if a heat related failure does occur (from aggressive tuning or ignoring high EGT), its usually found on the rear most cylinder. For a long time, that was just a fact of life, but now a Coolant Bypass kit from Fleece is the solution. It allows you to tap into a freeze plug on the rear of the engine, and provides a path for the warm coolant from the rear cylinder to flow right up front to the thermostat, which evens out the temperature across the block, and relieves pressure that can build up in the cooling system from excessive heat.

By having an equal distribution of coolant flow to the rear cylinders of your Cummins engine, you can safeguard against excessive heat developing at number 5 and 6 especially.

Speaking of heat, one part of the intake system on a Cummins that is the most restrictive is the intake horn. The stock part was designed to be about as aerodynamic as a brick, and the inside resembles a drinking straw. As we’ve discussed many times before, a restrictive induction system can lead to increased heat in the engine which could decrease longevity and fuel mileage, and this is no different on the Cummins. A few simple parts like a cold air intake, Monster Ram intake horn, free-flowing exhaust, and a cold air intake will together improve power and fuel efficiency, and lower the temperature of the exhaust gasses, all of which are great for your Cummins diesel.

Not As Bad As It Sounds…

By anybody’s standard, the 5.9 liter common-rail Cummins is a great engine and will give its larger displacement rivals a run for their money. There were a few teething problems, and the first CR Cummins did have a few inconvenient problems, but nothing that can’t be fixed with readily available parts and tools. And as long as you keep up on your filters and scheduled maintenance, you should expect several hundred thousand miles of trouble-free operation, no matter what path you take on your build. We’ll get into high-performance options another time, but the 5.9 can be built to be a heavy-hauling tow rig, a fuel sipping daily driver, or a fire breathing 1,500hp monster, and the best part is the inline design also makes them the easiest diesel engine to work on from the Big Three. Plus, if you drive one with the tow mirrors flipped up, you can earn one of those giant “C” decals to place in your back window…


2 thoughts on “What Breaks When: 2003-2007 Dodge Cummins

  1. Solid write-up, Josh! I considered myself savvy in my early 03 HO Cummins and this article showed me how little I actually know about the motor. Thanks for the straight-to-the-point, well written article.

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