Common Rails Common Problems: Typical Diesel Troubleshooting

Although common rails are an improvement from prior types of fuel systems, they’re not without some issues. Many of these problems, however, stem from things unrelated to the design of a common rail system, which common rail owners should be aware if they want to minimize expensive repairs. Be it common rail Cummins problems, fuel control actuator issues, 6.7 low fuel rail pressure codes, or the like, the following should help you with common rail diesel troubleshooting.

Scheid’s test cell evaluates the flow accuracy of a CP3 pump and common rail injectors. It evaluates the pump’s rpm and has a common rail attached with sensors, so the pulse width can be adjusted to three or four fuel set points.

Before getting into those details, we should first clarify how a common rail differs from other types of fuel injection. Then we’ll focus on a few common concerns encountered on common rails.

Note that space doesn’t permit covering every possible fault code on an OBD scanner, or an extensive list of SAE diagnostic procedures, as that would require a massive manual. Instead, we’ll show in the accompanying photos some of the basic steps and tools used to evaluate common rail injectors at an experienced shop, Scheid Diesel. This well-known company not only services a large number of hard-working trucks but also builds championship-winning drag racers and sled pullers. Thus, their technicians know their stuff and are a great resource for repairs and performance upgrades.

Subscribe Our Weekly Newsletter

OLD VS. NEW

Starting with the basics, a high-pressure common rail fuel injection system (HPCR) used in the GM and Dodge heavy-duty pickups since the early 2000s is quite different from the previously used Pump- Line- Nozzle (PLN) systems (like those found on a P-Pumped Cummins for example, but not like those found on HPOP equipped Power Strokes). Many older diesel injection systems only create about half the fuel pressure modern engines do, and older injectors send the fuel through much larger passages. Also, modern common-rail diesel injectors can fire two or three times per engine cycle, doubling the wear on the injector compared to diesels of the past—hence the need for more conscientious maintenance.

In a PLN system, an inline injection pump handles the following functions: delivery of a pressurized and metered quantity of fuel to the injectors once for each power stroke; control of fuel injection timing; governor control of engine speeds and fuel delivery quantity related to the engine operating conditions.

COMMON RAIL OPERATION

In contrast, with the HPCR system, the engine or power train control module (ECM or PCM) governs the rail pressure, fuel metering, injection timing and engine speed control. Several steps are involved, though, in getting the fuel to the injectors. In a Duramax or Cummins diesel, a low-pressure pump first draws fuel from the tank and pressurizes the fuel to around 10 psi.

Then a PCM-controlled inlet-metering valve (commonly referred to as the fuel control actuator or fuel-rail pressure regulator) controls the amount of supply pressure to the high-pressure pumping elements. This step regulates both the volume and output of the high-pressure (HP) pump.

From the HP pump the fuel flows to the rail where it accumulates prior to flowing to the injectors. Then the injector receives high-pressure fuel from the rail and injects atomized fuel into the combustion chamber as requested by the PCM.

1. To check the flow volume on the injector return line of this Duramax, a hose is attached to the injector return fitting and fed into a graduated cylinder.

2. Note the 20cc mark written in black on the cylinder. If the flow exceeds that amount per minute, then there’s likely some wear on the high-pressure seal of the injector.

INHERENT PROBLEMS

Given the complexity of the steps involved, several areas can be prone to failure, but often due to a single simple problem: contaminants. As Todd Emmert, machine shop manager for Scheid Diesel, points out, the problem usually boils down to three key words: “Fuel, fuel and fuel.” He’s not kidding here. Aside from debris and particulates, “There’s too much emulsified water in it.”

In other words, that gold tint you might see on fuel components doesn’t glitter. It’s actually a film or residue (what gasoline guys sometimes call “shellac” or “varnish” on a carb) created by corrosion from too much moisture. What this does to an injector is erode the injector’s valve seat in the control valve assembly, degrading the precise flow of fuel. It’s a critical component, since common rail systems have much higher injection pressures, and tolerances are measured out to five-place decimals. This minute thickness makes a human hair look like a tree trunk by comparison.

In the valve assembly, fuel passes through a tiny orifice at very high pressures. The opening is sealed by a check ball only 1mm in diameter. Contamination from water and debris have an abrasive effect on the orifice, grinding the surface, which quickly and inevitably leads to a poor seal between the valve and the check ball. This in turn results in poor injector performance including starting issues, reduced fuel economy and performance, and rough running.

The presence of water also impedes lubricity, resulting in metal-on-metal contact, Emmert observes. Where does the moisture come from? He says that biodiesel is a typical component of most No. 2 diesel (whether it’s labeled as such or not) and it tends to attract water droplets. But moisture can also come from condensation of outside air, leaky storage tanks at a fuel station, or even rainwater puddling on top of an auxiliary tank in the bed of a pickup

Sometimes the HP pump is mistaken as the cause of hot starting, low-power (or no-power) conditions. To avoid any unnecessary repairs, the first thing Scheid’s shop technician Daryn Clapp does is check for diagnostic trouble codes (DTC). The injection balance rate will indicate low compression in a cylinder by displaying the crank-speed values on the downstroke of the piston.

3. To diagnose a poorly running engine, a scanner (seen here, a GM Tech2) displays the OBD fault codes and also can display the injector balance rate.

4. The Edge tuner installed on this Chevy also provides data on the injection balance rate of the Duramax.

5. Shown here are all the high-pressure parts of a common rail system. These injectors are in various states of assembly. On the far left (top) are the solenoid, along with an air gap shim, armature, valve set with ball seat, seals and nozzle.

6. Here’s a close-up of the ball seat of the valve set. Look closely, and you’ll see a small ball on the table, directly below. This 1mm pellet is a critical component in the functioning of the injector. If the seat for the ball is even slightly damaged (usually by moisture or debris) the injector won’t function properly.

7. The yellow tint on the ball seat is Fool’s Gold. This tint is an unwanted film consisting of fuel residue and contamination and must be removed with solvents and ultrasonic cleaning for the injector to operate correctly.

8/9/10. Note the clean, silver-colored pin for the ball seat. An air gauge measures clearance, so that it doesn’t exceed 6,000 psi. Otherwise the injector would deliver too much fuel. This gauge measures in megapascals (MPa) and bar numbers, which can be converted to a more familiar psi number. For instance, the 24 MPa shown on the gauge equals 240 bar, which is the same as 3480 psi. Typical working pressure on a common rail system is about 1600 to 1800 bar, or 23,000 to 26,000 psi.

FINDING THE PROBLEM

If the fuel supply system is good, he then might do a quick test of the HP pump. He starts by removing the high-pressure discharge line from the pump, attaching a hose to the line and cranking the engine over until fuel starts flowing out. Then he collects and measures the fuel discharge by cranking the engine for ten seconds three times. If the cranking speed is around 150 rpm, the collected amount should be 70 ml, and at 200 rpm, it should be around 90 ml. If the discharge volume is low, a bad HP pump might be the problem.

The scanner might indicate a short in the solenoid on the injector, by indicating there’s too much resistance in the stator. That can be caused by thermal breakdown on top of the injector, or “just bad luck,” as Clapp puts it. There’s no fix for a broken solenoid—it has to be replaced.

He’ll also check the fuel supply system by measuring the return fuel from the injectors. The method used is fairly simple and gives a clear indication of an injector problem. The return line is disconnected and fed into a graduated cylinder. If the flow is too high (no more than 20 cc per minute) the likely culprit is a worn high-pressure seal.

In some cases, though, such as on a 5.9L Cummins, the issue might be caused by loose retaining nuts on the HPC (High Pressure Connector; part number 4929864), aka cross-over tube. Another possible issue on a 5.9-liter Cummins in particular is with the seal on the cross-over tube on the injector body can be reused if the sealing area of the tip is not damaged. Reusing a good HPC tube not reduces the cost of the repair, but also may save some troubleshooting time as well.

High return flows or connector tube/injector problems can be pinpointed to a specific cylinder by disconnecting one injector line at a time. Rechecking the return flow rate, rail pressure, or trying to start the engine can identify a leaking connector tube or injector.

11. This gauge is fitted on the intermediate assembly of an injector to measure the air gap between the solenoid and armature, which can be adjusted with a shim.

12. These four precision gauges are used for evaluating the air gap, and both the armature lift and nozzle lift of an injector.

13. Certain components are so precise that they can’t be handheld for measuring, as the heat from human skin will throw off the calibration. Instead, a micrometer holder measures the air gap shim (the ring-shaped item located top center). If the air gap is too large, the shim can be thinned slightly with 2,000-grit hand lapping.

14. On the Scheid test cell, the solenoids are clearly visible on the top of these injectors for a 5.9L Cummins head.

15. The lines on the back of the injector holders for the test cell feed into additional sensing units that check the return flow of the fuel.

AN OUNCE OF PREVENTION

So overall, what can a diesel enthusiast do to prevent problems on a common rail diesel? Filling up at a well-traveled, name-brand truck stop is a good place to start, Emmert advises. The fuel is likely to be fresher and of better quality.

Check your fuel filters and water-separators often (10,000 to 15,000 miles, or every other oil change). Make sure the water filter screens down to the correct micron level. Also, don’t wait for the “water in fuel” dummy light to come on. Emmert says he’s found a couple teaspoons of water in a separator long before seeing any warning on the dash. And the separator should be checked at a standstill, preferably after the engine has been shut down overnight.

Consider using some water-eliminating fuel additives. Scheid prefers Stanadyne’s performance and lubricity formulas, partly because this company makes pumps and knows how they function and can wear out. He says Scheid is also currently doing long-term testing of Stanadyne’s cleaner for common rails for benefits on DPF maintenance, and the company feels that additive can be of help, as well.

All told, keeping your common rail operating smoothly can be as simple as avoiding that old acronym about computers: “GIGO—garbage in, garbage out.” In other words, a clean (and dry) diesel is a happy diesel. DW

16/17. Scheid technician Daryn Clapp carefully monitors the test cell for functioning of the CP3 pump, common rail, and fuel injectors.

18. A digital microscope provides an extreme close-up of an injector valve seat to examine for any possible damage from water or debris.

19/20. This extra-fine close-up shot with a digital microscope shows what water and debris can do to an injector valve seat. New on the right, damaged on the left. There’s zero tolerance for this sort of damage. If it looks bad, it is. Even a small amount of scoring on the surface is like a finger sized hole in a levee, just waiting to develop into even bigger problems.

SOURCE:

Scheid Diesel . 800.669.1593 . ScheidDiesel.com