tips to diagnose a common rail fuel injection system

5 Basic Tips to Help Diagnose a Common Rail Diesel Fuel Injection System

Shift your thinking. Making the switch from working on conventional, mechanical diesel fuel injections systems to today’s modern High Pressure Common Rail systems required a change in your thinking and in the process you use to diagnose performance issues.

While it is common to believe that you “can’t teach an old dog new tricks”, we’re here to help you change the way you approach your diagnostic testing on a common rail diesel fuel injection system. Below are 5 basic tips you can use to diagnose a common rail fuel injection system.

Tip #1: Don’t Pinch or Restrict the Fuel Return Line

Those that have been around the block a time or two know that it was easy to “trick” the early Bosch fuel pumps (‘VE’ pumps, and ‘P’ pumps) into increasing the injection pressure by restricting the return fuel flow. You could get away with this type of mod because of the style of pump and the lower pressures they were limited to.

The same can not be said for an HPCR system.

Attempting to restrict the fuel flow of the CP3 pump in an HPCR fuel system will build up so much housing pressure that you will actually blow the inspection plugs and driveshaft seals right off! The CP3 pump relies on the fuel return line to relieve the excess pressure and send it back to the tank.

Remember, do not restrict the flow of the return fuel line when performing diagnostic testing. In fact, in some cases (with modified CP3 pumps) you will have to increase the size of the return line to handle the additional flow.

Tip #2: Don’t Blow It by Over-pressurizing the Fuel Inlet.

In some cases, more is better – like a Las Vegas buffet or a yacht in the Mediterranean, but that is not the case when it comes to HPCR fuel pressure. Excess inlet fuel pressure may create a similar problem as you see when you restrict the return fuel line. There is a ‘multiplication effect’ to the pressure going into the pump – feed fuel at too high a pressure and it will develop tremendous pressure (up to 40,000PSI) resulting in popped seals, a damaged CP3 pump or fuel injectors.

If you’re running a CP3 in a high-performance application, we strongly recommend installing a fuel supply pressure gauge to monitor the pressure at all times.

Here’s a quick reference guide for recommended feed pressures for CP3 and CP4 equipped engines:

  • Dodge/Cummins ’03 – ’16 (5.9L & 6.7L) no less than 8 PSI / no more than 15 PSI.
  • GMC/Duramax ’01 – ’16 (6.6L) no less that 8 PSI / no more than 10 PSI.
  • Ford/Powerstroke ’11 – ’16 (6.7L) no less than 8 PSI / no more than 10 PSI.

Tip #3 – Fuel In, Air Out: Prevent Condensation from Forming

We all know that diesel fuel is hygroscopic (how’s that for a big word?), which simply means that it tends to absorb water. So, it is no surprise that the primary reason for diesel fuel system failures is water contamination.

If you know your diesel engine will be sitting around unused for more than 30 days, it is a very good idea to fill the tanks up completely. Trust us, that might sound expensive now, but it’ll save you money in the future. When the tank is full of fuel there is less room for air which reduces the possibility of condensation.

You can further protect your engine by regularly changing the fuel filter. When water (condensation) collects on the walls of the fuel tank, it creates moisture droplets during the natural heating and cooling process of the surrounding air. The greater the temperature fluctuation, the faster the water collects. This can happen in the span of 28 – 30 days. The moisture will accumulate in the fuel and begin to create rust and algae inside the fuel tank, ultimately contaminating the inside all of the fuel system components.


During extended storage periods, we recommend using Stanadyne Performance Formula diesel fuel additive which contains a fuel stabilizer component and a mild biocide, both which extend the life of stored fuel and help to prevent algae growth.

Tip #4: Improve Filtration with Regular Fuel Filter Changes

Built to exacting standards, HPCR injectors are tested multiple times before they even make it into a box to be sold at the counter. If a diesel fuel injector does not pass the rigorous testing procedure, it must be stripped down to start all over again.

That’s why we choose Bosch. They use only the highest quality materials to ensure the longevity of your fuel injector (measuring components to 4 decimal places!) while also protecting these sensitive components from damage, excessive wear or contamination.

While there are currently no additives to eliminate water from diesel fuel (alcohol-based products, or methyl-hydrate products can’t be used with diesel), you can rely on your friend the water-separating fuel filter. We suggest changing your fuel filter with every other oil change.

One of the things we don’t like about OEM’s is their low standard of quality. They do just enough to meet the bare minimum filtration protection and nothing more. If protecting your investment is a priority for you (as it should be) and you want fewer trips to the dealer for repairs, improving your diesel fuel filtration system is kind of like buying an extended warranty plan. Your engine, and your wallet will be happy you did.

Tip#5 – Back to Basics: Return Custom-tuned Trucks Back to Stock for Diagnostics.

It’s a whole new world of diesel performance. Custom software tuning for late model trucks has taken customer expectations to new heights. If enhancing your truck’s performance is what you’re after, there are several well-engineered products on the market. However, put your truck in the hands of an untrained person and they could do serious damage in a short period of time.

To improve the accuracy and speed of diagnostic testing, we highly recommend returning any custom-tuned truck back to stock (or factory) settings. By masking or covering-up performance issues, custom software makes finding the actual problem much more difficult.

When diesel engine tuning is returned back to normal, the OEM test values, diagnostic check procedures and specifications in the shop manuals will more accurately reflect what the tests are showing, getting to the source of the problem more efficiently.

Sure, adding a tuning module to a healthy, stock truck will make it run stronger, they should never be used to make up for lost power on high-mileage or worn-out engines.

While these basic tips to diagnose a common rail fuel injection system are common sense, they do help ensure those old diesel dogs still perform a few new tricks.

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