5 Things to Consider When Diagnosing a Common Rail Fuel Injection System

Making the transition from working on the old-school, conventional or mechanical diesel fuel injection systems, to the modern, computerized High Pressure Common Rail, or HPCR fuel system, requires a shift in your thinking and in your diagnostic process.

We are all familiar with the phrase, ‘you can’t teach an old dog new tricks’ but we are here to smarten you up by giving you 5 thing to consider when diagnosing a common rail fuel injection system so the old diesel dogs out there can still perform a few new tricks.

Tip 1: Be Sure to Not Restrict or Pinch the Fuel Return Line

Those in-the-know when it comes to pump tuning understand that the early Bosch fuel pumps (‘VE’ pumps, and ‘P’ pumps) can be “tricked” into increasing the injection pressure simply by restricting the return fuel flow. The style of the pump and its lower operating pressure allowed this “modification”.
The same is not true for HPCR systems.

If you attempt to restrict the fuel flow to an engine with an HPCR fuel, the pump will build up so much housing pressure that it will literally blow the driveshaft seals and inspection plugs right out of the pump! The CP3 pump depends heavily on the fuel return line to relieve this excess fuel pressure that builds up inside the pump.

We can’t stress enough: DO NOT restrict the return fuel line when performing diagnostic checks. It’s important to note that in some cases (with modified CP3 pumps) you will actually need to increase the size of the return line to handle the additional flow.

Tip 2: Do Not Over-pressurize the Fuel Inlet.

In some cases, more is better – like making money or taking a holiday, but that is not the case when it comes to HPCR fuel pressure; too much inlet fuel pressure can create similar issues as restricting the return fuel line. There is a ‘multiplication effect’ to the fuel pressure going into the pump – feed fuel into the pump at too high a pressure and it will develop tremendous pressure (up to 40,000PSI), most likely popping seals, damaging the CP3 pump internally, or damaging the injectors. If you run a CP3 in a high-performance application, we recommend installing a fuel supply pressure gauge to ensure the correct fuel pressure at all times.

Quick Reference: 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: Fill Up That Tank to Minimize Condensation and Water Contamination.

We all know that Diesel fuel is hygroscopic, meaning it absorbs water. Therefore, it comes as no surprise to hear that the number one reason for diesel fuel system failures is, you guessed it, water contamination.

If your engine is going to be sitting unused for more than 30 days, we suggest filling up the fuel tanks completely. This ensures the tank has the least amount of air space possible which reduced condensation. It is also a good idea to change fuel filters more frequently if the engine sits around a lot.

Water, or condensation, forms on the walls of the fuel tank creating little moisture; the greater the difference in temperature, the faster the water collects. It takes approximately 28 – 30 days for condensation to form inside a fuel tank of a vehicle that has not recently been started. This moisture will accumulate in the fuel and begin to create rust and algae inside the tank, ultimately contaminate the inside all of the fuel system components.

Stanadyne Performance Formula diesel fuel additive is a great fuel stabilizer and we recommend using it when your vehicle will be in storage for extended periods of time.

Tip 4: Improve Filtration – Change Fuel Filters Regularly

HPCR injectors are meticulously built to exacting standards and are tested many times before they even make it into a box to be sold at the counter. If an injector does not pass these rigorous tests, it will have to be torn down to start all over again.

We really like Bosch because they use only the highest quality materials to ensure a long reliable injector life – measuring components to 4 decimal places. Protecting these extremely sensitive components from damage, excessive wear or contamination is critical.

While there are no additives to eliminate water from diesel fuel (alcohol-based products or methyl-hydrate products are not for use in diesel fuel), your first line of defense is the water-separating fuel filter. We recommend changing the fuel filter with every second oil filter change.

One of the concerns we have with OEM is that they install a fuel filter system that meets only the bare minimum standard for filtration protection. Improving the fuel filtration system is like buying an extended warranty, helping to avoid frequent trips to the dealer for repairs and savings you more than a few dollars.

Tip 5: Take Your Custom-tuned Truck Back to Stock During Diagnostic Testing

With custom software tuning for late model trucks now on the market, the diesel world has been catapulted to a new level of performance and customer expectations. While there are quite a few well-engineered products on the market to add performance and mileage improvements to your truck, if put in the hands of an untrained person, you could do some serious damage in a short period of time.

We always recommend returning any custom-tuned truck back to its stock or factory configuration to improve the accuracy and speed of diagnostic testing. Custom software tunes can disguise problems and make identifying failing components more difficult.

When the software tuning is returned back to stock, the OEM test values, diagnostic check procedures and specifications found in shop spec manuals will more accurately reflect what is happening with your engine so you can get to the source of the problem quickly.

Tuning modules should never be used to boost lost power issues on high mileage and/or worn out engines. While adding a tuning model to a stock truck will certainly make it run stronger, it should not be used to compensate for a poorly running diesel engine.

While not rocket science, these tips will help ensure that the very important basics are considered when working on an HPCR diesel fuel system, ensuring your time and efforts are put to their best use.