Solving Common Rail Injection Issues

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Solving Common Rail Injection Issues

We’ve gotten this call countless times:

Customer: Hello, I just put a set of injectors in my Dodge 6.7L Cummins, Chevy 6.6L Duramax, or my 6.4L/6.7L Ford pickup and I am still having the same issue. The injectors I received must have been faulty.

Our response: Are the injectors re-manufactured? Do they have new nozzles? Are the nozzles O.E. (in most cases Bosch)? Did you check rail pressure? High pressure pump supply pressure?

If the injectors were re manufactured by a Bosch authorized Fuel Injection center, send them back to the shop which you purchased them and had them tested. These facilities are equipped with the latest Bosch Common Rail Testing technology. If you bought injectors from a generic reman facility or site, or a company without fuel system expertise and test equipment, they may or may not deny responsibility and claim installation error.

Before you pull the injectors back out of the engine, here are a couple common issues we see with the Diesel Common Rail Injection systems to be checked:

Check your desired vs. actual rail pressure

Many scanners with live data including the newer snap-on scanners will plot out desired fuel rail pressure vs. the measured fuel rail pressure. Take the truck for a drive (on the highway if you can) and put the engine under a load. Does the desired fuel rail pressure match the actual fuel rail pressure at idle, under heavy acceleration, and on hills? If the actual rail pressure is not able to reach the desired rail pressure, check for a high pressure fuel leak. Check to make sure the pressure relief valve in the rail didn’t relieve pressure or is hanging open. If it did, I would suggest having a qualified Diesel Technician look at possible reasons as to why this would happen. An injector nozzle hanging open or an injector with an eroded ball seat and high fuel return will also do this, but we will assume this is not the case assuming they were purchased from a Bosch Authorized fuel injection center.

Make sure the MPROP is functioning properly (aka FCA, fuel control actuator, fuel pressure regulator)

The MPROP controls the amount of pressure the High pressure fuel pump (or CP3) outputs. The ECM receives electrical signals from the MPROP which will be interpreted as a percentage. Make sure the MPROP is functioning properly by watching this percentage fluctuate as you drive down the truck down the road. If you don’t have access to this information, take your vehicle to a qualified Bosch Drive-in service center or other reputable shop for repair.

Check fuel supply pressure to the high pressure fuel pump

Plumb a mechanical gauge into the fuel supply. If you are seeing a vacuum or near zero fuel supply pressure, you may have a fuel restriction between the tank and the high pressure pump. In some cases, the supply pump may be nearing the end of its’ useful like. Take the vehicle or truck on a test drive and monitor this supply pressure. Sometimes, data looks great at idle but under load is when the issues arise.

Check the injector harness for failures

The electrical signals the injectors receive are sent through the injector harness which is exposed to all of the oil and heat underneath the valve cover in many cases. There are some exceptions. Be sure to inspect the harness for cracks in insulation. Is there motor oil leaching into the harness from one of the connection plugs? Is the harness shorted anywhere?


If you have ran through all of the possible solutions above or you don’t have the equipment or resources necessary to perform these diagnostics, take your vehicle to a qualified Bosch Drive-in service center or other reputable shop for repair.

Diesel Pro is a Bosch authorized drive in service center and a Bosch authorized Fuel Injection service center. We can perform all of the above diagnostics and also test all Diesel common rail injectors using Bosch test equipment. We are happy to assist you with these services.



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