KD Fuel Pressure Gauge
for the Duramax 6600
By Jim Bigley

As diesel fuel injection systems become more complex and more sensitive to fuel quality, owners have become more tuned-in to the nature of diesel fuel systems. Being able to check fuel pressure (vacuum) can help an owner diagnose a future engine running problem or allow him to monitor the condition of the fuel filter in an on-going basis. This not only can save you a lot of frustration in the high-stakes game of vehicle service, but can save you money on fuel filters by not replacing them till the fuel restriction gauge tells you it's time.

For whatever reason, you can get a bad load of fuel at any fuel station, or you can be exposed to poor fuel quality on a chronic basis. Unless you have a fuel pressure gauge, we recommend changing the fuel filter at the recommended service interval (up to 15,000 miles) or whenever your engine experiences a running problem, no matter how long it's been since the last service interval. This is part of why a fuel pressure gauge is important. Over the course of your diesel ownership, a fuel pressure gauge will save you money by not having to guess about the condition of your fuel filter.

Diagnostic Trouble Code P0093 (2001) P1093 (2002+): This DTC indicates that the difference between the actual fuel rail pressure and the commanded rail pressure is greater than 20 MPa. While this could also indicate a "Large Fuel Leak Detected", which is related to excessive fuel return flow (bad fuel injector(s) or loose fuel return fittings), this could also indicate a restricted fuel filter or fuel supply line. This code could occur when more than 5" of vacuum is present at the fuel pressure test port shown below. The KD Fuel Pressure Gauge can be indispensable when troubleshooting these and other fuel related problems.

The KD Fuel Pressure Gauge comes with a quick-release Shrader valve matching that used on the Duramax fuel pressure test port. Also included as part of the gauge assembly is a Shrader fitting plug, to help prevent contamination while the gauge is not in use. Remove the plug by retracting the Shrader collar, then pull the plug out of the fitting as shown.

GM equipped all Duramax engines with a Shrader valve fuel pressure test port. At the factory, this is used to bleed the fuel system prior to running the engine the first time, and it provides dealership technicians a convenient way to check fuel pressure as part of any related diagnostic procedure.

Unscrew and remove the black plastic cap, then snap the KD Fuel Pressure Gauge onto the exposed port by retracting the Shrader collar on the gauge. Replace the black plastic cap once your tests are complete.

When should you change your fuel filter? Current recommendations are to first establish a baseline for fuel filter restriction after installing a new fuel filter. Then, change the fuel filter once the gauge indicates an increase of 3"-4" of vacuum above the baseline. In our tests, GM's newly recommended dual-element fuel filter produces about 2" of vacuum when new, which will slowly increase over time as the filter becomes more restricted.
     This photo shows the amount of vacuum restriction present after about 8,000 miles on the newest dual-element fuel filter. Readings are taken with the engine idling, a full tank of fuel and with the truck sitting on level ground. A full tank reduces the number of variables that could affect your readings.

The Duramax fuel system operates without an electric fuel lift pump. As such, a slight negative pressure (vacuum) will be present in the fuel system, and is why we need to measure vacuum (in inches of mercury). This is the amount of suction necessary to pull fuel from the fuel tank through the fuel lines and fuel filter.

This photo shows the fuel restriction present in our Lil Red Duramax project truck. After 18 months and 4,000 miles, we're still running the original single element OEM filter. We'll be installing a new dual-element fuel filter before long.

The KD Fuel Pressure Gauge can also measure pressure up to 7.5 psi. This is useful for those who install an aftermarket electric fuel lift pump for a performance application or when installing an aftermarket auxiliary fuel filter. Kennedy Diesel offers complete electric fuel lift pump kits as well.

The KD Fuel Pressure Gauge is a high quality tool. From the glycerin-filled gauge face, to the quality machined fittings, to the sturdy padded hard case, this highly recommended instrument is something every Duramax owner should have in his toolbox.   TDP


Kennedy Diesel
Call: 715-255-9433 Fax 715-255-9499

The Fuel Pressure Gauge can be found on Kennedy's Duramax Parts/Tools page.

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