Low rail pressure is one of the most common cause of a limp mode in the 2001 – 2010 6.6 Duramax. DTC codes P0087, P0093 or P1093 may set due to low rail pressure.
Low rail pressure could be caused by a restricted fuel filter, air in the fuel, a bad pressure relief valve, excessive return fuel from the injectors or a worn CP3 pump. The least likely cause would be a fuel rail pressure sensor.
Fuel Supply and Fuel Filter
Check the fuel supply side first. Connect a gauge to the test port and check for excessive restriction. The “Filter Life Indicator” may falsely set if the rail pressure is too low. We often see this set within 500 to 2000 miles of installing a new filter. Vigorously shake the fuel filter into a clean container and look for contamination. If the filter is not restricted, or severely contaminated, then you likely have low rail pressure, but it may not be low enough to set a DTC. Or the filter life indicator may set with a low rail pressure code, leading you to incorrectly assume the filter is the cause.
Rail Pressure Relief Valve
The pressure relief valve, or limit valve, if bad, will usually leak at higher rail pressures. The relief valve is in the system to prevent excessive rail pressure. Excess rail pressure could cause an injector to fail and dump the rail pressure into one cylinder, which could cause a hydro lock and engine damage.
Connect a clear line to the relief valve return, ramp up rail pressure and watch for a leak. If there is any fuel leaking from the relief valve, then it will need to be replaced.
Even if you replaced it with a block off plug, you should still check for a leak. We have seen several block off plugs leaking into the return, because they did not seat correctly into the rail.
High Pressure CP3 Injection Pump
On vehicles with more than 200,000 miles on the high-pressure pump, we commonly see the CP3 pump cause low rail pressure. It fails to maintain enough rail pressure when the vehicle is pulling a load up a hill, and your Duramax may go into a limp mode. If the actual rail pressure is between 2176 psi or 2900 psi (depending on model year) below desired pressure, this would cause a P0087 or P0093 or P1093 code to set.
Depending on year of vehicle: P0087 will also set if the rail pressure drops below 3263 psi. P1093 will set if the difference between commanded rail pressure and actual rail pressure is more than 2900 psi.
Worn CP3 pumps that are not producing enough rail pressure under load usually do not cause hard starting. Buy Duramax CP3 Pump
Excessive return fuel from the injectors can cause low rail pressure. If the injectors are worn enough to cause excessive return and low rail pressure, they will often cause a hard start. Perform the “Enhanced Injector Return Flow Test” to verify excessive return. Buy LB7 Injectors – Buy LLY Injectors – Buy LBZ Injectors – Buy LMM Injectors