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P0172 trouble code and fuel in oil plus rough idle and engine run-on

GM has a fix for a P0172 trouble code and fuel in oil, rough idle, and engine run-on

GM has issued a service bulletin #PIP4783B to address a P0172 trouble code and fuel in oil, rough idle, and engine run-on the GM vehicles listed below. Before you perform any diagnostic tests, pull the engine oil dipstick, and check for the presence of fuel. GM has determined that a P0172 System Too Rich (Bank 1) may be caused by a defective high-pressure fuel pump that’s leaking fuel into the crankcase when the plunger is depressed.

GM advises you to start your diagnostics by isolating the fuel tank fuel pump and performing a leak down test on the fuel system to rule out a pressure loss from the primary (in-tank) fuel pump. If the primary fuel pump holds pressure, close off the fuel line to the primary fuel pump, and conduct a leak down test on the high-pressure fuel pump. If the high-pressure fuel pump shows a pressure loss during the leak down test, it must be replaced, along with the fuel inlet hose.

But why would a leaking high pressure fuel pump cause a P0172 trouble code, rough idle and engine run-on

A P0172 trouble code indicates the engine computer has detected a “System Too Rich” condition, meaning there is too much fuel in the air-fuel mixture. That’s usually caused by too little air or a leaking fuel injection. However, in this case, if the high pressure fuel pump is leaking raw fuel into the oil, the hot oil would vaporize the fuel and the PCV system would carry those fuel vapors back into the intake manifold where they’re burned. That extra fuel would be noticed by the oxygen sensor and the PCM would cut back on fuel, causes a rough idle and engine run-on because it’s getting fuel from the oil instead of the fuel injectors.

Vehicles affected by P0172 trouble code are listed in Service Bulletin #PIP4783B

2010-2012Buick Lacrosse 2.4L engine
2011 Buick Regal 2.4L engine
2012-2017 Buick Verano  2.4L engine
2012-2016 Chevrolet Captiva 2.4L engine
2010-2017Chevrolet Equinox  2.4L engine
2010-2017 GMC Terrain 2.4L engine
high pressure fuel pump that leaks and causes a P0172 code
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