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Diagnosing An Idle Issue Without An Engine Light or Error Code, Pt 3

EGR valve

In this final segment of this series, we'll look at the PCV and EGR systems and how they can negatively affect your idle. In our first part, we looked at air intake. In the second part, fuel injection. In this part, we'll look at some accessories that can be affecting the fuel:air mixture.

The first is the PCV, or positive crankcase ventilation valve. We start here because the test for it is simple and doesn't often require the removal of anything to conduct. To nutshell what a PCV valve is: it's the valve hat opens and closes to relieve pressure from the crankcase (engine block). That pressure is relieved into the air mixture for combustion, usually into the intake manifold. The PCV valve is a very simple device that opens and closes based on pressure and engine RPM.

To check the PCV valve, simply find its vacuum connection and pinch it off. If cutting off its vacuum supply causes the engine to run better at idle, you've likely found your problem and should replace the PCV. Not very scientific and not always 100% guaranteed, but if the PCV is the issue, this is the most likely way to find it.

Our next look will be at the exhaust gas recirculation (EGR) system. It's not common for the EGR to cause an idle issue unless the valve has become stuck in the "open" position. This is a relatively uncommon problem, but it can happen.

In essence, the EGR pipes some of the exhaust gases back into the air intake so that they can be reburned, which improved emissions. Normally, the EGR only opens when the engine is running under load (moving the car) and at operating temperature. There are both manual (mechanical) and electronic EGR valves. We've talked about testing an EGR valve before.