i replaced my 2002 XJR egr valve, brand new one this time but still check engine light came on again after about 30 miles. Obd scanner says the same thing P0400. I don't know what to do next. Thanks.

 

Hi Maniqz


Did you have any symptoms before you replaced the valve? Hard starting, poor acceleration, etc.? Or was it just because the check engine light was on?

 

Just the check engine light. The car is running normally. In fact prior to this new egr valve, I had 2 egr valves replaced but they're both used, so I thought it's better to go with a new one.

 

The P0400 will be set if the vacuum in the intake isn't behaving as expected when the valve opens. So indeed it can be the valve itself or obstructed pipe, but more often air leaks in the intake system which can cause a different reading and thus evoke the P0400.

You can start with checking the stft/ltft values at idle, if you still have a good maf/o2 sensors it will show if you have a possible leak.

Added: A leak can be vey small, if you have for instance a leaky supercharger actuator that sits on the same vacuum tube as the MAP sensor will have a big effect on the readings but smaller on the stft/ltft readings.

 

thanks avos but how do I check the stft /ltft level at idle?

 

1st start with checking of the vacuum tube is able to hold vacuum. Take it of from the intake elbow (so leave the MAP/Bypass valve actuator attached) and suck air from it and see if the vacuum holds.