1997 XK8: I know I may be rehashing old threads. When the engine is at operating temperature (190 degrees F), codes P0171 and P0174 accompany a short-term rough idle when the car is at a stop following normal cruise. Occasionally, the CEL will illuminate, but not always. When increasing engine speed, the car runs normally. I have looked for vacuum leaks and haven’t found any obvious ones. I have replaced the MAF with an off brand one from Amazon. I have not had a smoke test performed. Thoughts?

 

Well, start with the smoke test!

 

To check, watch fuel trims at idle (warmed up engine) - likely they will be quite big (>10, say). Rev to 2500 or so and if they drop you for sure have a leak.

 

Go back to the correct and genuine Denso MAF sensor.

Check the accordion section of the black plastic intake pipe for splits and cracks, particularly its underside (which bakes in the heat of the engine bay and gets extremely fragile).

Check the two VVT solenoid O-ring seals to see if they are weeping oil. If they are, they can also be losing air there.