This is a reccurent problem that I have discussed in the past but it never got solved successfully. After I start to drive my 1999 XK8 and the temperature reaches the operational level - "restricted performance" comes on (on a scanner P0174 code). After driving about 3 miles the warning goes away and stays away for another 70 - 80 miles and a routine repeats. I have checked vacuum lines, breather lines from valve covers, sprayed a starting fluid around intake system - not any leak symptom found. Did anyone experienced a similar problem?

 

Assuming your car has VVT (Variable Valve Timing), what is the age and condition of the exterior seals around the VVT solenoids? They typically last just two or three years, then get hard and brittle and begin to leak oil and air....

Also, what is the condition of your plastic air duct that runs between your air filter box and your throttle body (specifically the underside of the accordion section? They tend to crack and split due to engine bay heat after four or five years....

 

Check the MAFS, clean or replace it. On your year, the MAFS sensor is made by Denso and is shared with a few other vehicles such as the Mazda 3, a few Fords. In the Non-OEM replacement parts sticky there is a post I made with the exact cars it is cross listed with.

 

VVT solenoid seals were replaced last year, no oil leaks around them.Air duct is not cracked. What puzzles me is that I get only P0174 code (bank 2 lean) never p0171)

 

In my experience, there are so many areas where an air leak can develop that the only clean way of debugging this problem is to do a smoke test, i.e. inject smoke under low pressure into the intake, and see where it comes out (the fuel pressure regulator vacuum line is great for this). You can of course have this done, buy a pre-made smoke tester (check eBay, Amazon) or even make one with stuff you have lying around. I made one a while back by re-purposing an old soldering iron to burn rags full of mineral/baby oil in a jar. The pressured air came from a bicycle pump. Cheap, gets the job done.

Other thing is to check the (long term) fuel trims over OBDII. The code comes at 25% adjustment, which is a lot. Maybe the other bank is very near it, so basically just as bad. A cheap ELM327 and a phone app will tell you.

Best of luck, keep us posted.