P 0174 only and restricted performance occasionally
 

This problem was discussed here many times, but usually with P 0171 code in a tandem. My car - 1999 XK8 - produces only P 0174 and intermitently when driving "restricted performance that lasts usually a few miles and goes away. Sometimes it comes two times during a 30miles trip, sometimes after 300 miles. I recorded STFTs. STFT bank 1 : 0.79% idling, -4.71 at 2K rpm. STFT bank 2: -4.7% idling, >18% at 2k rpm. LTFT -1: -4.72 idle. 0.72 2k rpm, LTFT - 2: 18.9 idle. 1.57 2k rpm. Any hints where I should look? Thanks, dneu

Unfortunately, it's hard to help without knowing the year and model of your car. If you open the User CP in the upper left of the page you can include that in your signature.

+1

then record the LTFTs

Try giving the MAF sensor a clean and a new air filter first .....

There does appear to be a large air leak on bank 2 at idle. If it persists after the mafs cleaning look to the seal of the oil filler cap and the part load breather tube which runs from the front of the cam cover to a T at the throttle body. The o ring on either end could be the problem. Carefully spray some throttle body cleaner at the joint and see if you get a change, but keep in mind it's flammable and you're close to the exhaust manifold.

Starting blind, focus on the driver side (bank 2). Replace all o-rings in all those norma connectors. Replace the VVT seals (likely old and showing oil). Replace o-ring on the dip stick. Then see how it goes.

If you want to start not-so-blind, do a smoke test and fix the leaks.

Depending on how cheaply you can find an AFM, you might want to just replace it. I replaced mine for $15. If you decide to clean it, use the proper AFM cleaner, and make sure you inject it in the recess (the "pill" is the air temp sensor), and let it sit for a bit, completely soaked, upside-down. Rinse, repeat.

Best of luck, keep us posted.