Lowest Fuel trims achieved only in the 1200-1500 RPM range?

Question

This car still driving me crazy. I wrote here before about getting the code P0171 and some suggested a vacuum or exhaust leak. I sprayed some flammable connector cleaner around the intake manifold, throttle and PCV valave area with no noticeable change. I also added the Techron fuel system cleaner but still no change. I started tracking the fuel trims behavior for a while and i noticed that the lowest fuel trims ( Closest to zero ) was achieved only around 1200-1500 RPMs. For example long fuel trim sets around positive 15.6% while idling then drops to 7% around 1300 RPM then it will climb to 14% around 2000 RPM. I know that higher fuel trims at idle suggests vacuum or exhaust leak but would think that fuel trims will always go lower with higher RPM but not in my case as the fuel trims dip around 1200-1500 RPM then they start to climb around 2000 RPM. Is it fuel delivery issue or extra air issue?
Thanks!

Answer 1

Keep looking for an air leak. I just fixed a lean bank code on my car that would illuminate at some speeds and not others, similar to your situation. It was due to a very slight vacuum leak around the PCV valve and the PCV valve itself. The spray solvent trick did not work for me, since the leak was very tiny and I couldn't discern an rpm drop. The leak was in fact so small I doubt a smoke test would have identified it. I was amazed at how such a small leak -- actually several leaks way smaller than a pinhole -- could cause a code.

I once read that about 80% of vac leaks are related to the PCV system. In my case, the code disappeared for a time if I just jiggled the pcv valve. The air hose that connects the valve was brittle due to age, so it didn't fit tightly. I sprayed the hose with PB B'laster and added a hose clamp to remedy the situation. Dirt had also accumulated inside the rubber grommet into which the PCV valve is fitted. As it turned out, the PCV valve was also the wrong part number, probably installed by a mechanic years ago. While researching my problem, I read further that a loose o-ring on the dipstick and worn crankcase oil filler cap on the valve cover can both upset the vacuum of the PCV system. Some cars have plastic connectors & elbows in the PCV circuit that develop minute cracks from heat stress, so be sure to pressure test them with the car off. Since PCV continuously sucks oily vapors from the crankcase, it is prone to sticky buildup and, as such, benefits from a few shots of carb cleaner through the PCV valve and tubing (with the PCV valve removed) at idle.

Looking back, I missed the whistling sound that the leak was generating. It was mixed with the usual engine sounds at idle, and I had grown accustom to it. Once I became aware of what I was hearing, it was still very difficult to pinpoint the location of the whistling because it seemed to magically change location depending on where I positioned my head under the hood. Some people use a cardboard tube (from a roll of paper towels) over their ear to narrow the acoustics.

As you can see, my code was triggered by a combination of several small problems. No wonder it lingered so long before triggering a code and was so frustrating to fix!