Unmetered air entering from a vacuum leak or other source is rather finite in its overall amount as compared to the total airflow into the engine. At lower total airflows, this "extra" source has a more profound effect than it does at higher airflows. Fuel trims that are way positive at idle, but more normal at 2,500 rpm, are a sure sign of an unmetered air source as being the cause of the lean condition.

Now to Find the Source

In the old days, many techs used carburetor cleaner sprayed around the intake area to find a vacuum leak. When the cleaner was sucked in, the rpm would increase and/or the engine would stumble. Propane was also used to find vacuum leaks by slowly passing the propane around suspect areas and looking for a change in engine condition, or for the change in oxygen sensor voltage that indicated a brief, rich condition.

Instead, I disconnected the new PCV vacuum line. I then attached a hose to the nipple at the throttle body and added smoke.

And None Came out!

Impossible. I knew there was an unmetered air source here. Time to think a minute. Oh yeah, the hose for the PCV valve is untested. It may be new, but that doesn't always mean it's OK. I switched my tester nozzle over to the PCV hose and hit the start-smoke switch. Instantly, smoke began rolling out from under the upper intake plenum, in the area of the PCV valve. Now we're on to something.

To make sure, I closed off the throttle body port that normally connects to this hose. I started the engine and watched the reaction of the fuel trims. STFT almost immediately went negative, and LTFT started to drop, telling me that now the PCM was trying to correct for its previous misinformation. No more unexpected air.

I pulled out my light and peered deeply into the heart of this Jaguar. The PCV (or "vent valve," as referred to by Jaguar) routes from the throttle body to the PCV valve, then to the oil separator. The separator is mounted to the top of the block, just below the fuel injector rail. This separator is connected to the PCV plumbing by a rubber boot, and that's where I suspected the leak was coming from. My "prescription" was to remove the upper intake plenum, and replace the leaking boot.

I removed the upper intake plenum and injector rail and found exactly that. The boot had deteriorated in the same way as the PCV hose the previous tech had replaced. The repair was straightforward, no surprises, and finally it was time to button it up and verify the fix.

After repairing any fuel management code (system lean or system rich), there is one last step that is vitally important, and that's to reset the Keep Alive Memory (KAM) and restore LTFT to center ("0"). If this step is not performed, the PCM will have to relearn fuel trim on its own, and in the meantime, your customer is going to have a whole new set of driveability issues to deal with.

With KAM reset, I took a test drive while recording fuel trim at idle and cruise. I also made a few Wide Open Throttle (WOT) passes to make sure the MAF sensor was reading accurately. All went well, trims were normal, no codes pending, and I pronounced the Jaguar "cured."

Pete Meier is an ASE certified Master Technician, member of iATN, and full-time tech in Tampa, FL. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Pete directly at http://www.autoservicetech.com/.