Motor Age Garage

Jan. 1, 2020
Medical diagnostics and automotive diagnostics are not as different as you may think

Anemic Jaguar

You don't feel well. You're sitting in the exam room waiting to see your family doctor. After exchanging pleasantries, the doctor opens his exam with the classic question, "How are you feeling today?" Next, he (or his nurse) performs a few basic checks on your "systems": He takes your temperature and blood pressure, listens to your heart and breathing, and looks in your mouth, nose and throat, all to get a basic diagnostic direction.

After considering your symptoms and this basic information, he may order more specific system tests to narrow down your possible ailments before finally making his conclusion and prescribing the cure. When that's all done, he sends you to the receptionist to schedule a follow-up appointment to make sure the cure worked.

Sound Familiar?

We essentially follow the same procedure when diagnosing our customer's complaints. We interview the customer and the car to find out how it's feeling; we perform basic tests on the systems and gather preliminary information before moving on to more specific tests that help us narrow down the cause of the complaint. Then we reach our conclusion, make our repair, and verify the result.

The only difference, as one physician once told me, is that we can turn our machines off while we work on them — his have to stay running!

The Jaguar in my bay provides a great opportunity to follow this diagnostic procedure and reinforce the importance of taking a logical approach to any automotive repair.

Gathering the Basics

The first step in any diagnostic procedure should be observation. This step gathers the basic information you need to get diagnostic direction. It includes interviewing your customer and learning as much as you can about his current complaint. How does the car drive now? When did the problem start? Has it ever happened before, and if so, what was done about it? The questions you ask the customer will depend on the nature of the complaint, but the more you know up front, the easier time you'll have finding the problem.

In the case of the Jag, all the customer could tell me was the Malfunction Indicator Light (MIL) was on. He didn't experience any change in performance or driveability.

So now it's time to interview the car and see what it can tell me. My first step is to take a look at the car. What is its overall condition? How many miles are on it? Are there any modifications that may affect the performance of the car or impact the complaint? Do I have access to any history on the car? (Previous repairs, maintenance history, etc.)

Looking in our history log, I found an entry where the customer had just been in a few weeks ago with an MIL complaint with codes P0171 (System lean, bank 1) and P0174 (System lean, bank 2). The previous tech had replaced the Positive Crankcase Ventilation (PCV) hose from the valve to the throttle body because the rubber elbow that attached the plastic hose to the valve had deteriorated and split. The car itself was in good condition, with no obvious defects seen in the visual inspection.

Time to Have a Talk with the Jag

The next step is to gather basic information from the onboard memory in the Powertrain Control Module (PCM). I'm still in the observation phase, remember. When I connect to the Data Link Connector (DLC), I first ask the computer the status of all the readiness monitors. Monitors shown as "not ready" or "in complete" may not have run because of a detected fault that is also a prerequisite for that monitor. In addition, incomplete monitors may be hiding an additional problem that will show up later, and result in a comeback.

Then I ask the PCM what failed. In the case of this Jag, the same codes were stored as the last time the car was in: P0171 and P0174. So whatever else was a cause the last time, it is not the only cause. There is still a problem.

Wonder what the freeze frame information can tell me? I access that next to see what conditions were present at the time the PCM saw and recorded the failure. Long Term Fuel Trim (LTFT) was pegged at close to +40 percent on both banks at idle, telling me the PCM was working overtime trying to maintain a proper lambda reading.

Some Basic Tests are Next

Just like the doctor, I wanted to run some basic tests. I started the car and allowed it to reach operating temperature, then reviewed the current data stream for anything out of place. What instantly came to the forefront was a repeat of what I had seen in the freeze frame data: LTFT was at its limits, and Short Term Fuel Trim (STFT) was trying to add even more.

This car's fuel management system uses a Mass Air Flow (MAF) sensor to read airflow into the engine. Because a MAF sensor measures actual air mass entering the engine, that's a tool for me to use. I increased engine speed to 2,500 rpm and took another look at current data. LTFT was now closer to normal, and STFT was normal, switching nicely across "0." The oxygen sensors looked a lot happier as well, also switching normally.

What does that tell you?

Is All the Air Being Measured?

A MAF sensor, unlike the Manifold Absolute Pressure (MAP) sensor in a speed-density system, measures actual air mass entering the engine. However, it can only measure the air that is flowing through the sensor. Air entering the engine from any source downstream of the sensor will not be measured. This air can come from a vacuum leak, a torn intake boot between the sensor and the throttle body — even a poorly sealing oil dipstick tube.

The PCM doesn't know that this extra air is coming in, and doesn't compensate for it by adding the appropriate amount of fuel. The oxygen sensors, however, are definitely going to tell the PCM that the mixture is too lean, and the PCM responds by increasing fuel trim. This cycle continues until the mixture is restored to lambda=1. If the LTFT corrections necessary to achieve this goal exceed a specified threshold, a "system lean" code will be generated.

Of course, there are other factors that can cause a "system lean" code. But all the evidence is there to point to unmetered air as the cause. Do you know what it is?

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.

But today's complicated intake systems require another approach for accurate diagnosis. So my "prescription" is an intake manifold smoke test, using the shop Evaporative emissions (EVAP) system tester. This test uses the smoke generator of the EVAP tester to fill the intake tract with smoke at low pressure. Normally, I connect to the vacuum line at the brake booster, but in the Jag's case, it was a little harder to get to than I wanted to deal with.

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

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