A fuel pressure regulator can cause negative idle trims in two ways: by not regulating fuel pressure or by leaking fuel through
the vacuum line at the diaphragm.
In our last issue, we learned how the Engine Control Module (ECM) adjusts air/fuel ratio using data from the oxygen sensor
and its own programming. We also learned how that data is combined and reported on a scan tool. In this issue, we'll discuss
how you can interpret this information to understand what the ECM "thinks" is happening.
One of the initial observations to make is the response of the lambda sensors, both pre-cat and post-cat. Because the pre-cat
sensors are the feedback to the ECM on how it's doing, it is critical to optimum performance that they are on top of their
game.
Is the Lambda Sensor Telling the Truth?
One method of testing the sensor's ability to react is the "snap throttle" test. Monitor the sensors on your scan tool (a
recording or graphing function is best here) while you snap the throttle to wide-open (WOT) a few times. When the throttle
opens suddenly, a huge quantity of air is ingested. This will cause a momentary lean condition that should cause the pre-cat
sensor signal to go low. Conversely, when the throttle is suddenly closed, the correction the ECM just tried to make for the
lean condition, combined with the sudden loss of air, will cause the signal to go high (rich). The post-cat sensors will also
react similarly, but not always to the same extremes as the pre-cat sensors.
OBSERVE, DEFINE, EXPERIMENT
Look for a low of approximately 0.100v and a high of approximately 0.900v. Make sure to run this test after the engine has
gone into closed loop to ensure the sensors are on-line.
If you have a lab scope, you can watch the sensor's signal under these same conditions, as well as measure the time it takes
to "switch." A rule of thumb for a conventional sensor is a lean-to-rich switch of less than 100 ms. With the right scope
leads and a clean screen, you can actually tell whether a misfire is caused by a fuel/mechanical issue or an ignition issue
by monitoring this signal, but that's another article all in itself.
You can also ask the ECM what it thinks of the performance of the sensors by looking in Mode $05 — Mode $06 on later model
On Board Diagnostic (OBD2) vehicles. Here, the ECM will tell you the results of its own functionality testing, as well as
the test standards. Mode $06 test descriptions are becoming more and more available, so check your service information provider
for specific TID and CID identifiers.
FUEL TRIM CORRECTIONS
If you see a sensor that is steadily reading low (or lean), you may be getting air in the exhaust that is not coming from
the combustion chamber. One giveaway that air is entering the exhaust is lean codes (or positive trim corrections) combined
with an engine that feels like it's running the opposite, or too rich. The air entering the exhaust is "fooling" the lambda
sensor, causing the positive corrections, when in reality it's feeding too much fuel to the combustion chambers. If you have
access to an Exhaust Gas Analyzer (EGA), look for abnormally high oxygen (O2) content as compared to the other gases.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. 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.
Articles by Peter F. Meier
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