I was called to a shop with a complaint of a hesitation problem accompanied by a MIL lamp on a 2000 Toyota Corolla with a 1.8L engine. The shop had already replaced the fuel filter, cleaned the injectors, cleaned the MAF sensor and even
sold an O2 sensor as a maintenance part replacement. The vehicle was returned to the shop with the same symptoms. Nothing had changed except the customer’s patience with the still illuminated MIL lamp. The shop did not want to put any more money into the vehicle without being 100% sure the vehicle would be repaired so they decided to call me in for a second opinion. I am frequently asked how I can come to a shop and solve problems that have eluded these shops for days or even weeks, in just an hour or two. The key to my, or anyone’s, success is to keep going to any training that is available to you. This allows you to understand the system that you are diagnosing so you can apply the correct tests in the correct
sequence. This will allow diagnostic success. It is also very important to have the right tools for the job at hand.
Repairing vehicles quickly and efficiently can be compared to a tri-pod; each leg must support the overall diagnostics.
One of the legs is the information required for the job. The second leg is the correct tools for the job. The third leg
is for the technician to be able to apply their understanding of the system being diagnosed. If any of these legs fail,
the diagnosis will also fail. I am always in search of new information and new tools that will help me diagnose modern
vehicles. I had just received a new tool that would help with the diagnosis of this 2000 Toyota Corolla. This new tool
just hit the market and I can only tell you that this story is not a sales pitch but rather an industry alert about how
I am keeping up with technology by venturing down new avenues to better my skills. This tool is not just another OBD2
Generic scan tool but a drivability scan tool designed to do the work for you and will definitely point you in the
right direction. Enough said! Now let’s get down with our diagnostic dilemma.
I plugged my PC into the DLC; this was the same as I had done many times before. However, this time there was a difference. The scan tool that I was using was my new EScan with Sharpshooter technology. First I pulled the diagnostic trouble codes (DTC) which was a PO171 (lean operation). There were no pending codes present. I then pulled the freeze frame so I could see what the operating conditions of the vehicle were when this DTC was set. When tracking down problems this is very important; since these failing conditions have to be repeated several times in a row in order to lodge a DTC, once a pending code is set the PCM adjusts all the criteria so it is within 10% of the original pending code criteria before it will run the second DTC or testing program. This information arms the technician with the exact conditions that were present when the pending code matured into a DTC. By viewing the freeze frame data the operating conditions under which the vehicle failure occurred can be seen. This will allow the technician to understand what the operating conditions were at the time the vehicle failed. Now if needed the technician can put the vehicle under the same operating conditions while monitoring the data.
By analyzing the freeze frame data (Figure 1)
you can see that the vehicle was traveling under a 38% calculated load at about
46 MPH at 2500 RPM’s. The long term fuel trim reached 37%. This was an
indication that the PCM had a report from the oxygen sensor of a lean condition
or low oxygen sensor voltage. When the oxygen sensor voltage drops below 0.45
the PCM commands more injector on time. It accomplishes this by fuel trim. Fuel
trim is a modifier to the original fuel injection equation. The PCM takes in
data (inputs) that allows it to calculate how much air is entering the engine.
It then commands the injection on time (output) so the air/fuel mixture will be
correct for the operating conditions of the power plant. The oxygen sensor then
reports the air/fuel status to the PCM. The short term fuel trim then commands
the injector on time in the opposite direction of the oxygen sensor report. If
the report from the oxygen sensor is lean (low voltage) the fuel trim moves to a
positive correction factor (adding fuel). If the report from the oxygen sensor
is rich (high voltage) the fuel trim moves to a negative correction factor
(subtracting fuel). This will allow the oxygen sensor to change its output and
then start to move toward 0.45v. As the O2 voltage crosses 0.45v the short term
fuel trim again changes its direction. This again moves the oxygen sensor in the
opposite direction. When the O2 voltage moves from a high value (rich) to a low
value (lean) and continues this cycle, it is an indication that the PCM has fuel
control. Under these conditions the PCM keeps the air/fuel ratio at 14.7 to 1.
When the PCM loses this movement or loses fuel control and the oxygen sensor
voltage drops below or goes above 0.45 for an extended period of time; the fuel
trim will try to counter the oxygen sensor voltage. At the point the oxygen
sensor voltage remains without having fuel control and the short term fuel trim
crosses a predetermined point, the long term fuel trim will start to move in the
opposite direction of the oxygen sensor voltage. The long term fuel trim will
continue its movement until the oxygen sensor voltage crosses the 0.45 voltage
threshold or the long term fuel trim hits its programmed limit. Once the fuel
trim’s movement goes beyond a preprogrammed limit the PCM will store a failure.
In this case it stored its lean limit. In other words, the oxygen sensor’s
voltage remained below 0.45 volts (lean) long enough for the PCM to add enough
long term fuel trim to the base fuel calculation to set a code. This could be
caused by a fuel delivery problem, injector problem, oxygen sensor problem, or
even a MAF sensor range problem.To find the cause of the fuel control problem I placed the EScan in graphing mode selecting the appropriate PIDS of choice. I power braked the engine in drive at about 2200 RPM’s (Figure2)
.
I could immediately see the graph for the O2 sensor flatten to about 100mV. At
the same time the trims rose to almost 20% before the LT trim rose again to
about 34% to get the O2 sensor to start switching. The graphing of PIDs are very
helpful to get a good visual of what is actually going on. It allows you to get
a better understanding of how the PIDs hold a relationship with one another so
that you can better hone your skills to come up with a proper diagnosis. Okay I
now confirmed the lean problem but we still need to dig deeper. I next moved on
to the EScan‘s Sharp Shooter section of the tool. Here is where all the action
begins and the excitement increases. I first selected the Fuel Trim grid to
again verify the trim problem. This grid is designed to fill cell blocks, which
mimics the way the onboard ECM would monitor cell blocks for proper fuel
control. The visual aspects of the color grid reveals the fuel trim problems at
a moments glance. By again power braking the vehicle at about 2500 RPM’s under
load (Figure3)
you can see the same results as in the graphing mode but with a quicker
understanding of where the problem lies just by looking at the color of the
blocks, green indicates the fuel trims are good while yellow, orange, and red
are indications of a fuel trim problem. The engine was not missing like it had
partially clogged injectors but rather bogged down under a load like it had a
low output fuel pump or a bad MAF sensor. Rather than spend the time hooking up
a fuel gauge it was easier for me to move on to the next Sharpshooter test. This
would be the Volumetric Efficiency test screen.An internal combustion engine is only an air pump. The efficiency of how much air the engine can pump can tell you a lot about the condition of the engine. If the engine has an intake or exhaust restriction or the camshaft is out of time, the amount of air the engine is capable of pumping will drop. This efficiency can be calculated if you know the engine size, engine RPM, and the barometric pressure. This equation is referred to as the volumetric efficiency of the engine. The EScan is set up to take all the math out of figuring out the volumetric efficiency of the engine. So you basically plug in the engine size and it does the rest. I plugged in the 1.8L engine size and allowed the tool to calculate the TPS% and set myself up for a test drive. It is important to drive the vehicle under a load at WOT from a dead stop to get a good accurate reading. The tool graphs the expected (red line) and actual (yellow line) readings as you drive the vehicle. I made three hard accelerations from a dead stop (Figure4)
and you can immediately see that this vehicle had a volumetric efficiency
problem. The VE table was showing readings that were low by about 30% indicated
by the red cells. By knowing this was a fuel trim problem this ruled out a
clogged exhaust or a valve train efficiency problem. If the volumetric
efficiency of the engine was low and the MAF sensor had reported the correct air
flow of the engine, the fuel trim chart would have been green. This indicates that the PCM did not need to correct the original fuel injection equation. Since the fuel trim chart was red, this was an indication that the trim had to change the original fuel
equation and add fuel to try to maintain the proper air/fuel mixture. Since the VE showed a very low reading and the trim chart showed very large corrections, this had to be a MAF sensor related problem and not a fuel pump problem.
I removed the MAF sensor and inspected the air cleaner housing for any restrictions and leaks and found none. By looking at the MAF sensor (Figure5)
many people get confused by the intake air temperature sensing device on the
outside of this MAF sensor and mistake it for the MAF sensing element. They go
ahead and clean the intake air sensing unit and then reinstall the sensor back
into the MAF sensor housing. This is just what the shop had done. By looking
through the top side of the sensor (Figure6)
you can now see the MAF sensing device located inside the MAF housing. This MAF
sensor element was still very dirty so I went ahead and cleaned it with
electrical cleaner. After the MAF was cleaned I removed the ECM battery fuse to
reset the fuel trims back to zero. I now placed the EScan Troubleshooter tool in
Volumetric Efficiency test and drove the car again. The results in the graph (Figure7)
were night and day. The calculated and actual readings basically mirrored
themselves and the VE table showed readings within the 10% range, shown in
green, and the car ran great. To verify my fix I loaded the Fuel Trim chart (Figure8)
and
you could see the results were night and day as well. The shop had come very close to replacing the MAF sensor which would have been another unneeded part. I can only tell you as technology keeps advancing and vehicles become harder to repair; it is more important than ever to have all three legs of the tripod when working on modern vehicles. I value my time and it is the visual associations that PC software of this type can offer that can only enhance your diagnostic strategies to get to the root of the problems quickly while learning in the process. This tool is not just another scan tool. It is a tool loaded with ECM algorithms to help pinpoint drivability problems. I hope this story has enlightened you to better understand how a simple sensor can set off codes unrelated to itself and take you on a diagnostic adventure!