Transcript No Code Diagnosis

No-Code Diagnosis
A Step-by-Step Guide to Success
Jim Halderman
Topics to Be Discussed
• Where to start diagnosis?
• Why following a procedure rather than just
performing tests works best
• How to look at scan tool data (PIDs) in a
sequence order to reduce diagnostic time.
• How to use fuel trim to diagnosis fuel
delivery problems.
• Many case studies as examples
Where do you start?
• Step #1; Verify-If you can not verify the
customer concern, you can not verify the
repair
• This is hard for some customers to
understand. “ Don’t you believe me?”
• Have the owner drive is instead of the
technician
• Is the troubleshooting procedure explained to
the customer?
Step #2
• Visual inspection
• An older technician once told me that “the
vehicle will tell you want is needed”
• True?
Turbo Ford Case Study
(ran rough after cylinder head replacement)
Checked Air Vane Sensor
Verified the Cam Timing
Verified Ignition Timing
Checked the Valve Clearance
Checked Injectors
Tachometer fluctuated
when running
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What can cause that to happen?
Bad coil?
Poor connections on coil?
Bad ignition control module?
Bad tachometer?
Poor ground?
Grounded Cylinder Head
Ran OK with the Jumpers
Turbo Ford Story Conclusion
• Cylinder head was not properly grounded to the
block
• Sealant was used on the head bolt threads
• The ground from the battery was connected to the
block; not the head
• Spark plugs need to be properly grounded
• Poor ground caused feedback to the tachometer
Step #3
• Check for diagnostic Trouble
Codes (DTCs)
• Could be performed before
step #2
• Check for pending codes
too
• Check that all monitors have
run
• Could there be a driveability
problem without a DTC? yes
Monitors
Continuous: Misfire, Fuel System, and
Comprehensive
Non-Continuous: Evap., EGR, O2
Sensors, O2 Sensor Heaters, Catalyst,
Heated
Catalyst, A/C System, Secondary Air, and
Warm-ups
NOTE:
In emission areas, a specific number of
monitors need to be “complete” or
“ready” in order to perform an emissions
test.
Monitors vs. DTCs
• If a monitor cannot
run, then a DTC
cannot be set
• Always check to see
if the all of the
monitors have run
and passed
• Some require certain
temperatures
Step #4
• Check for any technical service bulletins (TSBs)
• Why not use the resources of many before
you?
• I would also suggest using www.iatn.net
• Identifix (free to NATEF certified programs)
Step #5
• Check scan tool data
• Look at the “high
authority” sensor
information
Step #6
• Narrow the problem to a cylinder or system
• The systems could be the fuel, ignition or
emission control system
• The cylinder could be just one cylinder or a
bank of cylinders
Step #7
• Find the root cause
• The root cause may not be obvious but has to
be found and repaired to prevent a comeback
Step #8
• Verify the repair
• Use the same conditions used to verify the
problem to verify the repair
• Clear DTCs (not if going to an emission
test????)
• Write the story on the work order
• The three Cs (Complaint, Cause and
Correction)
Dash Warning Light On?
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Check Engine
Check Engine Soon
Maintenance Required
Service Vehicle Soon
Air Bag
Side Air Bag
Trac
P0304 Example
No-Code Diagnosis
• Many times are tough to locate
• Keep the basics in mind
• The primary purpose of OBDII is emissions-not
driveability!
2004 Prius Case Study
• Poor fuel economy (25
mpg instead of normal
40+)
• No codes
• Scan data (PIDs) looked
normal
• Found right front disc
brake caliper stuck.
• No drop in performance
noticed by the driver
Rough Idle; Surge; No Codes
7
5
6
8
Base line for Sensor Values
(Except as mentioned)
• Normal operating temperature (cooling
fans cycled twice)
• Idle (closed throttle)
• All accessories off
• In Park or Neutral
• Closed loop
Skewed Sensors
• A skewed sensor gives variable readings that
appears to be accurate
• However the sensor may be contaminated or
dirty and sending incorrect information to the
PCM
• Does the PCM know the sensor is skewed?
Data Stream Step #1
• Before starting the
engine, connect
the scan tool.
• This step is very
important,
especially if the
driveability concern
is hard starting or
cold driveability.
Data Stream Step #1 (continued)
• Key on/Engine off
(KOEO) and look at
the values for ECT
(engine coolant
temperature) and IAT
(intake air
temperature).
• Basically, the same
sensor and the two
temperatures should
agree.
ECT = IAT
• The two temperatures
should be the same
(within 5 degrees).
• Both should measure
the ambient air
temperature.
• If the two indicate
different temperatures,
the one closer to the
ambient air temperature
is the one most likely to
be correct.
ECT = IAT (continued)
• The ECT sensor has a
higher authority than
the IAT and is therefore
more likely to be the
cause of a starting or
cold running problem.
• The ECT is the only
sensor used by the
PCM when the ignition
key is first turned from
on to start.
Quick and Easy Metric
Temperature Conversion
• Double the Celsius degree number
• Add 25
• Should be close to the Fahrenheit
temperature
• Example: 50 degrees X 2= 100+25=125
• Actual= 122
Data Stream Step #2
MAP= BARO
• Another sensor to check is the
MAP sensor because it is a
high-authority sensor,
especially on speed density
controlled engines.
• The MAP reading at KOEO
should be atmospheric
pressure (about 29.50 in. Hg.),
depending on altitude and
weather conditions.
• An easier value to remember
is that it should be about
4.6-4.8 volts
MAP Sensor Authority
• The MAP sensor is a high-authority sensor
on an engine that uses the Speed-Density
method of fuel control.
• If the exhaust is rich, try disconnecting the
MAP sensor.
• If the engine now runs OK, then the MAP
sensor is skewed or giving the PCM wrong
information.
MAP Too High or Too Low
• The sensor could be
skewed.
• Check the power
and ground of the
sensor.
• If 5-volt reference
(Vref) is low, check
other sensors that
also use the
reference voltage.
Data Stream Step #3
IAC Counts
• After the engine
starts, observe the
IAC counts or
percentage.
• The IAC is used to
control idle speed by
changing the amount
of air bypassing the
throttle plate (just like
depressing or
releasing the throttle
pedal).
IAC (continued)
• On a warm engine (cooling fans cycled twice),
the IAC counts should be 15-25 counts or
percentage.
• If the IAC commanded position is low, a vacuum
leak (speed density engines mostly) could be
indicated.
• The extra air decreases the vacuum and the
MAP sensor reads this drop as an increase in
load. The PCM adds fuel, increasing the engine
speed.
IAC Too High
• If the IAC position is
higher than normal.
This could indicate a
dirty throttle plate(s)
or a vacuum leak on a
MAF engine.
Higher IAC
• A vacuum leak on a MAF
engine is actually false air not
measured by the MAF sensor.
This can cause the engine
speed to decrease due to the
leaner-than-normal air-fuel
mixture. The mixture causes
the PCM to increase engine
speed and commands a higher
IAC position.
• Note: Some minor vacuum
leaks can cause the IAC to drop
just like on a speed density
engine.
Data Stream Step #4
MAF Sensor
• Look at the amount of air entering the
engine.
• There should be:
– 3 to 7 grams per second (g/s)
– About 1 volt (analog MAF sensor)
– About 0.5 lb. per hour
– About 13-19 kilograms per hour (kg./hr)
Normal MAF Readings
• Use a scan tool to look at the grams per
second.
• Warm the engine at idle speed with all
accessories off. Should read 3 to 7
grams per second.
• GM 3800 V-6 should read 2.37 to 2.52
KHz.
• If not within this range, check for false air
or contamination of the sensor wire.
MAF Sensor Diagnosis
• If the MAF sensor wire were to become
coated, it cannot measure all of the
incoming air.
• A normal warm engine at idle should be 3 to
7 grams per second.
• Rapidly depress the accelerator pedal to
WOT. It should read over:
– 100 grams per second (scan tool) or
– higher than 7 kHz (digital MAF sensor)
– 4 volts (analog MAF sensor)
Visual Inspection
• Look for a very dirty
filter.
• Look for a K & N filter
that has been overoiled.
• Look for fuzz on the
sensing wire from
fibers coming off of
the filter paper.
Fuzzy MAF
Mass Air Flow (MAF)-False Air
Usually affects operation in drive; may run OK if driving in reverse.
MAF Sensor Authority
• High-authority sensor
• If in doubt - Take it Out!
• If the MAF sensor is disconnected, the
PCM substitutes a backup value.
• If the engine runs OK with the MAF
disconnected, then the MAF has been
supplying incorrect information.
Data Stream Step #7
Fuel Trim
• Short-term fuel trim
(STFT) is used by the
PCM to correct for
exhaust readings that
are slightly rich or
slightly lean.
• STFT can add or
subtract fuel quickly.
• STFT can add or
subtract only a limited
amount of fuel.
Long Term Fuel Trim
• Long-term fuel trim
(LTFT) can add or
subtract more fuel
than STFT.
• LTFT is slower than
STFT.
• The purpose of LTFT
is to keep STFT
within plus or minus
10%.
LTFT and STFT Diagnosis
• I suggest that you ignore STFT.
• STFT is what is happening this instant.
• LTFT gives a history of the exhaust
mixture and is more helpful when it comes
to a diagnosis.
• Add the two together to get total amount of
added or subtracted fuel.
• LTFT = +8%, STFT = -3%, total = +5%.
+LTFT Too High
• Look for a
vacuum leak.
• Look for a
cracked exhaust
manifold.
-LTFT Too High
• Check for too
high fuel
pressure.
• Check for a
leaking fuel
pressure
regulator.
Fuel Trim Example
• LTFT = +12%
• STFT= +2%
• How is the engine running now?
Fuel Trim Example
• Answer:
– The engine is operating normally now
because the PCM has compensated for a
slightly lean air-fuel mixture by increasing the
injector pulse-width by about 14%.
– Look for a vacuum leak or low fuel pressure.
Pin Down to a System or Cylinder
• Compression Test
• Cylinder Leakage Test
• Running (Dynamic) or
Relative Compression
Test
Pontiac Grand Prix Case Study
(1997 3100 V-6)
• Misfiring at idle on cylinders one, three, and five
only
• No DTCs
• When first started, I noticed slow, jerky cranking
even with the ignition disconnected (this was not
a customer concern).
• What would you do first?
Visual inspection
discovered a corroded cable
Injector inspection showed that the wiring for
the front and rear banks were switched.
ATF was used through the intake to check to see
if the valves were sticking.
Spark testers were installed to check the ignition
system for proper operation
Based on information from the archives of www.
iatn.net, the intake gasket area was checked for
possible leaks
Spraying around the injector
O-rings caused a misfire
Checking the misfire counters confirmed that we
had discovered the root cause
Final Result
• Replacing the injector O-rings solved the
misfire on cylinders 1, 3, and 5.
• Switching the injector harness back to
the correct bank of the injectors solved
the slow, jerky cranking
What I Learned
• I learned to use the misfire counter
whenever working on a possible
engine misfire
• The misfire counters picked up
misfires that could not be detected
otherwise.
Fuel Delivery System Check
Test Drive Analysis
• Low power complaint
– Drive will consist of WOT.
– Please check the oil level first!
– Watch MAP and O2 sensor reaction.
Must maintain at least 850mV
Test Drive Analysis
• If MAP does not go high
(4.6-4.8 volts); there is
an intake restriction.
• Look for clogged air
passages.
• If O2S voltage goes
low; the fuel system is
likely going lean.
• Look for a weak pump
or a clogged fuel filter.
Case Study
Chrysler mini van no-start
• Customer stated that it
would not start unless
he pounded on the
dash. Finally, the vehicle
wouldn’t start at all.
Step #1-Verify
Engine did not start
No spark
No Squirt
Step#2- Visual Inspection
• Yep the customer
pounded on the dash
• So hard that the
needles fell off
Step #3 Check DTCs
Step #4
• Checked for TSBs-nothing that was related
• Checked archives of www.iatn.net
• Bingo- found several fixes for wiring being
burned near the CPK sensor
Step #5 Scan Data
• No engine RPM while
cranking
Step #6- Narrow Down
Step # 6-Continued
Step #7 Root Cause
Step #8-Verify Repair
Summary
• Check monitor status and DTCs early in the
diagnostic process
• Always follow the same routine while
diagnosing problems
• Use all resources possible including:
– iatn.net
– Identifix (free to NATEF certified programs)
– Factory and/or aftermarket service information
Contact Information
• [email protected]