Embed Size (px)
Citation preview
Hit the arrow key to start!
EXPLORING COMBUSTION BEHAVIOR
THROUGH THE EYES OF A SCOPE
SECOND EDITION
SECOND EDITION
We know thatTHE OBJECTIVES OF THE
COMPUTER AND SENSORSare aimed at... .
MAXIMUM
COMBUSTION
EFFICIENCY...
…and it happens right here!
GAPGAP
PRESSUREPRESSURE
HCHC
Internal K.V demand =
ONLY PRESSURE
& FUELWILL VARYWITH RPM.Therefore
KV demandvaries
accordingly.Constant
Variables
GAPGAP
PRESSUREPRESSURE
HCHC
If we must compare… If the scan-tool looks at O2 to measure fuel efficiency, it
does so after combustion is
completed.
The scope looks at fuel efficiency
before combustion,during combustion
and after combustion
Plus… it does so per cylinder!
Fuel delivery equal for all cylinders.
Perfect conductivity from point of ionization to
the end of the firing time.
Hydro-carbons are all consumed when
the plug stops firing.
SUPER-IMPOSED SCOPE PATTERN
Is this is a lean fuel mixture?
1. Obviously a single cylinder problem.
2.Computer control problem ruled out.
At the ionization of the pressurized gasses, the
AIR-FUEL-RATIO was not lean.
For the first25% of thefiring time,the A.F.R.
is still normal.
The coil energy is prematurely absorbed,
due to lack of conductivity.
This is not a leanmixture, but rather an absence of sufficient
hydrocarbon.
1. Road Test? (Customer complaint- Missing at Hi RPM.)
2. Look at paraded pattern?
3. Scope O2 sensor?
4. Current ramp injector?
5. Scope injector pattern? -
6. Cylinder balance test? -
Are we ready to diagnose?WORN CAM LOBE
CLUESCLUES
Correct A.F.R. at start.
Verify: Power performance test - low vs. high speed.
Lower KV demand Why…?
Not enough HC to maintain firing...
1. Road Test? (Customer complaint- Missing at Hi RPM.)
2. Look at paraded pattern? DONE
3. Scope O2 sensor?
4. Current ramp injector?
5. Scope injector pattern? -
6. Cylinder balance test? -
DONE
1. Road Test? (Customer complaint- Missing at Hi RPM.)
2. Look at paraded pattern?
3. Scope O2 sensor?
4. Current ramp injector?
5. Scope injector pattern? -
6. Cylinder balance test? -
Verify: Power performance test - low vs. high speed.
Verify: Power performance test - low vs. high speed.
RULED OUT:Lean fuel mixtureLow compression
CONFIRMEDLower fuel volume
What is the What is the
SNAP-TEST?SNAP-TEST?
A sudden acceleration & A sudden acceleration & deceleration.deceleration.
Ideal objective:
When 2000 RPM is reached simultaneously with less than
4 inches vacuum.
Ideal objective:
When 2000 RPM is reached simultaneously with less than
4 inches vacuum.
Force the highest possible KV demand under any driving condition.
HOW On ACCELERATION, at W.O.T., before it RPM increase, timing is still near TDC. works This, plus high volumetric efficiency &
a lean mixture, create an extremely high KV demand.
Force the lowest possible KV demand in the combustion chamber.
HOW On DECELERATION, at closed throttle IT and high vacuum, before RPM drops, a WORKS maximum timing advance, plus rich fuel
mixture, create the lowest possible KV demand.
@ idle
@ 2000 RPM
8 KV
6 KV
The objective of the snap-test is to fool the computer.
Before the snap-testthere was no
apparent problem at low or high speed.
NEXT CASE
SNAP ACCELERATIONis too fast for the fuel trim to respond and the lean
injector is exposed.Note the high KV demandand the short firing time.
Snap-test result
At any steady speed, the computer is back
in control and coveringup for the lean injector.
@ 2000 RPMVerify
All cylinders aredriven lean except
# 3 IN F.0.
None are lean atthe start of the
spark line.
Lower KV supportsricher fuel mixture.
CONCLUSION!!!
@ 2000RPM
Escaping gasses passing the flame front.
Conclusion… RESTRICTED EXHAUST!!
…And the scopeshows reducedHydrocarbon…
We must conclude
there is alsoreduced Oxygen.
If the evidence clearly indicatesTHIS IS NOT A LEAN FUEL MIXTURE…
On all
Cylinders
Without dropping the exhaust.
Without drilling holes.
Without removing O2 sensor.
Without a test drive.
Allow a evaluation for each positive confirmation.
Vacuum gaugeresponse sluggish.
More turbulence onscope at high RPM.
Cylinder balance test worse at high RPM.
Inhibiting EGR valveimproves performance.
30 %
Minimal effect onO2 sensor.
30 %
30 %
20 %
20 %
130 % CONFIRMATION
Inhibiting EGR valveimproves performance.
What do weexpect to see?
Of course, in order to see any effect, we must be at a speed when the EGR valve is functional
and that is not at idle!If the EGR normally re-circulates about 7%, just
imagine how much exhaust is dumped back into the intake with backpressure.
Temporarily inhibiting the EGR function for test purposes should not increase RPM with more
than 5%. (100 RPM @ 2000)
Compare cylinders and isolate the odd one.Make note of difference at low versus high RPM.
Determine at what speed is worst pattern.Select the worst and the best cylinder.
Perform power test at that RPM on both.Record test results on those two cylinders.
KV DEMAND - Higher - Lower - Equal.
FIRING TIME - Shorter - Longer - Turbulence -More slope down left - More slope up right -
Good conductivity - Poor conductivity.
REASON IT OUT!Put all the information together and
pinpoint the problem based onlogical deduction.
PROVE YOUR POINT!Disable injector, EGR valve or O2 sensor, etc.Perform snap-test. Compare low & high RPM.
Enrich fuel mixture, etc.
Dissecting and accumulating
information.
Conclusionbased on analysis.
Experimenting at various speeds & loads.
Disconnecting or disablingto observe reaction.
Experimenting at various speeds & loads.
Disconnecting or disablingto observe reaction.
Verify and qualify to pinpoint malfunction
or component.
Verify and qualify to pinpoint malfunction
or component.
If this is a representation of all cylinders superimposed, there
is no need for further diagnosis.
1. Adequate ignition to burn all the fuel.
2. Fuel delivery equal for all cylinders.
If a SNAP-TEST reveals no problemand the O2 sensor verifies computer control,
all requirements of ignitionand combustion efficiency are met.
Includes a 125 page
notebook.
You have just watched a sample of scope pattern interpretation.
