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For those who is not familiar with OBD2 Mode 6, this tool can be a very good very early preventive maintenance and damage prevention.
This damn OBD2 Mode 6 for Mercedes is not available for its actual protocol or 101 full information
Example, what is the actual definition for Mercedes 1 drive cycle ? I can't find it.
OBD2 full info, a typical/generic one, but may not apply to all cars, is as attached from Snap ON.
Drive cycle available from Ohio EPA and BMW, as attached.
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What is the definition of misfire in technical speak ?
One requirement in the OBD legislation is misfire monitoring. CARB defines an engine misfire as a “lack of combustion in the cylinder due to absence of spark, poor fuel metering, poor compression, or any other cause.” (California Air Resources Board, 2016).
"The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. A single misfire will cause a subtle change in the speed of the crank." https://www.nyvip.org/PublicSite/OBD...%20the%20crank.
How does our ECM detect misfire ?
Most common used signal for misfire detection is the angular velocity measured at the flywheel. The oscillations in the angular velocity signal, related to the cylinders combustion, to detect when one or several cylinders fail to fire. http://liu.diva-portal.org/smash/get...796/FULLTEXT01
Above is a normal oscillation or speed surges. Read from a 24 tone teeth flywheel ( crank count). Above is a low resolution tone teeth quantity. 1 teeth = 25 degrees crankshaft angle
Don't expect to see super smooth RPM chart with 1 or 2 RPM difference only during each cylinder's piston travel, there is no such thing.
Anyhow, the peak signals are what ECM probably reads.
I have googled like MAD, no exact information on what sudden RPM decrease is logged as a misfire ?
30- 40 - 50 or what RPM ?
What RPM is MB using for its Mode 6 OBD2 misfire ?
I had last year , from OBD2 Mode 6, a detected misfire of 512 counts from a 10 drive cycle, at cylinder 2. Its too low to make it as a DTC material.
The formula is below from the attached Snap On document :
If I use simple averaging and all equal, it simply means 512/10 drive cycle = 51.2 misfire counts per drive cycle.
If 1 drive cycle is worth say 20 minutes and say 10 minutes of the section gets misfire monitoring and average RPM is say 1,500 :
600 seconds ( 10 minutes) x 12.5 4 strokes events per second ( 1,500/60/2 ) = 7,500 combustion event at all cylinders.
51 misfire from 7,500 = 0.0068% occurrence only and this does not made it to any Misfire category yet.
EMISSIONS-related misfires are generally in the range of 1 to 3 percent at all engine speeds and are a two-trip code.
The misfire must either be detected in the first 1,000 revolutions at start up or on four different 1,000-revolution checks while running.
CATALYST-damaging misfires are anywhere from 4 percent to 30 percent and depends on RPM and load under which the misfire occurs.
NOTE : Misfire monitor can be triggered not by "bad" combustion event only.
At high speed when the car jumps over bridge joints , it can be calculated as misfire when the load on the wheel ( and engine ) come and go from the tires being lifted up during the jump. https://www.jstor.org/stable/44611399
and from MB document called ENGINE SMOOTH RUNNING FUNCTION. Also attached as PDF.
Scan date Feb 2023
I find it odd that it is the same cylinder 2 and at 512 counts....again.
I am torn between a glitch or a real very mild misfire at cylinder 2 ?
The only time the car engine has potential very-very-very mild misfire kind of non-ultra smooth running, is when I started from cold.
The first 5 seconds sometimes I can sense "improper" burning of fuel in term of engine rhythm smoothness, but I never bother when and if it is within those 5-10 seconds of cold start.
Fuel Trim memory is from last engine kill and as always a very hot engine and the ambient temp sensor at the bumper would be corrupted anyway, when I need to pass thru the traffic jam to reach home.
Our engine does not go into fuel management close loop till like 2+ minutes or so, if from cold start. So non-ultra smooth engine running first 5-10 seconds is normal.
STATUS 2 is when Fuel Management in Closed Loop, aka Oxygen sensors is involved.
Example of OBD2 Misfire Count in use when Misfire DTC not triggered...yet. Yep, time consuming test drive yada yada.
So, I am on a tough mission to find this cylinder 2 misfire I can't detect from my azz + ear sensor
I looked at my archive to see what is my normal RPM variation between each cylinders piston up-down movement.
NOTE : The cylinder numbers in brown channel D are based on current ramp of the Ignition coils. Its powerful power consumption, 19.9 amps peak...WOW.
We M276 and probably the same for most MB engines, are fortunate the crankshaft tone wheel is a 60-2 or a 58 teeth and is considered a high resolution one. 1 tooth = 6 degrees.
Since the way our piston move up and down is kinda unique in respect how the crankshaft is designed and as piston moves down it looses velocity too, piston or crankshaft speed then varies at
approx 28-29 RPM per 90 degrees of crankshaft revolution at 650ish RPM idle speed.
Mean Piston Speed can explain what I describe above : https://en.wikipedia.org/wiki/Mean_piston_speed
I guess the ECM will take the peak signals only as current RPM, which in my scope capture above it would be 672 to 677 RPM in one 4-stroke cycle I shown as 720 degrees crankshaft spin.
Peak between peak RPM between slowest and faster cylinders in the one 4 stroke firing cycle ( 720 degrees) I shown, is only 5.6 RPM difference. I also measured at other waveform pages, max 7 RPM.
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Here is an example of a REAL misfire seen by a scope, from crankshaft speed too but without a misfire DTC.
This is the older Pico6 software, which is not as advance for frequency* ( *kind of RPM, but not ) calculation like the Pico7 I use.
Hence you will find it weird that the engine above seemingly doing only 410 - 420 Hz frequency and if we think it as idling RPM, that is too low.
Also on the Pico6 the frequency lines does not auto-connect like on my Pico7. And then, the frequency indicated in Pico 6 from a crankshaft tone teeth is lower than actual engine RPM,
where Pico7 can calculate actual RPM from frequency , because I can input the data perimeter of my crankshaft teeth arrangement.
A crankshaft tone teeth in the above Mitsubishi engine with fewer* total teeth ( *counted as 36 teeth or 1 teeth = 10 crank degrees ), frequency reading resolution can't be as high as MB type of 60-2 tone teeth.
Mario said the misfire is from the cylinder 4.
I calculated and labeled its peak between peak RPM of all 6 cylinders in one 4 stroke firing cycle ( 720 degrees) is 410 to 419 or 9 RPM difference, still Mitsubishi ECM algo does not yet define it as a misfire worthy DTC.
Maybe a typical ECM algo will trigger DTC only if at 20ish RPM difference peak between peak RPM ?
Mario initially was using secondary ignition trace of cylinder 1, to mark cylinder 1 to create the 720 degrees crankshaft revolution per one of 4 stroke cycle.
Later he read all cylinders. The misfiring cylinder 4 has too short a burn line of 600 micro second while the good ones are +-1,200 micro second on average.
Secondary Ignition burn line can show a misfire if ignition caused.
Above : The bad cylinder 4 firing line ( secondary ignition ).
The video is to show few ways to identify a fish-bite and intermittent misfire exact cause , where DTC never yet pops out...YET.
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Well, let me scoped my ignition coils secondary ignition to see what's up with cylinder 2.
I never ever scoped them yet.
I did a test drive today, as part of my once a week MUST DRIVE THE CAR protocol
As I explained before, road bumps and more so at high speed will create a false Misfire.
I also got to learn on how the average 10 drive cycles is counted. Any current drive cycle when completed will be 10% of the last 10 drive cycles.
8th Feb 2023 Test Drive as CURRENT DRIVE CYCLE
So the ECM OBD6 misfire count for last 10 drive cycles, uses 0.5 and higher as 1.
C column current drive cycle MISFIRE divided by 10 Drive Cycles is the D column .
I guess if , say cylinder 1 has 15 misfires, divided by 10 and I get 1.5, the Last 10 Drive Cycles misfire will be counted as 2 misfires.
Indeed the drive cycle need to be quite long as per typical OBD2 generic drive cycle as explained by Snap ON.
I did test a super short distance of 1.4KM from my home to the TOL entry and killed the engine. This short drive from warm up to engine KILL is 560 seconds or 9.3 minutes.
OBD2 Mode 1 - Readiness Monitor does not count this short distance as drive cycle, but counted as ignition cycle. So today's test gets 2 ignition cycle counts.
I did all kind of test, lug the engine at 7th gear MANUAL at low speed 70KM/H and on a mild gradient, I throttled to 85% ( no kickdown ).
High speed jump, high speed jump and hard braking when I know the car will jump/land.
The test was 63 minutes. Distance wise in excess of 50KM for sure.
The misfire counts during this test drive is good, as all cylinders experience the FALSE "misfires"...they should.
I worried my 512 misfire count could be from certain bi-drectional test I did last year.
Believe it or not, last few month of 2022 after I cleaned the intake valves and yada yada, I doubt I done 10 drive cycles on the car.
Will update when I see those unique 512 misfire counts again.
02-07-2023, 11:55 AM
