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Have they tried running a separate wire directly between the DME and injector #1 or 5 to bypass the engine harness?
Yes, unless I'm mistaken we've fully bypassed the harness for each of those injectors at separate times.
Originally Posted by Ahsai
Also, was the signal measured at the injector or the DME when they used the scope?
At the DME, so the missing signals are not getting sent rather than being lost along the way.
Both of those facts are what leads me to wonder if "the disease" is elsewhere in the harness wiring and these misfires are simply the different DMEs manifesting the symptoms in different ways (meaning there's nothing wrong at all with those injectors or their wiring).
When the DME detects a fault in the circuit it stops triggering the injector as mentioned above. It sounds like you have confirmed that DME A stops pulsing cyl5 injector(and DME’s B-D stop pulsing cyl1 injector).
This could still (and bare with me as this is reaching) be caused by DME A sensing an injector out of tolerance (actually cyl 1) but mistaking it for 5 and stopping cyl 5 injection. That is why I asked if you unplugged injector 1 with DME A causing a known circuit fault does it register cyl 1 AND 5 injector circuits?
I have to believe DMEs B-D at this point...which all blatantly point to a component that you have not replaced or swapped positions of?? In all my years working on these cars I’ve almost never seen a control unit incorrectly diagnose an electrical circuit fault due to a completely separate circuit unless someone has hacked the harness wiring (previous rat damage repair or screwed/drilled into harness wires?)
When the DME detects a fault in the circuit it stops triggering the injector as mentioned above. It sounds like you have confirmed that DME A stops pulsing cyl5 injector(and DME’s B-D stop pulsing cyl1 injector).
This could still (and bare with me as this is reaching) be caused by DME A sensing an injector out of tolerance (actually cyl 1) but mistaking it for 5 and stopping cyl 5 injection. That is why I asked if you unplugged injector 1 with DME A causing a known circuit fault does it register cyl 1 AND 5 injector circuits?
I have to believe DMEs B-D at this point...which all blatantly point to a component that you have not replaced or swapped positions of?? In all my years working on these cars I’ve almost never seen a control unit incorrectly diagnose an electrical circuit fault due to a completely separate circuit unless someone has hacked the harness wiring (previous rat damage repair or screwed/drilled into harness wires?)
I've wondered about that exact scenario (ECU A misidentifying the problem cylinder), but my indy thought it unlikely when I was picking his brain about it a couple of weeks ago. Hopefully this injector swap will tell us something!
Sorry for the delay, my indy’s been working on the car in little bursts with long gaps in-between as other cars come into the shop. Here are the latest updates:
First, while talking with him the other day we realized there’d been a slight miscommunication early on in this process: the [now replaced] cam sensor that had been wicking oil into the harness was actually a cam actuator rather than a cam sensor (this likely makes zero difference whatsoever to my misfiring problem, but I wanted to correct it here just in case).
We swapped the #1 and #2 fuel injectors with zero change in the behavior of the car: ECU A continues to misfire on cylinder #5 and ECU D “misfires” (injector fault codes and the noticeable slight shudder of a misfire, but no P030_ misfire codes) on cylinder #1.
We also realized that there’d been a misunderstanding on my part regarding what harness wiring we had/hadn’t bypassed. Turns out we hadn’t done ANY bypassing of the wiring for the #1 injector up until this point, so we completely bypassed the injector circuit (battery—injector—DME) and retested with both ECUs A and D: still no change.
Lastly, and this may be an important clue, we continued to learn more about what *exactly* triggers the P020_ fault codes. Back on the 26th I shared a message from my indy saying the three conditions that cause an injector fault code (quoted below), but it wasn’t until I spent some time with him at the shop I understood exactly what it meant on the waveform about an “induced voltage.”
"the ECM uses the induced voltage from the injector circuit to indicate that the injector is operating correctly electrically. It does not signal the mechanical condition of the injector . When the ECM opens the ground circuit to the injector (ceases current flow) a voltage is induced into the injector winding. This voltage shows the electrical integrity of the injector and associated wiring. Absence of this induced voltage results in a fault." The other two cases that will cause this fault are short to ground and or no voltage. We can see by the wave form that it is not the latter two cases. After reviewing the waveforms again there is no indication of any diminish in the voltage spike (induced voltage) when the injector current flow stops ie there is a spike on every waveform of a good injection pulse prior to a failed pulse/no signal.
It's not referring to the actual fuel injector signal pulse like I originally thought: it’s referring to the tiny spike at the end of each signal pulse (a couple examples are circled in red in the upper left section, plus I approximated where the missing injector signal would be):
What’s interesting is that if we had an actual injector circuit problem we’d expect there to be something different-somehow about the “test pulse” (what I’m calling the induced voltage that checks the circuit) immediately prior to any given misfire. As in: after the injector fires normally (downward box wave), the following test pulse would be somehow misshapen (different voltage/shape/etc to indicate a circuit issue), which the DME reads as a problem and thus the next injector pulse is never sent. Instead, on every waveform image we’ve taken, the test pulse prior to a missing injector signal looks completely normal in size/shape (see the bottom right image above, where I used purple to circle the test pulse and show where the missing injector signal goes).
So… At this point we’ve got two theories. Either,
A) We have FOUR bad DMEs.
Or
B) There is something going on electrically in the car, likely COMPLETELY outside of the six fuel injector circuits, that’s causing internal glitches within the four DMEs that they are each manifesting in the form of missing fuel injector signals. If the fuel injector is never signaled in the first place there wouldn’t be any, er, “rebound voltage” to form the test pulse, so the P020_ “fuel injector circuit” faults are merely symptoms of the missing injector signals rather than symptoms of what is causing the missing injector signals.
I realize that my explanation above may be poorly-worded/confusing, so for the sake of clarity here's how I described it when emailing the theory to my indy:
Maybe we've been looking at a large chunk of this issue backwards! We've known the misfires are a symptom of the missing fuel "squirts," which are a symptom of the missing signals from the DME to the injector, but we've been assuming for a long time that the P020_ codes are part of the disease itself. What if they're not? Here's why I think they may be just as much of a symptom as the misfires:
Remember when you said we'd expect the "test pulse" prior to a misfire to be misshapen if there was a problem with the circuit, but it's not? (Red circle below). The assumption there is chain of events like this: "a circuit fault is detected, so the DME doesn't signal the injector to squirt fuel, leading to a misfire"
What if instead the chain of events is "the DME doesn't signal the fuel injector (reason unknown), which means there's no induced 'rebound voltage' to compromise the test pulse, thus the DME reads the lack of a test pulse as a circuit problem"? Thus, the "P020_ injector circuit fault" is actually the DME reading the flatline in the purple circle and the injector circuit has little/nothing to do with what is causing the DME to fail to send injector signals?
It'd take us largely back to square one, especially because it's happening with multiple DMEs, but it'd make sense of why all our injector-related testing has been a dead end!
I would start with a visual inspection of the flywheel -that is the basis of all the timing .The issues appear to be around cylinders 1 and 5 .Firing order is 162435 -Depending on which dme cylinder 1 or 5 is flaky suggests flywheel .Also is cam deviation ok ?
Last edited by dcdrechsel; 05-11-2019 at 09:03 AM.
Reason: more info
I would start with a visual inspection of the flywheel -that is the basis of all the timing .The issues appear to be around cylinders 1 and 5 .Firing order is 162435 -Depending on which dme cylinder 1 or 5 is flaky suggests flywheel .Also is cam deviation ok ?
I'd previously discounted the flywheel idea because of the way the problem moves based on the DME installed, but I decided to look back over the flywheel discussion from a couple pages ago and focused on these posts:
Originally Posted by jumpy chunky
Shawn,
I thought I would try to get back to you before you spent more money on having your newest DME coded to your Porsche.
You have done an extensive amount of work to your car trying to figure out the cause of the misfire, but what I'm
proposing will take even more. My thinking is this, since you have had this fault ever since you brought home your
car and that your 996 had the IMS bearing done just 2,000 miles before you took ownership the fault may lie in the
installation of your new bearing.
A mechanic with no previous knowledge of the process of bearing replacement might have been surprised by the
weight of the dual mass flywheel. There is a chance that he dropped or some how damaged the toothed portion
of the wheel when dismanteling.
Since cylinders 1 and 5 fire first and last in the fireing order that may account for both misfires showing up at that
point on the fly wheel. Also the fact that the misfires are more pronunced when heated up, the crankshaft
position sensor would have a harder time picking up a signal from bent teeth throught hot air as that is denser
than cold air.
So with all that being said I'm not sure which direction you would want to take, or which is less expensive.
Examining the flywheel is a big job, but you may want to check everything else associated with that job since
the car is new to you. The flywheels do wear out and I think they recommend replacement every other clutch job.
Thats about all, A. P.
Originally Posted by Porschetech3
. Thiß is a good thought and I had considered the possibility of a damaged flywheel teeth but I discounted it due to missfires are not on paired cylinders.. On the m96 cylinders 1 and 4 use the same teeth of the flywheel so they would be equally likely to missfire. Takes 2revolutions to fire all 6 cylinders.
...particularly this comment from @jumpy chunky "Also the fact that the misfires are more pronunced when heated up, the crankshaft position sensor would have a harder time picking up a signal from bent teeth throught hot air as that is denser than cold air."
This is admittedly a reach, a LONG reach, but is it possible that there's a bent tooth related to cylinder #1 on the flywheel that would only be sensed intermittently/randomly (glitchy sensor? only slightly bent?) AND cause:
-ECUs B/C/D to misfire cylinder #1 (by failing to send signals to inject fuel) regardless of whether the engine is cold/warm (correctly reading the location of the bent tooth, but not always reading it as bent because they each vary in how often they misfire) BUT THAT
-ECU A would read normally on a cold engine (normal operation of all cylinders) and then misread as a cylinder #5 tooth when the engine is warm (causing cylinder #5 misfires)?
That's a whole lot of overlapping weird malfunctions, so even if it happened this way once or twice I'm really struggling to imagine a circumstance where it'd reliably switch to cylinder #5 whenever ECU A is installed many times over the span of several months (you'd almost have to have a slightly bent cylinder #1 flywheel tooth AND an unreliable sensor reading it AND a failed original DME that would misinterpret which tooth the sensor is unreliably sensing), but that's why I want to run it by you experts before we consider a job as big as inspecting the flywheel.
The other main idea that we're considering at the moment is a replacement of the entire engine harness based on the theory that this is an electrical/wiring glitch somewhere other than the fuel injector circuits. It's also shot in the dark without any direct evidence of bad wiring, but maybe slightly less Rube Goldberg-esque than the flywheel/sensor/DME theory? I don't know.
So new guy here,, but I read though this one,, and thought I'd toss something out,,
I had a similar issue(Not on a Porsche) , the root cause was iron filings stuck to the
flywheel in the notches for the crank position sensor. Result of a starter swap that had
dropped some magnetized shrapnel where it would touch the sensor. I've also had a flywheel
dropped, and it dented the edge of one of the grooves and it would not fire without the
CPS being at minimum clearance. Oh and A bad CPS sensor can play head games with the ECU.
You can get a good look at the flywheel teeth by removing the starter and rotating the engine by hand. It may be worth the trouble just to eliminate that as a possibility since this issue is so alluding.
But as I mentioned before cylinder 1 and 4 use the same teeth, cylinder 6 and 3 use the same teeth, and cylinder 2 and 5 use the same teeth.Takes 2 revolutions to fire all 6 cylinders(order 162435), on one revolution cylinders 1-6-2 use all the teeth, on the next revolution cylinders 4-3-5 use all the teeth..By all logic, is you have a bent tooth that effects cylinder 1 then it should effect cylinder 4 just the same. But stranger things have happened, so may be worth having a look. Dr. h.c. Porschetech3
Question for Porschetech 3 -in the lsx world there is a crank shaft position sensor relearn procedure for synchronizing cam and crank .Usually used when misfires are random or a new ecm (dme ) is installed .Does pwis have a similar procedure ?
Question for Porschetech 3 -in the lsx world there is a crank shaft position sensor relearn procedure for synchronizing cam and crank .Usually used when misfires are random or a new ecm (dme ) is installed .Does pwis have a similar procedure ?
Good question. There is a learning procedure/duration for the misfire monitor. When/if the DME is reset/new the misfire monitor is less sensitive untill after the learning procedure/duration (will not show misfires as easily, but will show injector/coil electrical faults). This is done to help with "false misfires" due to rough roads/pot holes ect. effecting flywheel speeds. The DME "learns" the flywheel teeth exact profile discrepancies and cylinder assigned. The synchronizing of cam to crank is done by cam position sensors.
Good question. There is a learning procedure/duration for the misfire monitor. When/if the DME is reset/new the misfire monitor is less sensitive untill after the learning procedure/duration (will not show misfires as easily, but will show injector/coil electrical faults). This is done to help with "false misfires" due to rough roads/pot holes ect. effecting flywheel speeds. The DME "learns" the flywheel teeth exact profile discrepancies and cylinder assigned. The synchronizing of cam to crank is done by cam position sensors.
Thank you .Good idea on checking the teeth thru the starter hole .My thought on the flywheel was not a tooth but erratic dampening which is pure speculation .
Is it possible that the dual mass flywheel could wobble if the integral dampener is failing, resulting in erratic readings by the sensor? Yeah, I know, off the wall, but thought I'd ask.
Is it possible that the dual mass flywheel could wobble if the integral dampener is failing, resulting in erratic readings by the sensor? Yeah, I know, off the wall, but thought I'd ask.
The flywheel teeth/ sensor ring is part of the fixed primary mass, so would not wobble. The secondary mass/ clutch can wobble or be improperly dampened or even become fixed/ stuck. I have seen dual mass flywheels fail in all those sceinerios, but didn't result in injector codes, the results are vibration, knocking noise, transmission ringing sounds when lugging. I have never had it cause a misfire/ injector code and would think it would be random if it did. If any of these other symptoms are present I would certainly replace the DMF and see if it solved all problems..
04-27-2019, 12:19 PM
