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First off, when I start chasing any issues, I stop and verify mechanicals immediately.
I'd say a running compression test is in order for this one, before you chase your tail any further.
I have seen injector codes and misfires from cylinders that had no injector, or coil faults, but had mechanical issues on that same cylinder. A running compression test is the only way to go here, static tests won't show the issues.
I had a Chrysler V6 that had been to 2 other shops for a random misfire on a specific cylinder issue come to me. You would have sworn it was an ignition issue from the "feel" of the misfire. Of course, the other shops had covered all of those basics. A static compression test yielded normal results. A running compression test however would show an anomaly every time the misfire occurred. Long story short the engine had a valve guide issue that was hanging a valve intermittently causing the skip.
What bothers me with your scenario though is the P0201 and P0205 codes. These are circuit codes and will not be set from a misfiring cylinder for any reason other than something related to the injector circuit. How I think you could best isolate this misfire further is to get an injector connector, and make a separate sub harness to run to cylinder 1 injector and all the way back to the DME. It doesn't have to be pretty because this is purely for testing purposes. I would pull the 2 pins from the DME connector and replace with my sub harness with the correct pins for the DME on the ground side and a known good +12volt on the other side and see if the issue is resolved. If so, go back to the original wiring and scope the injector on both sides and see if you are losing the power side or the switched ground side. Once you determine which is failing you now get to hunt and find where the harness is wounded. I know of some techs that think the separate harness is an acceptable permanent repair, but I personally dislike that because if the damage is caused by something like a chafed bracket there is likely going to be other damaged circuits in the future. Better to find it now and keep it from happening again. If your misfire and injector codes are still there after this, you need to test the grounds to the DME itself and verify they are good all the way to the DME if you have isolated the failure to the switched ground side. Once that's done, you have pretty much effectively ruled out everything but the DME itself.
A static compression test yielded normal results. A running compression test however would show an anomaly every time the misfire occurred. Long story short the engine had a valve guide issue that was hanging a valve intermittently causing the skip.
That's a classic.. Same thing happens with a lifter in these engines.
Yes, if one ECU works fine, and the other doesn't;t, then it doesn't sound mechanical... Never hurts to check, though.
What bothers me with your scenario though is the P0201 and P0205 codes. These are circuit codes and will not be set from a misfiring cylinder for any reason other than something related to the injector circuit. How I think you could best isolate this misfire further is to get an injector connector, and make a separate sub harness to run to cylinder 1 injector and all the way back to the DME. It doesn't have to be pretty because this is purely for testing purposes. I would pull the 2 pins from the DME connector and replace with my sub harness with the correct pins for the DME on the ground side and a known good +12volt on the other side and see if the issue is resolved. If so, go back to the original wiring and scope the injector on both sides and see if you are losing the power side or the switched ground side. Once you determine which is failing you now get to hunt and find where the harness is wounded. I know of some techs that think the separate harness is an acceptable permanent repair, but I personally dislike that because if the damage is caused by something like a chafed bracket there is likely going to be other damaged circuits in the future. Better to find it now and keep it from happening again. If your misfire and injector codes are still there after this, you need to test the grounds to the DME itself and verify they are good all the way to the DME if you have isolated the failure to the switched ground side. Once that's done, you have pretty much effectively ruled out everything but the DME itself.
Thanks Matt! What you describe sounds a lot like the bypassing we've already done, but I forwarded your suggestion to him anyway just in case there's some tidbit that we've somehow missed or overlooked.
Have you scoped the fuel injectors?
How do the waveforms look?
It's on our list! My post this morning (~8:30 your time in TX) was roughly in the order we're planning on doing things, so when we get to the scoping stage I'll definitely let you know. I'll try to get pictures of the waveforms to post on here if at all possible!
Whilst I am not suggesting it's your issue based on the misfires changing with the ECU. I had a persistent misfire that was ok when cold but would trigger when hot or revved that turned out being a sticking valve lifter which totally resolved the issue. looking at the service records that came with the car it was clear the previous owner and various shops had tried to fix the misfire with new coils, plugs and sensors to no avail.
Lifters do stick on 996.2 3.6 engines, you replace them with a revised part.
Whilst I am not suggesting it's your issue based on the misfires changing with the ECU. I had a persistent misfire that was ok when cold but would trigger when hot or revved that turned out being a sticking valve lifter which totally resolved the issue. looking at the service records that came with the car it was clear the previous owner and various shops had tried to fix the misfire with new coils, plugs and sensors to no avail.
Lifters do stick on 996.2 3.6 engines, you replace them with a revised part.
Fascinating: I'd heard only the Turbos had trouble with sticking lifters! That's good to know even if it's not related to my current issue. Thanks!
It's on our list! My post this morning (~8:30 your time in TX) was roughly in the order we're planning on doing things, so when we get to the scoping stage I'll definitely let you know. I'll try to get pictures of the waveforms to post on here if at all possible!
No joy from our cam sensor/four coils/injector swap/MAF-unplugged work. Yesterday he hooked up the his scope and recorded three different runs measuring the fuel injector signals from ECU A to cylinder 5 and to a known-good cylinder. He didn't include any pictures, but here's what he said:
"I graphed/recorded three different runs and every misfire showed a no pull to ground from the injector driver on the original DME [ECU A]. I graphed the offending cylinder [#5] alongside a good cylinder and the results are a tell all: no DME output ie no pull to ground just a flat 14v line between the square waves of a good cylinder whether it was a single misfire or multiple misfires. All three tests had identical results. We need a good computer. [Ran out of time, so] I did not put ECU C in and run the same test to watch #1 injector."
I've emailed him asking a couple of clarifying questions such as where in the circuit the signal was being measured, but it sounds like whenever there's a misfire it's because the ECU isn't sending a signal to the injector at all. I've asked him to test ECU C and cylinder #1, but I'm not sure when he'll get a chance because they've got a full case load.
If the results are the same with ECU C (no injector signal altogether), are we back to the "bad ECU" theory with three bad ECUs in a row? Considering those crazy odds, is there anything else that would make sense of this? I'm tempted to send ECU C down to Specialized ECU in Florida for testing (it shouldn't be able to pass their tests like ECU A did because it causes misfires much more quickly), but even if they confirm the problem repairing/replacing it through them would be more expensive than buying yet another eBay DME and having it programmed.
Haven't heard back from my indy on those questions yet, but I went ahead and ordered yet-another DME from eBay. It has a 30-day return window and will take a few days to arrive, so I figured it would make sense to get that process moving while we try to decide if that's the direction we should go.
Any ideas? I'm thinking of posting a thread in the East US Region board asking if there's anyone who'd be willing to let us borrow their car for a couple of days so we could try swapping a known-good DME (+ immobilizer + key: I think you need all three) into the car, but I'm not optimistic we'll get any volunteers.
Yes, no help ie still misfire and p020 codes. I didn't do the injector swap, not enough time. The scope recordings were performed with the original DME [ECU A] on cylinders 1 and 5 simultaneously as a dual trace. The scope was hooked directly to the DME. Remembering that cylinder 1 would set P0201 (with ECU C) fault right away and cylinder 5 was fine I only scoped those two circuits. I recorded 3 separate drive cycles and cylinder 1 always showed a picture perfect square wave from voltage to a solid pull to ground from the DME while cylinder 5 on every misfire the pull to ground by the DME would either start to pull to ground poorly or no pull to ground at all. See attachment . I question whether or not the computer place in Florida can repair an injector driver
For the sake of clarity: all four images below are with ECU A (the original), which is the one that is more intermittent about the cylinder #5 misfires (yellow line) and has no problems with cylinder #1 (green line). I'm hoping he'll have a chance to scope it again with ECU C installed, where we'd expect to see cylinder #5 running smoothly and cylinder #1 barely firing at all.
That's just crazy !! The odds for having 3 bad DME's is unheard of..That said it looks to me your problem.
I was specialized in drivability for over 20 years for FoMoCo and was awarded CTP's top Technician award for the southeast region for many years in a row during the advent of electronic fuel injection, and then for 10 years with Mitsubishi and then 10 years with Porsche and can say that I have never seen 3 failed DME's /ECU's in row like that.
I can also say that 90% of the failed DME's/ECU's were due to wiring shorts that damaged them, or bad engine grounds that sent starter ground amperage through them on startup instead of through the ground cable.Your starter is spinning fine/strong/fast right?
Since you have now gone through all the ground connections and tested, cleaned and tightened them, maybe you will have no more DME failures and one more good one will do the trick.
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.
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.
. 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.
. 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.
Yeah, thanks for the info. Kind of a reach not sure of flywheel layout. Hope a fourth ECU does the trick.
04-09-2019, 12:16 PM
