Timing Issues with Built Motor and LS Coils
09-20-2021, 07:55 PM
#1
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Timing Issues with Built Motor and LS Coils
So I've been getting close to perfecting my tune on my built 1995 1.9l and have started to notice intermittent misfires under load/RPM. I've experienced the issue using two different sets of coils (D581 and D585) both with sequential spark under many conditions. Recently, I noticed that timing error % starts becoming highly erratic when the misfires happen and I imagine this is the cause. I see spikes as high as +/- 12%. My dwell is taken straight from some of the threads here that have mapped genuine GM dwell tables into MS3 versions, but it seems possible that dwell has to do with it because timing error seems to decrease when dwell steps down. I've attached my tune and log, anyone else experienced anything like this?
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09-20-2021, 11:22 PM
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I pulled some dwell and took it for a drive again. The timing error is no longer as dramatic but I am still getting misfires. I'll pull another 0.1 across the board and see if I get better results, do you have a table I can look at in the meantime to compare?
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09-20-2021, 11:25 PM
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Interesting... in that case I will assume it is unrelated unless I see something that indicates otherwise. I do wonder though, if ignition timing based on crank angle predictions and if so wouldn't it also affect ignition timing?
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09-21-2021, 12:23 AM
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You're not getting it. The large timing error number is caused by your misfire event. If the engine misfires then engine speed will slow down/accelerate more slowly than expected which will cause the actual engine position trigger event to come later than predicted.
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09-21-2021, 12:26 AM
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That clears up my misconception here. Timing prediction is derivative, misfire events cause projected timing error to increase due to decrease engine acceleration. I will fully ignore that since it is clearly just a side effect of the larger problem. Pulling plugs tonight to see if there are any indicators there.
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09-21-2021, 01:54 AM
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4.5ms might be too much depending on what your battery correction table looks like.
Raw data straight from a 5.3 truck ECM (6.0 data is extremely similar):
I punched that all into Excel and produced the same data as a percentage of dwell at 14v, then distilled that a little further to give me two 2D curves to punch into the MS3.
Dwell vs. RPM:
Dwell vs. Battery voltage:
Without a 3D table like the GM ECU has, you're stuck with two linear 2D curves. The battery curves are correct for everything above ~5000rpm and a little low below that, but since these coils discharge with excessive dwell at high RPM, I felt it was better to stay on the conservative side everywhere. The tables above will never produce a dwell figure that's higher than the OEM table, which is what I wanted.
I have cranking dwell set at 4.5ms and the car cold-started in my 68*F shop, but YMMV.
Raw data straight from a 5.3 truck ECM (6.0 data is extremely similar):
I punched that all into Excel and produced the same data as a percentage of dwell at 14v, then distilled that a little further to give me two 2D curves to punch into the MS3.
Dwell vs. RPM:
Dwell vs. Battery voltage:
Without a 3D table like the GM ECU has, you're stuck with two linear 2D curves. The battery curves are correct for everything above ~5000rpm and a little low below that, but since these coils discharge with excessive dwell at high RPM, I felt it was better to stay on the conservative side everywhere. The tables above will never produce a dwell figure that's higher than the OEM table, which is what I wanted.
I have cranking dwell set at 4.5ms and the car cold-started in my 68*F shop, but YMMV.
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09-23-2021, 05:24 AM
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I've double checked my plugs and they look good albeit slightly rich as expected while tuning. Also ensured that they were all gapped to 0.040. Took the car for another drive and got the attached log with some hardcore misfiring under load and then what appears to be a loss of sync where RPM drops to 187 and the car enters cranking state for one log point.
My two suspicions as to the cause of the weird RPM signal are:
I am not really sure where to go from here to troubleshoot further. What would yall do to track down the cause of this misifre?
My two suspicions as to the cause of the weird RPM signal are:
- grounding issues (which I have already looked into a little)
- bad connection in my ECU connector (my tach signal already has this issue, likely just from being yanked too many times)
- CAS going bad
I am not really sure where to go from here to troubleshoot further. What would yall do to track down the cause of this misifre?
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09-23-2021, 05:12 PM
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log file seems very sparse, like resolution is bad - only a single update between 5000rpm and 187 and then back to 4000.
3 doesn't seem likely, CAS failures tend to be more pronounced. 2 is nearly unheard of.
Have you tuned the trimpots?
3 doesn't seem likely, CAS failures tend to be more pronounced. 2 is nearly unheard of.
Have you tuned the trimpots?
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09-23-2021, 09:57 PM
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Don't have pots, running MSLabs MS3 Basic. I agree, CAS failure would show issues elsewhere. That is just a snippet of the log -- look at the time -- RPM drops from ~5100 to 187 in less than a tenth of a second and then back up to 4400 in about the same time. I have also never heard of the ECU connector going bad, but I have confirmed that my tachometer output wire is definitely failing at the connector -- a simple jiggle will cause the tach to gain or lose signal. I've attached another log -- a full one -- with multiple more examples of this RPM drop occurring although no misfires because I was not pushing the car hard.
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10-15-2021, 03:35 PM
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Replaced spark plugs and the issue got marginally better -- still misfiring though. New genuine OEM (white epoxy) coils are on the way rather than the ACDelco black epoxy version I am currently running. When I put the new coils on I will recheck plugs and post pictures here to see if any one coil was acting up.
I have isolated my RPM signal loss issue to a loose ground in the ECU connector -- likely one of the sensor grounds as logs show all sensors keep the exact same values during the disconnect. It also only occurs when hitting a large bump or other jolt so I will tackle that issue later.
I have isolated my RPM signal loss issue to a loose ground in the ECU connector -- likely one of the sensor grounds as logs show all sensors keep the exact same values during the disconnect. It also only occurs when hitting a large bump or other jolt so I will tackle that issue later.
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10-16-2021, 10:31 AM
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Interested to see what you turn up @intenseapple ... been working with a similar setup that exhibits similar symptoms, his is equipped with an FM "Big Spark" kit so in theory it shouldn't be the coils (although I can't say for sure where they sourced them.)
It looks like yours is a more pronounced miss though, at least in your 15_modified log. His is more similar to your 17_modified log, and the behaviour is more of a drivetrain-jerking than an outright miss. Reducing dwell helps but does not completely eliminate the problem, it's just more subdued. I had it down to as low as 2.5ms fixed.
It looks like yours is a more pronounced miss though, at least in your 15_modified log. His is more similar to your 17_modified log, and the behaviour is more of a drivetrain-jerking than an outright miss. Reducing dwell helps but does not completely eliminate the problem, it's just more subdued. I had it down to as low as 2.5ms fixed.
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