So I'm helping someone with a misfire on Corvette Forum...

He's 8D, 1991.

Car is basically stock except for relocated MAT, disabled AIR and a heated O2. He has the ability to tune, so I had him make the necessary changes in the chip for these things. He's been sending me Tunerpro data to review too

I'm not sure if this is related to the misfire, but I notice that when he lets off the throttle, the spark timing literally drops out... Closed throttle spark timing is 20° for 8D, but I'm seeing the commanded spark timing drop down to 7°! It appears to recover when he applies throttle again. I'm also not seeing anything else in his data that would make the ECM pull timing like that... sensor readings are good, no knock retard kicking in, etc..

Could the ignition control module cause the ECM to command a spark timing that low? I thought maybe the EST wire having an intermittent connection, but he's not getting an error codes on that and I don't know why it would only manifest at closed throttle.

BTW, this is happening above 50C too when the Coolant bias spark table values go to 0, so it doesn't appear related to that or coolant temp.

 

Is it in DFCO when the timing drops out

 

Thanks for the rely BHR.

DFCO is not active, and in some cases the vehicle speed is actually 0.

I'm actually attaching the .csv datalog here in case anyone wants to take a look. The yellow highlighted areas are my doing for a reference point during my analysis.

 

Have you verified this is actually happening with a timing light and not a datalog glitch/error?

 

He'd have to get lucky to capture it as it's a very quick intermittent.

I'm skeptical its a glitch because everytime I've seen glitches happening in TP, other parameters glitch at the same time... you start seeing wildly improbable sensor readings and stuff... I'm helping some guy with a 93 LT1 too and his Tunerpro data is doing exactly that...

In the case of this other guy with the 91, the other parameters seem to be stable, and only the commanded spark timing is anomalous.

 

Is this something he is feeling seat of pants or effecting the motor? If not, I'd lean towards just ignoring it as a non-issue quirk.

 

He's chasing a misfire.

https://www.corvetteforum.com/forums/c4-tech-performance/4882937-strange-misfire.html

We seem to be narrowing it down to fueling on the passenger side. But the timing thing was an incidental finding i made while reviewing his data

 

I'd be swapping drivers side injectors with passenger side and see if problem follows. If all was fine before the injector swap, then it points directly at the injectors. Weirder things have happened with new injectors.

 

Agreed... the only thing is TPI is such a pain to get to the injectors, we're attempting to use data as much as we can to narrow it down. At that point, if he commits to a tear down to get at the injectors, it may be he decides it's worth it to just get them all cleaned or replaced before re-assembling everything.

If it were a Miniram.... I can replace all my injectors in about 20 min.

 

I'd like to help you but I can't work with a .csv!!! It's a pain in the **s to go through 5200+ rows by 141 columns. There is no highlight. I work with .XDL and TunerPro Monitor/Graph, way easier.
Do you have the original .XDL?
Do you have the BIN?
You can compress them and attach them
Thanks

 

Hah! I know what you mean... I always ask for the XDLs too.

When I get home, I'll zip them up and post it.

 

I'd put money on injector issue. The odds of something else random popping up at the same exact time only the injectors were swapped are pretty small. Unless a gasket or something isn't sealing or he messed up something during reassembly.

 

Me too, as well as he... something up with the one or more injectors on the passenger side.

Here's the xdl and associated bin. I had him in open loop at this point, trying to isolate closed vs open loop. As you can read in the thread it didn't make a difference.

 

As far as the SA issue, I don't see anything wrong with it based on your .XDL and .BIN

The "Cold SA" is deactivated around 130Deg.F as per your BIN
After that, the only time SA drops to the low10s* is in idle mode. Your BIN is programmed to retard up to 9.8* for RPM error in idle mode. -9.8* is reached at only 50 rpm error.

Personally, I always reprogram the SA main table and get rid of that annoying cold SA feature. Just a preference.

Peace

 

This alone is pretty excessive I would think. I have no more than 4 degrees on mine and that is only at 200 rpm error. Scales down from there to only 1 degree at 100 rpm and 0 at 50 rpm.

 

They came like that from factory!!!
I have mine reprogrammed
There is a lot of this factory stuff that is not optimal.

 

Good point... I forgot about the idle speed compensation via spark.

I actually zeroed mine out. I was able to get my idle stable enough without that.

 

Ain't that the truth....


Guys, thanks for taking a look at this.

 

late to this but curious how injectors effect timing glitch on screen or at the plug?

to see if it's real could you data log the ground side of the coil, like for a tach and see if it drops out?

 

The timing glitch was only noticed when he started reviewing they guys datalogs due to a misfire issue that popped up. The misfire issue popped up immediately after swapping injectors for new ones. Which throws major red flag at the injectors being the cause of the misfire.

I have no good answers for the timing glitch. If it isn't noticeable or causing running issues, to me, its a non-issue and just a glitch. He seems to imply it is quick enough of an event that it wouldn't be noticeable when checking timing directly.

 

Yeah, it's likely the injectors.

His theory is that he didn't clean out the tank and fuel lines well enough before installing his new injectors and now one or more on the passenger side are clogged with debris.

 

I switched last week from ULTM8Z's miniram bin to 5.0tpi bin. When idling I did see single digit SA figures. Also saw high IAC steps of around 60 at idle. Switched back to ULTM8Z's bin since it's superior. SA is now mid 20's and IAC is under 10 steps.

 

I think you are posting in wrong thread! Have no clue what you are talking about.

 

I guess I should have explained a little more. My spark appeared to be dropping out at idle after I switched to the 5.0 bin. It sounds like the same problem as what the poster was seeing. The problem was software not mechanical in my case.