Without enabling the fuel trims, the PCM can't adjust the A/F ratio to compensate for lean or rich conditions reported by the O2 sensors. But looking at your data log, the short terms appear to be handling any minor compensation, and indicate that it's running almost right on where it should be with the LTFT's locked at 128.

I don't see the fuel trims being locked as contributing to the idle irregularities.

Just before many of the surges in RPM, there is a very low RPM reading, like something was going to make it stall, and the PCM responds by immediately jacking up the IAC counts to bring the RPM up, and then starts to close the IAC at excessive RPM, but it takes a long time for the RPM to come back down. When it is idling, do you actually hear it or sense it slowing down, just before the RPM starts to surge?

The reason I ask is that there are numerous events of a single data frame spike of RPM to 2,000+, 3000+, and even a 4,119 RPM. That, plus the sudden low RPM readings that the PCM seems to respond to, could be erratic RPM info from the Opti.

Can I assume that you were opening the throttle periodically? Throttle opens, RPM goes up, that part's normal if you were actually playing with the throttle. When you do that though, you start to get the random non-0 MPH readings - again very spotty, with a, 2, 3 or maybe 6 MPH in a single frame, while every other frame is "0". At the end of the log, you are getting numerous MPH readings in the 1 - 4 MPH range, with an occasional "0" frame popping up at random. Was the car moving at all?

 

No sir, I wasnt touching anything car was in park the entire time. look at the tps values you can see nothing moves on the TB. It was sitting still in my driveway .
It started doing it again today. seems to come and go. if I swap out the iac it will last a couple days, maybe a week then start up again. I have a genuine gm part from a buddys engine that works great on his car. Today it was surging up and down, at idle in gear. I took a short video of the Tach, you can see it twitch just before each episode. Tach twitches engine revs up. higher and higher as time goes on. It only goes up a few hundred r's at first by the time I am done its going over 1000 rpms. I cant keep replacing IAC motors every week , this is nuts.

 

But the TPS values do show several instances that the TPS was moving.... at least the sensor was reporting it was moving. While it sits at an acceptable closed throttle value of 0.63V, there are times when it increases substantially, and the RPM climbs proportionally. Not looking at the file at the moment, but it was showing over 2,000 RPM.

 

I had a surge issue which I traced to the IAC. Cleaned the IAC AND the port where the pintle moves with TB cleaner and a gun cleaning wire brush. Never had another issue.

.

 

Im sorry you are correct, I wasnt thinking about the correct log. I was thinking of a different one I was comparing it to from a couple months ago, completely different scan and problem. (Sorry Fred, I was having blood drawn today and had been fasting since yesterday, I was getting a little loopy from hunger)

I am beginning to think it is something in the optispark. I was out driving the car and got a bit frustrated with it and now, after a 5.5 k rpm pass, there is a fluttering miss at 1500 rpms on occasion. I had thought I heard/felt it before but there is no mistaking it now. feels like you are running over rumble strips. I may have collapsed a lifter also.
So now I am going to go out and adjust the lifters, check the wires and such.
After all that it still was surging at idle....stuborn damn car....

 

I have adjusted the rockers and it seems to be ok now, maybe it just backed off a bit.

I am going to contact Summit to find out about returning this msd unit and either getting another one or a Delphi. I have replaced everything else, every other sensor I can think of has been replaced. This is the only part that was put on that I can think of that could cause this problem. Either on its own or through erroneous info provided to the pcm.

It is just so random. Once it starts its absolutely maddening. I can watch the Tach, as soon as I see it twitch, vroooommmm, twitch vrooommm.... where does the Tach pulse come from....? the coil, opti or pcm?

 

Thanks. but I have replaced the IAC like 6 times now, they last about 3-7 days then symptoms return.

 

To answer your questions, yes I was revving it a bit, no the car was in the driveway not moving. I am leaning towards the optispark also as noted above. Sorry man I was a bit out of it yesterday.

So if the opti is providing rpm readings to the pcm and that info is wrong, or tainted somehow. could I also assume that is why my Tach does the twitch just before the rpm spikes start?

 

That's what I'm thinking.

What brand is your Opti? The guy who solved his surging problem with an Opti replacement had an MSD Opti when the problem started, and that unit had just been rebuilt by NSD due to earlier problems with it. He replaced it with a Delco.

I hate to just jump to the conclusion that the problem is the Opti, because I can't stand it whenever someone posts an engine problem, and somebody else immediately posts "it's your Opti", and is usually wrong.

 

I have a MSD opti in it currently, figures. I have been on the fence about what to try next but I think that about makes up my mind for me. So, Delco or Delphi or does it matter?
I know how you feel on the , "throw parts at it approach" but I think we have done due diligence towards solving the problem. Ill report back after I swap it out. Might be a while due to work. Thank you for your help.

 

ok, well heres an update for ya. I ended up selling my motor! I am picking up an ls1 car to do a drive train swap. I am installing my lt into a budd's firebird it will have a delco optispark on it , from his old motor, when it leaves here. So at least we will have an answer about what was causing the problem. But my $'s on the MSD opti.

 

Interesting solution to the problem.....

 

interesting, expensive too, but I am confident it will cure my opti issues.lol.
thanks for the help Fred

 

Ok. I installed my old Lt into my buds firebird. We used his oem opti. No issues. I believe it was the msd opti.
With the oem unit it runs like a champ.
So it goes....

 

What I can't understand is why people continue to assume the MSD is somehow superior to the factory unit.