This has been driving me nuts for a while now. I've got the cold start where it idles just great. Starts up pretty well and just hums along fine while the engine is warming up. Unless I blip the gas pedal while it is still cold. Then it just goes bonkers and either just stalls out completely, or engine speed swings slowly high and low for around 19 seconds before stabilizing again. Once the engine cooling temps get above around 100 degrees, the engine seems happy with the throttle blipping and doesn't react so negatively afterwards.

Best I can figure out it's being caused by the throttle blade closing up too far after the gas pedal blip. Idle seems fine at 17.3 percent throttle and all vital signs seem OK, but the gas pedal blip will have it dropping down to 14.1 percent and it appears to be starving for air then. I've played around with the tune (EFILive) till I'm blue in the face, but nothing I do seems to change that 14.1 percent to ANYTHING different. I've played around with airflow because I presumed that this was the way that the PCM controls airflow into the engine. Nothing. So I tried playing around with spark advance, thinking I'm chasing a "chicken or the egg" kind of problem. STILL nothing.

So anyone got any ideas? It has to be something, and I'm sure I'm just over looking something stupid, but this chasing my tail is getting tiresome.

Oh yeah, I'm a complete novice at this tuning stuff, but trying my best to figure it out. I actually thought I was getting a little bit of a handle on this till this little nagging problem has been slapping me around. I've been able to smooth out a few driveability issues so I have had some success at my hunt and peck type of learning.

Anyway, here's a video I took several days ago merging the log file display with the audio of what the engine is doing. I had to cut and paste the log file a bit, as apparently it is not in real time to keep pace with the audio stream I'm using. But you'll get the idea.



Any help would be GREATLY appreciated.

 

I am far from an expert but I notice 2 things in that video. It seems to get better once it goes into closed loop. Maybe try setting it to go closed loop a bit earlier. Also, its hard to tell from that, but your idle spark seems pretty low. Also idle airflow could also cause issues like that.

 

Yeah, I think airflow is the key, but so far I have been unable to convince the throttle body to NOT drop down to 14.1 percent after the gas pedal blips.

Trying to fix a cold start problem is a real pain in the butt, because you only have one chance each day to try something new. Consequently, I'm at the frustration stage where I make a bunch of changes just to see if something, ANYTHING, changes. So I'm not sure if I am changing too many things at one time and they are cancelling out each other, or I'm just missing some enabler setting somewhere.

Regardless, when the changes make no difference, I just scrap the new tune and roll back to the previous one.

So I'll try another whack at it tomorrow morning taking your comments under consideration.

Thanks for your help.

Oh, BTW, if you go directly to YouTube to view the video, use the highest resolution and full screen it, the values on the log trace are a LOT easier to read.

 

Well finally! Something I did made a difference. I made changes to 13 tables that I thought might make a change to that stalling problem. Apparently one or more of them did the trick. Now I know this really isn't the recommended way to do tuning on a car, especially if you don't know what you are doing (like me) as most people will recommend only making a single change each time and then test the results. But this would take forever with the cold start issues. The iterative process is pretty drawn out because of the time necessary for the engine coolant to cool back down in between runs. So I can only get maybe two runs per day to check out the changes I make.

Anyway, here's where I am now...



The stalling has ceased, probably because the throttle blade doesn't drop down below 16.9 percent. But there is still a loping of the idle after the gas pedal blip. And apparently idle is revving up quite high as the engine warms up right before going into closed loop mode.

Some of the difficulties of this stuff is trying to figure out whether the chicken or the egg came first when looking at the results seen in the log data. You can see the RESULTS, but what is the actual CAUSE of what you see? Is airflow, or the AFR mix causing the timing to change, or visa versa?

So I'm going to make changes to each of those 13 tables mentioned above, in order, to try to get the idle to come back down to a more normal speed. The engine loping is still an issue, but it's not really a killer like the stalling issue was. I may or may not spend time trying to track that down and fix it immediately, as I want to move on to throttle response as soon as I can.

But at least now I feel that I am moving forward...

 

Without knowing your mods its difficlut to figure out however an egine that is stockish/boltons should not have any idle issies unless a sensor or the MAF table is out of wack--BUT if you have some major mods like heads/cam /LT headers/differnt intake manifold/larger T-body/ altered from stock MAF then you need to adjust some tables to avoid stalling and surging
When COLD the eng runs in "open loop" until the ECT reaches 140*
after warmed it stays in open loop on a timer for about 10-20 seconds
(start up ECT above 140*)
So it sounds like the stalling is while its in open loop
Typically with mods as I've described the way to corrrect the open loop fuleing is in the "open loop fuel table -normal"
You need to LEAN this AFR down-usually the stock OL fuel is richer like in the mid 13's As as example on my car to get it to NOT surhe i had to lean it down to 16.2 in the start up fuel areas i the "OLnormal table) to make it quit
Not to worry its not really that lean---But becuse of your mods your sensors (02 mostly) are rreading incorrectly and youu have to trick the ECM into running less fuel in OL--but in reality the OL fueling will be close to stock --On my car when i inputted 16.2 Ol AFR my wideband actually proved it was 13.5-14.0----

 

Actually my car doesn't have very much under the hood that is stock left in it. -> http://forums.corvetteforum.com/c5-t...lxr-heads.html

Custom built 427 by LME with STS twin turbos. Tranny and dif rebuilt by RPM with the tranny using C6Z gears. Fuel system replaced by a dual pump setup from Lonnie's and the fuel pressure regulator is MAP referenced. The MAF is still stock for a C5 Z06, however. 90mm TB from earlier C6Z (silver blade) and FAST 102 intake.

So I know no one could really give me EXACT values to put into any given table to help me out, I was just trying to figure out which tree (or trees) that I need to be barking up for the answers.

The odd thing is that if I don't touch the gas pedal at cold start up, the car starts up strong and idles pretty well. But as soon as I stab the gas pedal, it's like the PCM just goes into another room to get the values it needs for idle for 19 to 20 seconds, then comes back to idling OK.

The oscillation I have with the idle after it gets destabilized is not too bad, but I know it's going to nag at me that it's not *perfect*. But at least I was able to lower the lower limit of the dip in RPM so that it isn't stalling out on me. I've been looking over the log, and it seems that the throttle blade position is staying stable, so something other than varying airflow is apparently causing the variance in RPM speed. And it seems to become unstable for a period of time that is fixed somewhere.

The problem is that with the traces on the log that are showing the variance, are the showing me the symptoms, and I'm not sure where to go hunting for the cause.

But anyway, point taken about looking for the answer in the open loop tables. I'll try leaning it out a bit and see what that does for me.

Thanks...

 

Well, something I did in the previous tunes has the idle running too high, which is very noticeable when I push in the clutch to coast to a stop. Revs up to 1400 rpm or so, and then drops down to a more reasonable level (around 950) when I come to a full stop.

So I rolled back to an earlier tune and am going to have to try one alteration at a time.

Any ideas on why there is an apparent state change from idle when I blip the gas pedal? It acts like for 20 seconds it is accessing different tables after that event than it was using prior to the blip. So apparently the gas pedal blip is the trigger for that change.

Is there any way in EFIlive to actually show (as per something similar to a debug routine in most programming languages) which tables are being accessed by the PCM in real time?

 

I want to state up front that I am a novice tuner. I have the Banish DVDs and have been tuning my car for years now with success, however I by no means consider myself a "tuner." With that in mind, here is my .02:

Video 1: Your overspeed/underspeed tables are not aggressive enough for the camshaft you are utilizing. The vehicle is stalling out because you need to have more timing in the underspeed table. I was trying to pause the video at just the right time to see what the timing was when the car was stalling. I noticed that the timing was very low then would jump up to 15.5, when the car was stalling out. Adjust the underspeed table so when the rpms dip, the underspeed table will add enough timing to bring the idle back in line. IMO, the vehicle is dying because there is not enough timing being added to bring the vehicle back to life.

Video 2: I notice that when your idle dips to around 700 rpm you only have about 5-7 degrees of timing in. That is an underspeed problem (imo). Add timing to your underspeed table to bring up your idle rpm.

Without actually verifying what your overspeed/underspeed tables look like I cannot tell you that is the problem for sure, but that is the first place I would look. If I recall correctly, my tune has an upwards of 20 degrees of timing added when the rpms are only 200 under. The more aggressive the camshaft, the more timing is necessary to keep the idle in line.

Good luck

 

Another very important piece of information ( and I cant believe I missed it cause I basically got schooled on this from another member in another post): Your AFR is way too rich at idle. The fist vid it is all the way down in the 10 - 11 range. In the second video it goes up to around 12 but that is still too rich. Lean out your open loop EQ table in the appropriate areas and your idle quality should change dramatically. Sorry I did not catch that in the first post.

For reference, edcmat-l1 is the one who schooled me on this situation in this post http://forums.corvetteforum.com/c5-s...722-surge.html

 

Yeah, I am running a bit richer than normal at cold startup. I was initially working on trying to get the actual engine startup to be stronger and cut down the number of engine revolutions before it would catch. Back in the old days, cold start needed to have the choke closed which gave a very rich mixture. Plus we needed to pump the gas pedal a few times, which primed the carburetor from the accelerator pump. So I was trying to add fuel to that process. Probably just went a bit too overboard, and need to lean it out quicker after the engine actually starts.

In any event, I've been playing around with the Desired Airflow table (B4307 EFILive) and getting pretty good results now. Apparently I had opened up the throttle blade too far at cooler temps and what appears to be happening is that there is too much airflow after I punch the throttle, and airflow and spark advance are having a tug of war to try to get the idle to stabilize again. So I've got the unstable idle cut down from 19 seconds to just two seconds, and it's not stalling out on me. I am going to reduce the values in B4307 again to see if I can find where the sweet spot is located for each ECT value. I think that will also straighten out the spark advance, but I know I do have more work to do with spark advance. When driving at low speeds right around engine idle speed, spark advance looks like a darn sawtooth with a 16 degree difference between the spikes and valleys.

But I was able to get rid of that high idle problem I was having coasting to a stop light via that table, so it seems that some of my problems are stemming from that one particular airflow table.

BTW, the cam in my engine is pretty much a baby cam. It's a turbo grind and I specifically did not want it to be radical as I wanted low end torque with the turbos picking up the slack at higher RPM.

Duration - 232 228
Lift - .630
Lobe separation - 116
Intake center line - 109.25

Thanks for your help with this.