Off idle stumble...................

[Pete]

Originally Posted by tpepmeie

Jeff,
Boy do I know what you are talking about. I was totally perplexed by this for several years on my modified '91. Several "little" things can help, but not totally eliminate this. For what it's worth, my pristine stock 7000-mile 93 does the same thing. I am (almost) convinced that this is something all LT5's experience. My personal view is that it is a function of the lengthy inlet tract behind the throttle blades, and the plenum volume. There is surely an inertial "lag" of the airflow from when the throttle is cracked to when the engine responds, but I can't get my head around the engineering explanation.

Some things that help, but don't totally eliminate the phenomenon:

1) Denso IT-22 plugs, or equivalent, gapped at no more than 0.040" AC Rapidfires seem to contribute to a sag, in my experience.

2) properly adjusted TPS sensor, and throttle blades. I like less than 15 IAC counts at idle.

3) extensively re-worked Delta TPS, and Delta MAP Acceleration enrichment and some other values in the calibration.

This sag clearly shows up as a lean spike on a wideband O2 trace. The task, therefore is to get the fueling to respond quicker to the initial tip-in. But not too much fuel as to cause a rich bog after the tip-in. It will try your patience to get it just right.

Short answer... you're not alone. It is just something to live with unless you want to spend hours working with the calibration to improve it.

Todd

Yeap, that's our ZR-1's some guys snap there foot on the throttle expacting a pro stock engine response not in our Z's ported or stock.

Pete

[Z51JEFF]

Originally Posted by Pete

Yeap, that's our ZR-1's some guys snap there foot on the throttle expacting a pro stock engine response not in our Z's ported or stock.

Pete

Thanks for all the info guys.The cars a 91 with 7500 miles.Maybe do an injector upgrade anyway.The stumbles not bad but it is there.Sounds like some of it might be normal.

[tomtom72]

Morning fellas! Hope everyone had a nice easter? I had a good one, sunny & about 40* so me & my Z went out to take in the morning air.

Maybe mine has this off idle stumble & I never noticed it due to my never having had a pro stock motor! I don't remember having any stumbles until my OEM injector coils went away, at about 9500 miles. I used RC's as they were the only game in town at the time, 9/05. Ouch on the $ but at 30,000 I'm still smilin' as they're still good.

Your injectors will go eventually Jeff, but my Q's still stand about when the miss occurs.....all the time off idle, just when hot, just when cold, there is no miss at any other point in the rpm range??? Oh yea, usually if there is no SES light a scan really does tell ya what is working right and you have to reason it out from the data as to what is really the root of the issue.

Not trying to be a wise guy....don't take me wrong. I'm just trying to lend a hand.


Tom

[Paul Workman]

This thread got me curious, so when I was out for a quick buzz into town on some errands, I did snap to WOT a few times, and did not detect any stumble. RPM was just off idle when I went WOT several times, but no stumble.

The car was fully warmed up, and it had done a couple 7k run-ups prior to the off idle WOT "test", for what it's worth.

I had some stumbling problems when I got the car, and I ran down a few air (vacuum) leaks, re-torqued the intake plenum, etc, and later found a vacuum hose disconnected under the chin of the TB (90 ZR-1). The injectors are new, according to the guy I bought it from, but chewed plug wires (mice) was the final "fix".

If ya want, I'll take my scanner along next time and do some more WOT and print out the results; TPS, etc. and see how it compares to yours or some others, perhaps.

P.

[XfireZ51]

Originally Posted by Pete

Yeap, that's our ZR-1's some guys snap there foot on the throttle expacting a pro stock engine response not in our Z's ported or stock.

Pete

Pete,

What's interesting about the ZR calibration is that AE is based on MAP. There doesn't appear to be a table for Delta TPS%. There are some TPS% Constants in oorder for MAP AE to take place but nothing like the AE PW based on Delta TPS%. When I tuned my 84 using EBL, I relied more on TPS % change than I did on MAP % because it reacted faster and then I would decay out the AE. But that's on a wetflow TBI system. In talking to some other tuner friends it seems that GM may have used the VE tables to add AE. And the Decay for the AE PW in my calibration is immediate. Once the
AE PW is delivered, ITS OFF. Also, the AE PW is the same in the 92 AYWT bin regardless of Delta MAP. I'm sure part of the reason for this is the second injector. It makes up for lack of AE fairly quickly.
Is the stumble you have all experienced more likely at lower or higher rpms?

[tpepmeie]

Originally Posted by XfireZ51

Pete,

What's interesting about the ZR calibration is that AE is based on MAP. There doesn't appear to be a table for Delta TPS%.

Not true. There is a delta TPS AE factor. I do agree with your prior experience, the TPS-based enrichment does provide a better response off-idle.

For what it's worth, going from memory, the TunerCat tables/constants are not exactly labeled right for some of the AE items. What you see is indeed the Delta MAP AE table. If I recall the earlier calibrations have a rather flat enrichment regardless of D-MAP, while the later (BMCB) is more sloped. I think some of the other TC items are labeled wrong. I don't have it installed, so can't check it anyway.

A final "for what its worth", the very early development calibrations relied exclusively on D-TPS enrichment. The D-MAP feature didn't get put in until later, from what i can tell.

Todd.

[XfireZ51]

Originally Posted by tpepmeie

Not true. There is a delta TPS AE factor. I do agree with your prior experience, the TPS-based enrichment does provide a better response off-idle.

For what it's worth, going from memory, the TunerCat tables/constants are not exactly labeled right for some of the AE items. What you see is indeed the Delta MAP AE table. If I recall the earlier calibrations have a rather flat enrichment regardless of D-MAP, while the later (BMCB) is more sloped. I think some of the other TC items are labeled wrong. I don't have it installed, so can't check it anyway.

A final "for what its worth", the very early development calibrations relied exclusively on D-TPS enrichment. The D-MAP feature didn't get put in until later, from what i can tell.

Todd.

Todd,

From what I've been able to see there isn't a AE PW table based on Delta TPS%. I do see a constant for Delta TPS% to allow MAP AE but again no table. The stock 92 bin has a flat AE PW for MAP. Which cal year started using the "sloping" AE? I'd like to look at that.
With modded motors it may make sense to add a bit of decay and possibly increase the PW a bit at the lower Delta MAP. Probably want a bit more fuel going there at smaller Delta MAP since tthe secondaries are not likely to kick in.

[tpepmeie]

Originally Posted by XfireZ51

Todd,

From what I've been able to see there isn't a AE PW table based on Delta TPS%. I do see a constant for Delta TPS% to allow MAP AE but again no table. The stock 92 bin has a flat AE PW for MAP. Which cal year started using the "sloping" AE? I'd like to look at that.
With modded motors it may make sense to add a bit of decay and possibly increase the PW a bit at the lower Delta MAP. Probably want a bit more fuel going there at smaller Delta MAP since tthe secondaries are not likely to kick in.

Dom,
You're using TunerCat, right? There are probably 500 items not defined in TC's definition files. D-TPS Acceleration Enrichment is among them, constants AND tables. And, as I said, don't think the AE things are all labeled right in TC anyway. I will say that the initial D-TPS AE term is not a traditional table like the MAP AE is, but it is in there nonetheless.

The BMCB is one that I know has the sloped MAP AE values.

This is without a doubt the most frustrating part of any calibration to get right. It's a delicate combination of many factors to get it just right. And lots of trial and error, and trial again.

Todd

[XfireZ51]

Todd,

I use TunerPro RT. Tunercat has a 99 item limitation. Any chance you'd send or post the section of the code with the D-TPS table for AE?

[cuisinartvette]

Have you tried Jeff? Why not support someone who supports the forum,
bet he could take care of you.