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

[Z51JEFF]

If the car is idling,stab the pedal it has a slight stumble.The air filter is new.Any suggestions?

[bradslt5]

what yr car 90-92 .if so are your injectors the original ones? if so they could be getting ready to go as they all do from these yrs . they dont handle todays fuel. i am getting ready to replace the ones in my just bought 2nd zr1 a 92 with 40k miles . my first one had those gremlins until i put in new injectors now she runs like a top . do you have a scann tool / if so look at your blk learns mine has 117 l 127 r@ idle which means i got at least one leaker on the left . this may not be your problem , but the same thing happened to my blk car. accels are the way most are going about 580 from summit or rc injectors they are double what the accels are .i put rcs in the blk car but that was before i knew about the accels.if injectors have been changed do a fuel pressure test per the FSM maybe also put on a new fuel filter . this is just my guess hope it helps

[tomtom72]

Jeff,

I'm with Brad on the injector issues. However, I'm thinking that "off idle" is the only "stumble"? If yes, then I think that eliminates alot of things like, plugs, wires, coils, f/filter, fuel pumps (maybe) and injectors (maybe). What I mean is no other miss going up thru the rpms, after the initial off idle miss kind of rules out most all the stuff I mentioned.

.... then open or closed loop operation? because that may tell ya something....different sensors in both phases of operation come into play. Know what I mean?

Open loop = cold start, then is TPS set right? Is IAC doing the right thing? Are temp sensors doing the right thing? Are the plugs too cold? PCV's okay? Is there a small vac leak or does she idle @ the right #?

Closed loop off idle stumble, my 1 suspect is the TPS, then O2 sensors, then the recalls/TSB's on any EPROM issues ( unless this issue is "new" to your car? ). Again a small vac leak or clogged PCV's. Plugs too cold? Heat related failures of the injector coil & wires & coils & plugs usually result in a solid miss everywhere...ya know it just don't run well....you didn't mention that so I knid of rule most of the ing & fuel system out.

Oh, I'm with Brad on the scan as it may show what's up with the electronics.

JMHO
:cheers:
Tom

[bradslt5]

TOM, good catch i have been up all night and didnt even think of the tps and iac, he right on the injectors. mine started like his did but there is a very good chance that if original they could be going soon. the callaway zr1 i think has them going blk learns at idle 117l 128r when cruizing 120 l and 134 right . tom, did you use the accels how do you like them? i used rcs on the blk car . but for 1/2 the rc money am really thinking of using the accels. ah heck you know you have to do the plenum pull on every zr1 we get .just to ck out whats been done before me.i am getting very good atit:o have an excellent easter

[Paul Workman]

Jeff,

I'm with Brad on the injector issues. However, I'm thinking that "off idle" is the only "stumble"? If yes, then I think that eliminates alot of things like, plugs, wires, coils, f/filter, fuel pumps (maybe) and injectors (maybe). What I mean is no other miss going up thru the rpms, after the initial off idle miss kind of rules out most all the stuff I mentioned.

.... then open or closed loop operation? because that may tell ya something....different sensors in both phases of operation come into play. Know what I mean?

Open loop = cold start, then is TPS set right? Is IAC doing the right thing? Are temp sensors doing the right thing? Are the plugs too cold? PCV's okay? Is there a small vac leak or does she idle @ the right #?

Closed loop off idle stumble, my 1 suspect is the TPS, then O2 sensors, then the recalls/TSB's on any EPROM issues ( unless this issue is "new" to your car? ). Again a small vac leak or clogged PCV's. Plugs too cold? Heat related failures of the injector coil & wires & coils & plugs usually result in a solid miss everywhere...ya know it just don't run well....you didn't mention that so I knid of rule most of the ing & fuel system out.

Oh, I'm with Brad on the scan as it may show what's up with the electronics.

JMHO
:cheers:
Tom

Dittos on the scan. With as many things that will produce symptoms that are the same or similar, the scan will really narrow down what is and isn't working in very short order - giving you a starting place. The Z is my second Vette, and as it happens it was my second opportunity to have to troubleshoot a (stumbling) problem. In my case, it wasn't what the scanner said was wrong, but what it eliminated that eventually pointed me in the right direction. Scanners (or a laptop)...I feel they are essential as far as keep one's sanity when problems pop up. W/o it, I would be religated to "shotgunning" the problems, and me hates dat!!

P.

PS. BTW, the scanner didn't indicate any particular fault. The injectors ohm'ed out OKay, and a leak-down test showed they weren't leaking. Making a LSS, mice had ruined a couple wires resulting in off-idle stumbling and stumbling at other intermittent times (depending on whether the wires were wet from condesation or not!).

Good luck!

P.

[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

[Pete]

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]

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.:dancing

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.:o

:cheers:
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]

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


Todd,

Do the Delta TPS/MAP AE have any "lag filters" or coefficients? How about the VAFPR? EBL has them which allows you to adjust how quickly these params will react to the changes. Shortening the lag improved throttle response significantly. I would think that the fuel would arrive sooner than the airflow given that the injector is closer to the valve than the TB. By that logic I wouldn't expect to see a lean spike. But as you described the stumble is caused by AFR leaning out.

[tpepmeie]

Todd,

Do the Delta TPS/MAP AE have any "lag filters" or coefficients? How about the VAFPR? EBL has them which allows you to adjust how quickly these params will react to the changes. Shortening the lag improved throttle response significantly. I would think that the fuel would arrive sooner than the airflow given that the injector is closer to the valve than the TB. By that logic I wouldn't expect to see a lean spike. But as you described the stumble is caused by AFR leaning out.

Yes, there are filter coefficients for these inputs, and they can be adjusted to affect the timing of the enrichment pulse.

[XfireZ51]

Yes, there are filter coefficients for these inputs, and they can be adjusted to affect the timing of the enrichment pulse.

Todd,

So they work by affecting how quickly the params react to new inputs, correct? Are those the ones you changed and to what result? Or did you just add more AE?

[tpepmeie]

Todd,

So they work by affecting how quickly the params react to new inputs, correct? Are those the ones you changed and to what result? Or did you just add more AE?

Sort of. Both AE types are driven by delta-x inputs. I.e., delta-tps and delta-MAP. The delta values are partly determined by the filter coefficients. So by changing the way the delta-x is calculated, you can increase or decrease the amount of enrichment applied for a given (actual) change in input.

There are many variables involved in fine-tuning the throttle response and tip-in. Off hand, I would say I worked over maybe 12 different variables to achieve the response I wanted. It took a lot of experimenting.

Todd

[Z51JEFF]

I dont know if there related but the car has stalled several times.Which injectors are the best and whats a set go for?

[lbszr]

I dont know if there related but the car has stalled several times.Which injectors are the best and whats a set go for?


I doubt if fuel injectors would cause it to stall. Rough or miss or hard to start maybe. Have you done a leak down test after turning the engine off? I was still in denial that my oe inj. would last forever and it still ran good but noticed a little roughness. Found one inj resistance out of limit cold and 6 more out when heated up in the oven to 170. Anyway ended up with Accel and happy. It's smoother than it's ever been. From Atlanticspeed for about 530, heard afterward's with netregistry membership can get for below 500. 21# p/n 150121

[Z51JEFF]

I haven't done a leak down test but just plan on replacing the injectors.It looks like a pretty straight forward job.It would give me a chance to clean up some things while Im at it.Whats the deference between the RC injectors and the ACCELS besides price?

[tomtom72]

I may be mistaken.....but I think the RC's and the Accels are made from the same s/s injectors made by Lucas....I know the RC's start out as Lucas s/s injectors. I have the RC's in my 90 and think they have served me well. If the Accels were offered when I had to do my injectors, I would have bought them as they look to be the best deal for the $. My RC's were $1068/16 in 9/05!

:cheers:
Tom

Oh, I don't really know enough about injectors & EFI to say any more than I wrote. All I know is I have no off idle stumble...I went out to play with my toy this morning....for sure no off idle stumble that I can detect and I tried several times just to be sure! :mrgreen: :wink:

[XfireZ51]

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]

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]

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]

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.