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

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 have an excellent easter

[Paul Workman]

Originally Posted by 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

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]

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.