Can anyone help diagnose a stumbling issue??

[ZmanDan]

Okay here the story....car was running fine, decided I needed to get bigger injectors because it's a heads/cam car puttin' out 380 RWHP which I figure is around 435 at the crank....so I get the 32# injectors that Racetronix sells. I install the injectors and send the PCM away to PCMFORLESS for a reprogram (he did the original programming on the car). Soooooo, I get the pcm back and take the car for a test run. Car idles fine and cruising around it seems fine as well ....but when I romped on it and it got in the upper RPM's just a second before the shift light came on (got a 6800 pill in the shift light) the car starts stumblilng. I'm thinking maybe he lowered my rev limiter on me so I put a 6600 pill in and romp on it again and it did the exact thing...right before the shift light comes on the car started stumbling again......what could possibly be causing an upper RPM stumble light that? could it be something with the PCM tuning? What really confused me is why it didn't stumble til 6800 the first time but stumbled at 6600 the second time....I'm lost....

[Christopherrr]

let's start by letting us know what year your car is and mods...

[ZmanDan]

oh yeah I guess that would help...sorry...

Car is 1995 Z28 6M

mods:
cc306
GTP Stage II heads
58MM TB
Hooker LT's
32# injectors (just installed)
PCM reprogram
SLP CA
EWP

and not that it's relavent to the problem in anyway but:

Moser 12-bolt with Richmond 4.10's and Borla exhaust

[Squat13]

Okay here the story....car was running fine, decided I needed to get bigger injectors because it's a heads/cam car puttin' out 380 RWHP which I figure is around 435 at the crank....so I get the 32# injectors that Racetronix sells. I install the injectors and send the PCM away to PCMFORLESS for a reprogram (he did the original programming on the car). Soooooo, I get the pcm back and take the car for a test run. Car idles fine and cruising around it seems fine as well ....but when I romped on it and it got in the upper RPM's just a second before the shift light came on (got a 6800 pill in the shift light) the car starts stumblilng. I'm thinking maybe he lowered my rev limiter on me so I put a 6600 pill in and romp on it again and it did the exact thing...right before the shift light comes on the car started stumbling again......what could possibly be causing an upper RPM stumble light that? could it be something with the PCM tuning? What really confused me is why it didn't stumble til 6800 the first time but stumbled at 6600 the second time....I'm lost....

What type of Motor SBC, LT-1, LS-1. What about car type, Vette, TA, you should make a sig with all your cars date so we can halp you. If you got a LT-1 I bet the Opti Is breaking up on you at that high of a RPM. With the Opti ant high RPM the coil will not have enough time in-between Firings to charge, So at high RPM (6500!!!) The Opti is not sending a full charge to the plugs, so maybe due to the larger injectors and more fuel, the plugs are having a hard time firing in the cylinder because they arnt getting a full charge.

[Squat13]

Yeaf Bro its the opti. Get you one of the Dephil systems or one of the new MSD cap and rotors and a huge Coil. That should help

[Christopherrr]

Yeaf Bro its the opti. Get you one of the Dephil systems or one of the new MSD cap and rotors and a huge Coil. That should help

that gets my vote too...

[Squat13]

that gets my vote too...

Good Lookin out LimTang

[ZmanDan]

really?? I always figured the opti could take more punishment than that. Surely there's higher HP cars out there that are still using the factory opti.(BTW it has a new opti with less than 4000 miles on it) ...I saw a couple of guys on this board with bigger cubed LT1's . I wonder what they are using for their ignition systems? But what you say Squat definitely makes sense....

Joe 1320 knows the car, wonder if he can throw his .02 in on it.....

[Squat13]

Most guys running higher HP LT-1's with a Opti don't spin them to 6500 RPM. Then there are High Power LT-1's with convental SBC Distiributors, and then still there are the guys with these High Power LT-1's that go with a coil on plug set up like the Delteq that uses the crank trigger in the opti with 4 Caddy Northstar coils (1 per ever 2 plugs). The coil-on-plug setup is the best, then ever coil only fire twice insteed of 8 times during a full cam rotation.

Also its not the age or milage on the opti. but the amount of chage that is built up in the coil between fires, It is a flaw that is in all Opti's even brand new ones.

[Injuneer]

Are you saying the stumble did not show up until A) you added 32# injectors, B) had a pcmforless tune. Did you change anything else? Were you always shifting at (near) 6,800rpm?

If nothing else changed, you have to look at the injectors. I have seen the data logs from at least one setup with the 32# Racetronix injectors that had problems with fuel delivery. The injector constant was set for ~32# in the PCM, and it ran way lean. They ended up dropping the constant back to about 25.2# to get it to run rich enough. Using that approach, and not understanding why the injectors flowed less than claimed is a dumb approach, but no one else seemed to think it was a dumb solution, including Alvin from pcmforless. Interestingly, Racetronix contributed to the thread, but even they didn't seem to want to address the issue of why the constant had to be set close to stock. Nobody called them on it, and I stepped back, becasue Jack from Racetronix and I have had some problems in the past.

You need to check the A/F ratio at the upper end. Using the stock O2's might give you a rough idea of what is going on. Do you have a scanner?

[ZmanDan]

Are you saying the stumble did not show up until A) you added 32# injectors, B) had a pcmforless tune. Did you change anything else? Were you always shifting at (near) 6,800rpm?

If nothing else changed, you have to look at the injectors. I have seen the data logs from at least one setup with the 32# Racetronix injectors that had problems with fuel delivery. The injector constant was set for ~32# in the PCM, and it ran way lean. They ended up dropping the constant back to about 25.2# to get it to run rich enough. Using that approach, and not understanding why the injectors flowed less than claimed is a dumb approach, but no one else seemed to think it was a dumb solution, including Alvin from pcmforless. Interestingly, Racetronix contributed to the thread, but even they didn't seem to want to address the issue of why the constant had to be set close to stock. Nobody called them on it, and I stepped back, becasue Jack from Racetronix and I have had some problems in the past.

You need to check the A/F ratio at the upper end. Using the stock O2's might give you a rough idea of what is going on. Do you have a scanner?

yeah that's finding a piss poor solution without addressing the problem. My car ran fine with no stumbles when I had the stock 24# injectors, I always shifted at 6800 and it roared like a tiger. Not until I put the 32# injectors in and got the reprogram did I encounter a stumble and no I changed nothing else but the injectors and the programming. I didn't drive the car with the 32#'ers until after the reprogram....so I definitely agree with Squat that the ignition needs upgrading but before I spend $1000 + on a Delteq system I would want to know if I have defective injectors. Setting them back to near stock makes no sense either because I would have just left the 24#'ers in and been done with it......unfortunately I don't have a scanner....what would happen if I put the stockers back in now that the PCM is set for 32#??


also I want to thank you all for your input because I have got no responses anywhere else for any help. you guys rock!

[Joe 1320]

Hey Dan! Fred may be on to something here....... I have seen two injectors supposedly flowing the same numbers, yet the actual output was definately different. It's also difficult to tell whether the PCM is actually programmed correctly. I have to tell you I am not a fan of a mail order tune once you get beyond stock parts. Having a dyno tune with wideband 02 sensors is the real way to go. Dan, you are long past point that you really should have a scanner.... It might be a good idea to borrow someone's PCM, swap injectors and see if the stumble goes away. While you won't have anywhere near the power, you can at least verify that it is indeed the PCM or injectors.

BTW, give Dan a big congrats. He and his wife Carla, are expecting a baby!

[ZmanDan]

the bitch of it is noone around here is setup to dynotune our cars. I got a dynotune in April just to make sure everything was to par since I had had no tuning since the original mail order tuning I got when I first did the heads and cam......to do so I had to rent a trailor and tow the car allll the way to Tampa just for a dynotune. 4 hour drive just to get a tune, makes you reconsider the mailorder tuning sadly enough...

so what scanner are you referring to? Like a Hypertech? I didn't think they did anything but adjust shift points and little things like that...

[Joe 1320]

The hypertech programmers have more of a merit when playing around with fan turn on points, shift points, etc..... The feature that is a redeeming quality with that device is the optional scan cartridge. It plugs into the bottom of the unit and allows both a real time view and a data log capability of your sensors. If you have access to one, it will work. If you are looking to buy.....honestly, you should look into getting a laptop and scan software. Something like LT-1 edit. For OBDI it's not nearly as pricey as OBDII.

The coil problem can be fixed by the addition of an MSD. The problem that you will then run into is that above 6500 RPM, the rotor tends to start frying. The Delteq conversion would make sense in that case. The Delteq should anly set you back about $650 for the complete kit. Much less if you get the basic kit and pick up the coils from a local wrecking yard.