Okay... I have a 94 Z28. Right now it has Bosch Platinum Plugs and Taylor wires brand new Borg Warner Coil and GM wire. Anywhere from 1000-2500 RPM's it will stumble and hesitate badly.

FACTS
It DOESN'T stumble or hesitate during WIDE OPEN THROTTLE
IT DOESN'T stumble or hesitate until the about/past 180 degrees
IT ISN'T THE MAF because I unplugged it and it did the same thing
I Checked the wires in the pitch black and didn't see any arching
The wires are away from the headers with wire ties (I know they still can be damaged inside)

QUESTIONS
If it was the Opti wouldn't it do it all the time and not at certain temperatures?
If it were the O2 sensors wouldn't the car stumble during high RPM's when the car is floored?
If it were the fuel pump wouldn't it do it when the car is 1st starting to move?
Could it be a plug or plug wire even though there is no miss when i rev the car?
Could it be plug or plug wire even though it takes the car until 180 degrees or above to start stumbling?


Sorry to ramble on guys but I have some other repairs I want to do to the car and I don't want to spend ALOT of money replacing everything just trying to replace things that don't need to be replaced yet. This is killing me everytime I drive my Z I miss my 99 Z28 M6 1000x more please help

 

Hey, My opti did the same exact thing. Low RPM stumble only when warm but boy was it nasty sounding! Its pretty much a spark related problem but sounds like an Opti to me. Call Jason Cromer @ Sam Taylor for a new GM opti at a damn good price. 850-994-4277

 

Thanks for the reply. I was thinking of doing a vented conversion but I don't know. Your opti only stumbled down low??

 

being you noticed that your stumble occurs around 180 degrees i would assume your O2 sensors are going. if you get a scanner...you will be able to see fairly easily if they are working properly. the reason i would suspect O2s is b/c the only real difference between open loop and closed loop functioning is when a car goes into closed loop...it starts to recieve feedback from the O2 sensors. this all happens around 140 degrees. my car has a bad idle and i guess a slight hesistation and i KNOW i have a bad O2. just havent gotten around to fixing my problem yet

 

Thanks for the reply turbo. I have to replace these cause they have 170,469 miles on them

 

dude, check your EGR, i had your problem and that was the cause, finally got to it after spending 2k+

 

I checked the EGR. Still blocked off. Thanx for the reply

 

Some optis only have trouble when they get hot. The ignition control module can do that, too.
No, because the pcm does not use the O2 sensors for calculations at WOT.
Fuel pressure is varied by engine demand and controlled by the fuel pressure regulator.
Plugs and wires usually break down more under a load, not just when you rev the motor. Other board members have noticed bad luck with Bosch plugs.
Heat causes greater resistance. If you have a wire or a plug that is marginal, it could exhibit worse performance when hot.

 

shoebox

Can the ignition control module be replaced? The coild only has 500 miles on it.

 

Sure, it is mounted on the bracket with the coil. Some people have been able to isolate a heat related problem with the ICM by spacing it away from the head to reduce heat transfer. An easy way to do that is to remove the studs and assembly, then put the studs back in first and mount the assembly one the outer part of the studs. Or you can come up with a spacer and bolts or studs of your own design.

Make sure you have checked the connection at the opti for any corrosion on the plug or down inside the connector on the opti, itself.

 

I think I'm gonna try the O2 sensors 1st then if not order a cam and opti and call it a day!

 

My vote is to suspect O2s. Full throttle = Power Enrichment Mode. I think if > 80%(somewhere in that vincinity) throttle, O2 feedback is ignored by the PCM. So, if it runs well at WOT, but stumbles at part throttle, bad O2(s) is/are a distinct possibility.

 

I got the same problem, in addition to that I have pinging under load. Could this be bad opti?

 

Let me chime in, about 3 weeks ago I installed 30# SVO injectors to compensate for the 94% duty cycle my stock ones were giving me with my LT4 conversion. When we did a scan we noticed that my MAP reading was high and the left 02 sensor failed, well I installed new 02 sensors and the car still feels like it wants to stall when I start it. I contacted Bryan @ PCMforless and he said he viewed the scan and told me one of the fuel cells is dumping a large amount of fuel in, he said that I might have un-metered air going in somewhere when I did the injector install, perhaps I didn't seat one of the injectors fully. What do you gurus think? Also, would it be a good idea to get new spark plugs, I'm running NGK TR55VX plugs, is there a better plug to run. Any help is greatly appreciated. I'm getting very frustrated to where it's not fun driving anymore.

 

I had a low RPM stumble to about 1800 rpms? When it hit 1800 rpms it felt like somone turned on a switch to make it run better again. I also had a high rpm miss (bad!), even in first gear my car would run past 5200 rpm's. The low rpm stumble was the EGR system. The high rpm miss was the coil. Lucky I got them my first try huh?