Hey guys. I have searched and read so many threads on high misses that my brain is cooked. Here is my problem. The car stumbles or misses at 5200-6000. I have a 94 z28 w/A4 and stuff in my sig + 1.6 crane gold rockers(just installed), fpr, new opti, coil, accel 9000 wires, plugs, crane ignition box , 52mm throttle body, and a custom air intake. The problem surfaced at the track. I still ran a 13.4 @105.91 but my 1/8 was only 82 or so.

Anyway, I have run the car w/the fuel pressure guage on the windshield and the pressure did not budge. It stayed at 49.5 or where ever I set it until redline.

Anyone have any ideas?

 

It is the accel wires. They have proved to be the bane of me and several others. Get the MSDs and that should solve the problem. I am pretty confident in this since 5 others besides myself tried the accels and had this same mysterious miss, runs good up till then, then almost cuts out, right? The resistance on those wires is crap, and when I took mine off after about a month two were brittle and one had a hole all the way through the boot.

 

Did you change your valvesprings when you added the rockers? Do you have an lt4 knock module?

 

LT4 mod yes, Valve springs, I'm ashamed to say no. . I was thinking they may be weak and can't handle the added stress. One thing I forgot to mention. Before the rocker install, the car would be hard to crank on the first start after sitting a while (2-3 hrs). After that, no problem. That sounds like a fuel problem but I tested the pressure at WOT and no problem. These two things may not even be related. I am going to try the wires tommorow since I have nothing to lose and then I am going to check the grounds because the lights also dim at WOT. I forgot to mention these other issues in my first post.
So:
1. Stumble/miss at 5200 rpms at WOT.
2. Hard start after 3-4 hrs of sitting.
3. Lights flickering/dimming at WOT.

Thanks for your posts, I really appreciate you guys taking the time to help.

 

i also have been having this same problem. my mods are listed below. 6 spd car and it's not soo bad through 1st cuz of the short length of 1st between the first gear ratio of the 6 speed and 3.73 rear gears, but it really likes to stumble in 2nd at 5k+rpm. I also have a 2 year old opti, new accel coil, & newer msd 8.5 wires. i then repalced the plugs to bosch platinums and it didn't make any difference. still a stumble. 2 of my spark plug wires have holes in the insulators, but the core is untouched. i don't think this could be the problem?? i ordered taylor wires anyway just to see and i have'nt got them yet. anyways, I'm always open for suggestion on what is going on. the car drives and runs perfect in every aspect untill i pound on it (5k+ rpm) and then it stumbles. help!!!!!!

 

Is it not the absolute most helpless feeling in the world? I will pound my head through the wall if it turns out to be weak valve springs. The worst part is that I ordered them with the rockers but time was not on my side so I skipped putting them in. We'll see what happens tomorrow after I do some more playing around.

P.S. I am amazed at how many people experience this problem and how many different things it turns out to be. Here's a list I made reading other threads:
1. Coil or coil wire
2. Plugs
3. Wires
4. Opti
5. False knock
6. Bad fuel pump
7. To be continued.....

 

Well, I changed the wires and unfortunately the same results. What next? I am upgrading the fuel pump tomorrow but I really doubt this is going to do anything because I already tested the pressure. As was posted by a member here, I am starting to think the valve springs are too weak for the rockers. Any other idesa before I replace the valve springs?
Thanks again.

 

Just noticed the addition of what might be important information. The dimming of the lights at WOT might indicate a problem with the alternator or the battery. It certain would be something that I never noticed with my car. It would definitely show that there is some sort of draw on the system that is decreasing voltage which could cause the loss of electricity to the coil.

 

Nathan,
Yes I did change the battery to an Optima and a new alternator. I checked them and they are reading good. Well, as things get worse, the new hitch is that when you are in park/nuetral and stab the gas really hard, it misses and then catches itself. Like a delay.

 

Threw in the towel today. On the way home car was popping through the intake and backfired at full throttle. I dropped the car off at my brother's shop and am letting his guy change the valve springs and check for that miss afterwards. I hope everything is related. I really hope it isn't a bad opti since I have only put about 500 miles on the car since I installed the new one.

 

did ya ever get those valve springs replaced? did that solve the problem? I have the same problem with my 94 z as well. i just thought the engine wasnt gettin enough fuel at the higher rpms so it was cuttin out.

 

After all was said and done, it was the LT4 knock mod that cured the problem. I changed the one I had in the computer and put a new one in. That took care of the problem. The sensor must have been defective or took a dump around the same time as I installed my rockers. My other problem was that the coil waire was slipping off of the coil itself. That led to backfiring and no starts. I fixed that problem as well. Car went 2 mph faster in the 1/4 at 107.5, but I still got a 13.003. I need a torque converter and shift kit to get my 12's.

 

I had a similiar high speed miss in my TA after installing the hot cam kit. I did everything i could think of to cure the problem, 2 new opti's, msd coil, wires, plugs, nothing worked, i was at my wits end and someone at the track suggested i put the MSD 6 AL on it so i finally did after over 2 years of the ridiculous miss and wow, it solved my miss completely. Car pulls like never before. Evidently the stock ignition takes a dump over about 5800. mine would just start stumbling after about 6200 especially on the 2-3 shift. Don't know if this is your problem but it solved mine!! FINALLY

 

well I hope the problem will be cured through use of the LT4KM since it is only $27. Is there a major difference to the MSD 6A and MSD6 AL BESIDES the fact that the 6 AL has the rev limiter? Where did you buy yours?

 

I got the MSD 6AL so long ago!!! I actually bought it at an Englishtown N.J. Swap meet from one of the vendors. If you ge either one, make sure to get the adapter plug. It makes installation a snap. I think the MSD 6A is just as good, just no rev limiter. I never used the rev limit part of the box either. Do you have roller rockers? Headers? I never noticed the problem until after the roller rockers. I was a moron for not doing the LT4 module first. I wasted so much money, time and killed my hands.