I've got a high speed miss I can't seem to diagnose..

1993 383 Stroker LT1 Auto (engine specs in sig).


The miss occurs between 4800-6300 @ WOT ONLY!

So far I've done the following..

-Brand New GM Opti-Spark (this was replaced because of a no start condition - had the miss before and after this)

-Brand New Wires (while I was in there.. why not - Hoped it would cure the miss)

-Check fuel pressure as outlines in the factory service manual..

-Checked Fuel pressure at WOT (No drop in pressure.. steady @ around 49psi)

-Dataloged a WOT run... the inj DC showed 110% is some lcoations.. so I leaned out the mixture a little.. (this didnt help). I'm running the factory 1993 Multi-Tech injectors..


WTF? whats next?

 

What did you gap the plugs at? Also do you have a aftermarket ignition in place? If you have a MSD box, disconnect it and go back to stock and try it. I suggest gapping the plugs at .040 thats where I have mine in my blower engine with a Crane Hi-6 ignition.

If that don't cure it, the best thing I can advise is to take it to a dyno shop that has a good ignition scope. With those two tools a good dyno operator can spot the problem and cause the moment it occurs. Thats how I would find the problem and have done so many times. Its very hard to diagnois it while going down the street and making it happen.

I forgot to ask, has this been going on since the new Opti installation? If so and I don't want to sound like an ****, but are you sure it was indexed properly? The reason I ask is because I have seen the later Opti's installed so that the index pin wasn't engaged properly. It was forced on so that the pin was out of place. The other one I had that was misfiring like yours on a brand new 383 was found to have a camshaft with a short dowel pin in place. It would not index the Opti at all so the Opti was flopping around on the three cam bolts!

 

Make sure that your plug wires are in their wire guides which keeps your plug wires from touching grounded metal. All plug wires have microscopic pinholes and if the wire touches grounded metal, the spark can jump the pinhole instead of the plug gap. Next, look at the condition of your spark plugs and maybe consider installing new ones and set the gap to factory (.035), not wider, which requires a higher voltage to jump and more likelyhood of jumping a pinhole instead. Even though you have new wires, measure the resistance of each wire, it should be about 5000 ohms per foot and continuous when you wiggle each end connector. Most high speed miss is due to somewhere in the spark system.

 

Low speed miss is valves. High speed miss is ignition. Based on that I would not assume anything you have done to the ignition side is ok. with the others have a diagnostic run and you will find the problem. It may be plugs or wires or any number of things [I]but it will be ignition related

 

all this info is really helping! I took a second look at the wires today and ran an ohm meter on all of them.. they are all pretty close to 8000 ohms.. so they all looked ok.. however, the #6 cyl wire was 34k+ohms.. upon closer inspection i noticed it had been burned through .. it must have been touching the exhaust manifold heat shield near the plug boot.. anyways, I happen to have a few spare wires laying around (brand new) and replaced that wire and went for a ride.. it felt much stronger.. but still the miss in the 4k-6.3k rpm range.. I got to thinking amybe its because the old program was a program for 7 cyl. since this one wasnt firing.. soooo.. I did another datalog run and found that with the new wire I was getting quite a bit of ping (knock count) and the computer was pulling out a full degree of timing almost accross the whole board.. 85 kpa-100kpa from 2000rpm-6500rpm.. so I did as suggested by GS and took 4* of spark out of the whole range.. WOW what a difference!! no missing throughout MOST of the range.. the only place i get a miss is around 6300rpm in the 1st-2nd gear shift.. it might be the fuel cutoff.. but its too late at night to make another datalog run.. so i'll hook up the computer on the way in to work and see if its needs more timing taken out at the 5500-6500 range or I need to raise the fuel cutoff again...

This leads me to another question.. I have the stock Auto Tranny with no shift ki or nothin'.. why is it waiting so long to shift? 6300rpm in the 1-2 shift?? the tranny is strong and has about 20k miles since the last rebuild.. it seems the 383 just rev's a heck of alot faster than the stock displacment configuration does?? any ideas??

Lastly.. with a SCAT 9000 crank, ARP rod bolts, factory rods (reconditioned, relieved and polished with KB hyperutetic pistons.. whats a resonable redline to set the limiter (fuel cutoff) at???

 

Sounds like you are on the right track,
The bottom end sounds ok but most high RPM problems ocure in the valve train what lifters are you running? If comp Rs is the preset a 1/16 turn or less? are you running valve guides and hardened push rods? What springs and retainers? Lastly have you had the car on a dyno yet, if so where did it stop making power? The reason I ask is my 383 made peak power at 6100 RPMs and peak Tq at 4500 so there was no reason to set my rev limiter above 6500 RPM's and a dyno run should also show any problems with floating.

 

at what rpm does it occur? is it around the same RPM everytime?

Does it feel like you hit a rev limiter?
Could be floating the valves.

 

it feels like it bouncing off the limiter.. but the miss also happens at sub 5000k rpm range also.. which i doubt it floating new springs.. the valve springs were replaced when I rebuilt the engine with a set matched to the cam (recommended by CompCam).

 

Sorry, I'm betting on a bad spring or lifter-cause it sounds like it's running fine until you really put a load on it (hi-rpms) and that's generally where you'll float a valve if you've got a weak spring or lifter.
Just my 2 cents worth.

 

I would check the valvetrain. I had a rocker brake once on my factory LT-1, and I didnt think nothing of replacing it and the pushrod. Well after years of trying to figure out my high speed miss, I found it. It was the cam. When the rocker broke, it scarred the cam, and it was slowly going bad in my 93. Not suggesting the cam is bad, just suggesting checking out your valvetrain, with no codes etc, mechanical issues may be happeing.