96GS#007

Ok gang...This one has me stumped so I'm hoping I can get some opinions...

Car: '96 Grand Sport, LT4 with LT4 Hot Cam (sig shows other mods)

Here's the problem description:

Above 6000rpm, I have a miss. In 1st and 2nd, it's pretty severe, enough to slightly jerk my head forward when it hits. In 3rd gear it's still quite noticeable though not as extreme, and in 4th it feels like a "sag" in power. The engine will continue to rev. Sometimes it's a single "event" during the run. At other times, it'll stutter a couple times before continuing to rev. The interesting thing is that some days it'll do it (today) and some it won't (yesterday). The big difference was outside temp (today was cold...25* vs 70* yesterday, welcome to St. Louis).

Using AutoTap, I'm not seeing any knock counts or knock retard. I am seeing a few misfire counts across random cylinders in the 2000rpm+ range, but not above 6000. No DTCs are set.

What I've replaced to date and the reason for replacement:

Spark Plugs: Denso Iridium IT20 (ACs were missing pucks on 6 of 8)
Spark Plug Wires: Taylor 8mm Spiral Pro (found split OEM wire)
Valve Springs: New springs have 20% more pressure than LT4 springs (eliminate possibility of valve float)
Coil: Replaced with new MSD unit designed for '96s (running out of ideas)

I also added a Holley AFPR (and reprogrammed my PCM for that) to lower my injector duty cycle from 100% to the upper 80s (which should be fine for short blasts). Fuel pressure looks good, no drop off, and given that the stock LT4 can rev to 6400, I'd expect to at least be able to do that.

Opti? At low rpm/low speed, I can drop it in to 6th go WOT. Although I don't go anywhere fast, there's no miss or stumble. My first Opti went around 36k miles or so and was replaced under warranty. This one has ~34k miles on it.

If not the Opti, what? :confused: This thing has me bummed. Car runs like a banshee....to 6000 :(

Goody

Mine has the occasion to do the same event. I have a LT1 Auto, but it will do it on occasion when shifting from 1st to 2nd and/or when the car downshifts when going into passing gear mode. But, the temps are down around here and it hasn't done it as of late.
I've thought about it as fuel delivery problem. Want to replace fuel pump. Odd thing, I have the Holley AFPR too.....hmmm, get's me wondering. Is it possible that the AFPR is faulty at the upper RPM's? If mine returns with the warm weather I will swap the stock unit back on and see if it disappears just for giggles.

Dan Parker '96

Jim.. sent you an e-mail on the subject. But I'd like to know how the first Opti faired (failure wise.... physical condition and such).

We may be dealing with some core "out of round" issues.

Darkness

What's your injector duty cycle at 6000+??? if its about 90% then it might be form that. Just a thought :cheers:

96GS#007

At 6500 it's ~85%. Sometimes the miss happens as low as 6000. The stock LT4 fuel cutoff is 6400. Using LT1 Edit mine is now 6800, so I know it's not the fuel cutoff.

Goody; doubt it's the AFPR. Mine was doing it before the install. I installed the AFPR to reduce the injector duty cycle.

:cheers:


[Modified by 96GS#007, 7:32 AM 3/10/2003]

96GS#007

Dan,

The first Opti had something less than 40k miles on it...~36k or so. It was replaced under warranty by GM but I took it home for disassembly. As far as physical condition, the inside looked a lot like this one (slightly worse). Best way to describe what they look like would be...A conventional distributor cap & rotor that are due for replacement. For the first one, the symptoms were a low speed/low rpm/high load miss, sometimes multiple misses.

Neither were ever washed or hosed down. Neither were exposed to anything other than the usual road grime...i.e. no gushing oil leaks. Both are dry inside although the current one has both chunky and powdery black debris inside where the optical sensor resides. No codes were set in either case.

I'll bring it to the dyno day so you can see it.

Nathan Plemons

I bet you are getting a little bit of valve float anyway, I had the exact same thing. Originally when I installed my cam I had some IRS Terminator valve springs that weren't the greatest but they were better than stock LT4 springs.

Check out my dyno graph



I never really thought about it that much until I solved the problem. I thought it was the nature of the cam, other Hot Cam owners saw the same trend. Now look at the dyno with my new cylinder heads.



Notice how it doesn't fall off anymore? Everything else about the car is the same except for the valvetrain. I have new valves and titanium retainers with new huge valvesprings and look at the new graph. Very minor valve float is the only thing I can think of that would cause this so consistently on all LT1 / LT4 motors.

96GS#007

Nathan, I appreciate the reply!

I'd go for the valve float theory if it was only occurring above where an LT4 normally revs. I still have the stock lightweight LT4 valves (hollow stemmed, sodium filled on the exhaust side). The new springs have more 20% more seat pressure...120# closed vs 101# for the LT4 springs and 315# open vs ~250# for the LT4 springs. I had the new springs checked before I installed them and they were dead on (worst was +2# open). I'm using chromoly retainers, while not as light weight as Ti, they're not bad.

The first couple of months with the HOT Cam were fine. Just within the last 2 months has this started occuring so that's why I suspect a component failure.

:cheers:

Mr Mojo

Happened to me, the coil wire was rubbing on some metal, sometimes the car wouldn't start, when it did start, anytime I was over 5000 rpm, it would miss. Below 4000 rpm, car ran as if nothing was wrong.

96GS#007

No signs of arcing on the wire. Two different coil wires (OEM and Taylor), same result :(

Once I get it back together, I'll wrap the coil wire better. Anything is worth a shot. Thanks!

Dan Parker '96

I just want to remind everyone (although this may not be related to Jim's problem) of some information I came across in the past.

I remember that one Hotcam owner complained of Opti failures after performing the Hotcam swap. It turned out that the drive pin on the Hotcam was extended compared to the stock cam. I'm guessing the additional pressure caused some distortion of the Opti (internally) and contributed to failure.

I happened to remember this during my conversion and did just eye-ball the cams side-by-side. The pin depth looked pretty darned close to me so I didn't bother measuring with instrumentation. Hopefully I gauged correctly :)

But pin length on the cam is something to consider. And I'll try to dig up the actual information/posts concerning this situation.