96GS#007

Ok guys, I'm running out of ideas on what is causing my high (>6300) rpm miss. To recap, I can run through the rpms and 70% or 80% of the time I get a short duration "miss" (enough to physically feel as well as see in data logs(rpm drop)). Fuel cutoff is 6800 and my shift light comes on at 6500, so I know I'm not bumping the cutoff (that's what it feels like though). Generally I experience the miss just before my shift light pops on.

Here are the new parts (just did the heads, hence some of the new stuff :) )...

- Heads ported, port matched to intake, combustion chambers cleaned up, valves backcut, new seals, etc
- Crane 99845 H-11 Tool Steel Valve Springs (119# closed / 340# open)
- Impala SS head gaskets (gives an 11.2:1 cr)
- DelTeq Stage 1 igniton system (includes new wires, coils, module, therefore elminates the possiblity of the opti cap & rotor, plug wires, coil, ignition module)
- Plugs are like new (Denso Iridium IT20s) and gaps verified
- Opti is barely a year old, never been doused with water or oil, so I doubt the optical sensor would be bad (optical sensor is used by the DelTeq)
- New fuel filter

- No knock retard is present, no increase in knock counts (verfied using AutoTap and OBD-2 data logging software)
- No codes or any kind, no SES light
- Fuel pressure checks fine
- Injector duty cycle is ~83%
- All data logs look great (except the miss)
- CompCams ProMagnum rockers set to 0 lash + 1/8 turn

Other mods (HOT Cam, etc are listed in my sig).

Engine runs absolutely flawless and is smooth from idle up to the "miss point".

There's one thing I haven't changed....lifters. They're OEM and high mileage. I'm wondering if I have a couple pumping up. Thinking about switching to the Comp "R" lifters.

Not looking forward to having to short-shift at the track if I don't get this fixed :(

Any and all thoughts appreciated!

rick lambert

Well, I think you may have answered your own ??, I was going to guess 1.Bad lifter, 2.Weak valve spring,only a guess. :seeya

superlund

Just a guess and the Delteq should help with this but as cylinder pressure goes up / increased compression spark plug gap should go down.
On the LTX cars modified at LPE plug gap is reduced to .035
Even with my LTCC I kept my plug gap in the .035-.040 range, 11.54 comp ratio. Had new factory GM lifters w/ my Hotcam, no miss at all to 7K RPM

Something to consider....

tjwong

Jim is there any hint of backfiring into the intake when this happens? Perhaps a weak spring as posted earlier? Do you have a dyno there that can run a loaded test up to that RPM and have an ignition scope on the ignition system during the run to catch any kind of a ignition blip. It could be something as simple as a plug wire breaking down. Yes, I know you got new wires but that don't mean that you may not have a pin hole in a plug boot causing shorting to ground under extreme load and RPMs. Or a possible cracked insulator on a spark plug. A digital ignition scope could catch this blip and data log it for play back.

Does the dyno shop where you and Mike go to have a Sun ignition scope? If so connect it to the plug wires and dyno the car and see if you can capture it there, that is the best way to catch it, doing it on the street is next to impossible when the miss is at that RPM. Another possibility is getting someone with a TECH 2 to datalog it, using a TECH2 can isolate which wire or at least which cylinder the miss is coming from when performing a misfire count test with a TECH 2.


[Modified by tjwong, 7:41 PM 2/29/2004]

STL94LT1

Jim, #007 seemed to run fine to me today. Oh............wait a minute, I'm not use to reving much above 6000 rpms.

I really like the look of that DelTeq Stage I ignition system. You should do a write up and pictures for the forum.

See ya the 13th, hope you get your high rpm miss fixed by then.
MP

HammerDown

You didn't get any anti-seize on the plug electrode(s) by any chance, did you? Even if the plugs are brand new, this is always a possibility. If any amount got on the tips, the plug is a throwaway, because it will be squirrely for the rest of it's life.

Just something to consider.

95_6spdvette

My LT1 is all stock and its missing at like 5000 rpms at about the same percent of the time as you are stating, I have 72k miles on the car. Im gonna change the spark plugs and hope that itll fix it. I think we may be having the same problem and im hoping it gets fixed soon because the track where I live is opening march 23!

96GS#007

Lots of great suggestions, I do appreciate the help!

As you mentioned Tom, catching something at 6000+ on the street is near impossible. The lower gears flash by too fast and police officers don't take kindly to 100mph+ blasts in the higher gears.

AutoTap can log Misfire data like the Tech 2 (while it's happening, counter by cylinder, etc). I'll take a look at that. Not sure if the shop has an ignition scope.

No hint of backfire thru the intake at all.

Springs are brand new and all were tested prior to the install.

This happened with a set of NGK TR55 plugs, not to mention my previous set of springs and plug wires. No anti-sieze on the plugs.

My plug gap right now is at .045 I'll regap to .035 and retest. It'd be nice if it was something easy for a change.

Of course we're supposed to get rain the next couple of days :(

HammerDown

You call pulling plugs and re-gapping easy? You must be a pro at removing plugs on LT engines. Takes me half a day and a pint of blood :crazy:

vader86

I remember Nathan Plemons had a high rpm miss when he had some headwork done, and he turned out to have a bent valve.

BAM92

Anychance gas went bad waiting for mods to be completed. Happened to a friend hence the idea. Drained fuel and added fresh stuff and miss was gone.

emoshn

Like you said, gas shutoff? I have the exact same thing happeing on my LT1, but not as high as yours...my shift point is about 5600 (factory at any rate) at just the time it reaches that point I get a dead miss and what seems like a backfire. I thought it was my plugs or wires or the opti...so I changed all...Taylor 8mm reds, NGK TR55 (.045...which by the way was way tighter than the plugs I pulled) and vented my opti to the intake manifold. Nothing changed. went to better breathing with K&N, open lid...same, throttle body water bypass...same thing, 160 degree stat....same thing. So, I've been thinking weak springs, but if I have a fuel cutoff like you...that's exactly what it feels like.

I'm 59...when I was a kid, we used to take our dads cars out. We'd go to the top of a high hill and start coasting with the ignition of...after we got up to speed with let the clutch back out, but with the ignition still on, after about 3 seconds we'd turn the ignition on...BAM...nice blown out muffler. The point is...this feels exactly the same way...like the fuel got shut off and almost instantaneously (after a decrease in RPM's) got turn back on again. Does this make any sense?

If it is the fuel cutoff...how does one fix it, so that it doesn't shut off til say....never. :)

96GS#007

Yeah, generally I can do them inside of 90 minutes. The trick is the right set of extensions and universals, and removing the inner fender panels. With the fender panels off, only #6 & #8 are a PIA.

96GS#007

Gas is fresh, bought 17 gallons (93 Octane) Saturday night :) It was doing this last year as well, so I've been thru a LOT of gas :)


[Modified by 96GS#007, 7:15 AM 3/1/2004]

96GS#007

This started prior to the headwork. The miss is identical (before and after).

Dr. Evil

If you could get a chance to dyno with the autotap or tech 2 on it to watch the misfire data that would point yopu in the right direction. Someting else I would do would be to tape a fuel gauge to the window and see what fuel pressure does when it cuts out.

HammerDown

Oh well then, ok. Yeah, the other 6 take me about an hour to get, then #s 6 and 8 take 5 hours.
:D

Chevy Guy

Sounds just like a vavlespring problem. Even if they are new, a spring can be weak, or broken, minutes after being put in use. Or they can be improperly shimmed making them too tight or loose. Another thing to check is the fuel system leaning in the upper RPMs.

madmike

sounds like a valve staying open, sticky lifter.....this is interesting , i cant wait for the diagnosis... :smash: :seeya

96GS#007

What makes me shy away from the valve spring theory is that I had the identical problem prior to valve spring replacement. I had new LT4 springs and replaced them with something with more pressure (110# / 315#) thinking they weren't strong enough. Still had the miss. The current springs replace the replacements. The replacements are good to .600 lift (I'm running .525) and are installed at 1.762 to get the pressures I mentioned above. Closed height spec on the springs is 1.75.

I had thought about fuel as well. I richened my AFR fueling equations in the appropriate RPM range to bump my AFR ~.5 pt. Injector duty cycle is within normal limits, <85%. No change in results.

The only common denominators are the lifters and the spark plug gap (was NGK @ .045, now Denso @ .045).