High RPM miss can you figure it out?
06-13-2007, 10:47 PM
#1
High RPM miss can you figure it out? OK so heres the story. The car is a 94z28 with a fresh rebuild, and a gm847 cam. I have had my car for 6 years and it has always had a high rpm miss that starts right around 6200. I just had the motor rebuilt this winter, and it still has the exact same problem, so I doubt it is mechanical.
The miss seems to be directly heat related. So, I think its something electrical. If I start the car up, let it get up to running temp, it runs great no problems. Once the car has been running for more than 20min or so it starts to miss. The motor got a new opti this winter, didnt solve the problem. New 30lb injectors, new plugs, wires, nothing. This last weekend I tried a new coil, ignition control module, still missed. Tonight I got a new PCM, flashed it with my tune, and its still missing. Virtually everything is brand new.
This car has had the same problem for 6 years. Anyone have any guesses.
The miss seems to be directly heat related. So, I think its something electrical. If I start the car up, let it get up to running temp, it runs great no problems. Once the car has been running for more than 20min or so it starts to miss. The motor got a new opti this winter, didnt solve the problem. New 30lb injectors, new plugs, wires, nothing. This last weekend I tried a new coil, ignition control module, still missed. Tonight I got a new PCM, flashed it with my tune, and its still missing. Virtually everything is brand new.
This car has had the same problem for 6 years. Anyone have any guesses.
06-14-2007, 03:18 AM
#4
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Have you tried the ICM mod to move it out just a bit? That thing hates getting warm.
http://shbox.com/1/4th_gen_tech2.html#ICM_cooling
http://shbox.com/1/4th_gen_tech2.html#ICM_cooling
06-14-2007, 11:26 AM
#5
Originally Posted by Formula350
Opti (new doesn't mean good heh), heat soak, bad tune (no offense).
The car has a brand new AC delco Opti. I have tryed 3 different Opti's before I broke down a bought a new one. The car had the same miss with all 4 Optis ive had.
My car has had probably 20 different tunes in it. The car has had been dyno tuned 5 different times. On the dyno the car has a -30 hp spike when the motor misses. I would say its viturally impossible for there to be something wrong with the tune that is heat related.
06-14-2007, 11:29 AM
#6
Originally Posted by titter
Have you tried the ICM mod to move it out just a bit? That thing hates getting warm.
http://shbox.com/1/4th_gen_tech2.html#ICM_cooling
http://shbox.com/1/4th_gen_tech2.html#ICM_cooling
Yes, I have had the ICM spaced off of the head for about 3 years. Didnt help out at all.
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06-15-2007, 09:07 AM
#9
Originally Posted by Formula350
I know this sounds really dumb, but is it the rev limiter set too low? >_>
06-15-2007, 10:11 AM
#10
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Dumb question but does it miss 6200 WOT? I started thinking O2's but then I realized that at 6200, you're probably in PE tables and bypassing O2's.
06-15-2007, 10:13 AM
#11
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Oh something else I thought of, do you have newer model opti conversion? I have read that people get similar effects as you if the dowel pin is too long on the cam. I am assuming it throws the low res disc (or whatever it is) off balance.
06-15-2007, 09:42 PM
#12
Originally Posted by dhdenney
Dumb question but does it miss 6200 WOT? I started thinking O2's but then I realized that at 6200, you're probably in PE tables and bypassing O2's.
The car has brand new ac delco o2s as of a few weeks ago. But yea, they shouldnt be making to much of a difference at WOT.
06-15-2007, 09:46 PM
#13
Originally Posted by dhdenney
Oh something else I thought of, do you have newer model opti conversion? I have read that people get similar effects as you if the dowel pin is too long on the cam. I am assuming it throws the low res disc (or whatever it is) off balance.
Well the car got a brand new cam, and brand new opti this winter. I would say yea, that might be a problem but seeing that it is exactly the same as before I change anything I would have to say no.
I just bought a new fuel filter yeasterday, so im gonna throw that on and see if it makes a difference. I doubt it will, because I dont see how that would be a heat related issue. But, I really wanna get rid of this miss, so im gonna try everything.
06-15-2007, 10:37 PM
#14
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my car is experiencing the same problem. only at WOT. when I am 3/4 it is fine....
my rev limiter is set to 7 as well. my car was fine until abotu 4 weeksago then this started to happen. I drove ti all last year no probs......wierd.
my rev limiter is set to 7 as well. my car was fine until abotu 4 weeksago then this started to happen. I drove ti all last year no probs......wierd.
06-16-2007, 04:03 PM
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I have te same problem in my car at around 5k rpms, but Mine is still stock. It's been like that since I bought it 6 months ago, but it only does it once the motor is fully wormed up after about 20 min like you. I've also been watching vids of lt1's on u tube and streetfire and I;ve seen and heard some other lt1's do the same thing. It's annoying as hell wen it feels like you hitting a wall for a split second. My cai helped it out a little mainly because I think the engine runs a tad cooler with it.
Question for FOGGEDZ, how long have you been running that gm847? What rpm do you shift at when racing? And are you running stock bottom end. Someone was teling me that the bearings will fail past 6200 or so. But I want an aggressive cam without having to get into rebuilding the whole motor. For me I'd like to be able to shift at 6500, but is that ok for the engine or will it damage stuff??
Question for FOGGEDZ, how long have you been running that gm847? What rpm do you shift at when racing? And are you running stock bottom end. Someone was teling me that the bearings will fail past 6200 or so. But I want an aggressive cam without having to get into rebuilding the whole motor. For me I'd like to be able to shift at 6500, but is that ok for the engine or will it damage stuff??
06-16-2007, 05:39 PM
#17
Originally Posted by ryanisfast
I have te same problem in my car at around 5k rpms, but Mine is still stock. It's been like that since I bought it 6 months ago, but it only does it once the motor is fully wormed up after about 20 min like you. I've also been watching vids of lt1's on u tube and streetfire and I;ve seen and heard some other lt1's do the same thing. It's annoying as hell wen it feels like you hitting a wall for a split second. My cai helped it out a little mainly because I think the engine runs a tad cooler with it.
Question for FOGGEDZ, how long have you been running that gm847? What rpm do you shift at when racing? And are you running stock bottom end. Someone was teling me that the bearings will fail past 6200 or so. But I want an aggressive cam without having to get into rebuilding the whole motor. For me I'd like to be able to shift at 6500, but is that ok for the engine or will it damage stuff??
Question for FOGGEDZ, how long have you been running that gm847? What rpm do you shift at when racing? And are you running stock bottom end. Someone was teling me that the bearings will fail past 6200 or so. But I want an aggressive cam without having to get into rebuilding the whole motor. For me I'd like to be able to shift at 6500, but is that ok for the engine or will it damage stuff??
Previous years with the 100,000mi stock bottom end and a smaller cam I shifted the car at 6400.
06-16-2007, 05:53 PM
#18
I have been messing with the car all day, and I found one odd thing that doesnt make any sense. My car has a MSD 6a box and it has ever since I bought the car. I havent touched the thing in 6 years because it has always seemed to work fine. I decided this morning to try and bypass the 6a box, and rule that out as a problem. I unpluged the factory coil connector from the MSD connector, and pluged it directly into the ignition coil..... NO START. So, I completly pulled all the wiring out from the coil/ICM back to the computer, and tested it for resistance. Everything seems to be fine. I look at the factory wiring diagrams and everything checks out. So, I hooked everything back up bypassing the 6a box still will not start. I just can figure out how the car wont start with the factory wires pluged directly into the coil like it would be from the factory. But, when you plug the same wires into the msd 6a box, and use the MSD to contoll the coil the car fires right up. Anyone have any thoughts on that?
06-16-2007, 05:58 PM
#19
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mine does the same thing..........once it warms up lol mine also has a chip its a 93 do u guys think if i got a new chip that it would solve my problem?????oh and sorry to high jack your thread man
