VERY strange problem with 30th LT4 Z. HELP!
VERY strange problem with 30th LT4 Z. HELP! What I have is a '97 30th Anniversary LT4 330HP Camaro SS. Car now has a 396 stroker with AFR cylinder heads and what looks like an LT4 intake, Holley billet throttlebody, hydraulic roller cam, comp cams roller rockers and hooker longtubes. Customer brought the car to me because the original shop that put the motor in couldnt figure out how to tune it very well and didnt want to deal with it..
As it started out, it made 360ish rwhp by about 5200 rpm and the graph was VERY erratic and just broke up real bad to 6800. It broke up so bad at one point in the graph it lost over 100rwhp and got it back within 200 rpm. I was going to suggest a new opti-spark but the customer told me a brand new one was put on at time of engine build. (I was still skeptical as I know the history behind these). The dynograph looked like serious valve float issues so I pulled the heads and had the springs checked and at the same time I replaced the lifters. Springs checked out ok. Put it back together and on the dyno and got the same results. Car wants to make power up to 5300 and then just falls off very erratically all the way to revlimiter. Checked fuel pressure under WOT-good. Changed spark plugs and wires with NGK and MSD. Still same results on dyno. Added MSD 6AL ignition box (brand new), back on dyno, same results. Decided to go ahead and get rid of the factory opti-spark and go with one of the MSD billet units as many had suggested. Back on dyno and same results as the factory unit but now we noticed that the first few pulls are MUCH cleaner. By the third pull, it goes right back to the breaking up really bad from 5300rpm all the way to rev limit. So someone emails me about doing the "Ignition Module Cooling Mod" where you space the ignition module away from its bracket to keep the heat from the cylinder head away from it.. Well we did that AND we spaced the coil itself away from it. Same results. So I decide to go ahead and replace the PCM. It was the last thing on the list. Got one from O'reilly, took it to the dealer to get flashed, tuned it on the dyno with LT1Edit, and got the same results once it warmed up. I forgot to mention that the crank sensor had already been replaced also with a new GM piece, and all the grounds on the engine that would be removed by the shop that did the 396 install would have disconnected we checked and are ok. Should I pull the timing cover to check the reluctor wheel to see if there is anything wrong with it? One thing I did notice on the Tech2 is that around that RPM where it starts breaking up, it shows it on the scan tool screen that it is actually LOSING RPM signal and dropping down then coming back up, as the same way the dynograph shows the rwhp doing. I got it up to 402 rwhp at 5300 rpm before it starts breaking up so far (which I feel is low for an AFR headed 396) Im not sure where to go from here. Air fuel is good. I am going to post the dyno sheet tomorrow. If anyone has heard or seen this, any help or recomendations would be greatly appreciated.
As it started out, it made 360ish rwhp by about 5200 rpm and the graph was VERY erratic and just broke up real bad to 6800. It broke up so bad at one point in the graph it lost over 100rwhp and got it back within 200 rpm. I was going to suggest a new opti-spark but the customer told me a brand new one was put on at time of engine build. (I was still skeptical as I know the history behind these). The dynograph looked like serious valve float issues so I pulled the heads and had the springs checked and at the same time I replaced the lifters. Springs checked out ok. Put it back together and on the dyno and got the same results. Car wants to make power up to 5300 and then just falls off very erratically all the way to revlimiter. Checked fuel pressure under WOT-good. Changed spark plugs and wires with NGK and MSD. Still same results on dyno. Added MSD 6AL ignition box (brand new), back on dyno, same results. Decided to go ahead and get rid of the factory opti-spark and go with one of the MSD billet units as many had suggested. Back on dyno and same results as the factory unit but now we noticed that the first few pulls are MUCH cleaner. By the third pull, it goes right back to the breaking up really bad from 5300rpm all the way to rev limit. So someone emails me about doing the "Ignition Module Cooling Mod" where you space the ignition module away from its bracket to keep the heat from the cylinder head away from it.. Well we did that AND we spaced the coil itself away from it. Same results. So I decide to go ahead and replace the PCM. It was the last thing on the list. Got one from O'reilly, took it to the dealer to get flashed, tuned it on the dyno with LT1Edit, and got the same results once it warmed up. I forgot to mention that the crank sensor had already been replaced also with a new GM piece, and all the grounds on the engine that would be removed by the shop that did the 396 install would have disconnected we checked and are ok. Should I pull the timing cover to check the reluctor wheel to see if there is anything wrong with it? One thing I did notice on the Tech2 is that around that RPM where it starts breaking up, it shows it on the scan tool screen that it is actually LOSING RPM signal and dropping down then coming back up, as the same way the dynograph shows the rwhp doing. I got it up to 402 rwhp at 5300 rpm before it starts breaking up so far (which I feel is low for an AFR headed 396) Im not sure where to go from here. Air fuel is good. I am going to post the dyno sheet tomorrow. If anyone has heard or seen this, any help or recomendations would be greatly appreciated.
RPM Signal That loosing RPM singal is a big problem. I'd start by checking the wiring from the opti harness back over the intake and then to the ECM. Did you ever replace the ICM, these go bad alot? Also I'd verifiy that your raw TPS voltage is reading correctly ie. 4.8-5.0 Volts @ WOT.
That loosing RPM singal is a big problem. I'd start by checking the wiring from the opti harness back over the intake and then to the ECM. Did you ever replace the ICM, these go bad alot? Also I'd verifiy that your raw TPS voltage is reading correctly ie. 4.8-5.0 Volts @ WOT.
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That loosing RPM singal is a big problem. I'd start by checking the wiring from the opti harness back over the intake and then to the ECM. Did you ever replace the ICM, these go bad alot? Also I'd verifiy that your raw TPS voltage is reading correctly ie. 4.8-5.0 Volts @ WOT.
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From: Catawissa PA
Don't forget about the stupid stuff that can give the ECM fits. make sure you have good grounds on the engine/body. might even want to add a second ground from the body to the ICM mount. When I relocated my ign. for the supercharger I got all kinds of odd codes due to a bad ground.
What about something as simple as knock retard? Do you have a scan of the car?
What about something as simple as knock retard? Do you have a scan of the car?
