rob69zombie

iTrader: (7)

Does it have an u/d pulley on it?

dtfastbear

My engine does not have an underdrive pulley. I do have the corvette pulley instead of the F-Body pulley, though. Same diameter but shallower.

rob69zombie

iTrader: (7)

Damn. Seems another guy has a stumbling issue but said his started after he put hid's u/d on. Mine started after I put mine on too; although it wasn't immediately after the install, I still plan on taking mine off and doing some more dyno pulls to see if it fixes it.

rob69zombie

iTrader: (7)

https://ls1tech.com/forums/generation-iii-external-engine/1399482-my-ls1-breaking-up-when-hits-5k-rpm.html
Here's his thread.

dtfastbear

We spent a bunch of time on the dyno today. We haven't solved the problem, but we did a lot of experiments.

1 - I gapped the plugs down to 0.035" and put 100 octane fuel in the tank. No difference.

2 - We watched the timing with an actual timing light on the pulley while doing a couple pulls. The timing was not changing in any appreciable way, so we know that the dip is not cause by the PCM or anything else pulling timing.

3 - We unplugged the knock sensors and did a pull. No difference. We weren't seeing any knock retard (or knock at all) on HPTuners, but we wanted to make sure.

4 - We swapped on a "known good" set of coil packs from my Orion's car. He just had his car on the dyno earlier in the week and had no issues. INTERESTING INFORMATION HERE: The car bucked, stumbled and backfired the whole way through the rev range with these coil packs! WTF?!?! The car did pull the whole way through the rev range and the dip was still there, but this is very interesting... Why wouldn't the car like those coil packs?

5 - We put the original coil packs back on and the car ran fine, but still had the dip.

6 - We put the plug wires on from my 2001 Yukon. No difference.

7 - We dropped the muffler off completely. No difference. Well, the car was louder and made more power, but the dip was still there...

8 - We watched the voltage AT THE COIL (the #2 coil) and it was getting 14v the whole way through the stumble. Perhaps we should try this at each of the 8 coils??

9 - Here's the second most interesting bit of data for the day - I did a pull where I really backed out of the throttle as it approached and revved through 5200-5800 and it STILL really broke up. That tells me that it is RPM related, not LOAD related.

*sigh* Another trip to the dyno and while we've got some information, I don't feel like we got any closer to the solution. Any ideas, no matter how crazy?

Cheers,

Dean

rob69zombie

iTrader: (7)

I'm changing pulleys to check that. Sorry all of your work didn't fix it.

orion4096

iTrader: (5)

I saw this in person and the car breaks up like it's randomly cutting spark in the 5k rev range. It's incredibly consistent and you can pretty much lay back to back dyno runs on top of each other within a few hp/tq. I went home and put my ls1 coil packs back on my car and had 0 issues. We have identical ls1 camaro 00/01 dwell settings in our tune.

More random ideas:

• Check WOT TPS voltage.
• Try plug wires without metal boots. I saw a post about someone seeing shorts that weren't random caused by this.
• Put an oscilloscope on the coil pack input and check dwell and power while revving.
• Check for a slipping belt. I know someone tested V somewhere (output of alternator? input to coils?), but if it wasn't with an oscilloscope we might miss something.
• You had your alternator bracket modded. Any chance that might be a problem?

Schwanke Engines

iTrader: (1)

Does it do it when it isn't on the dyno?

You may need to think about valvesprings.

rob69zombie

iTrader: (7)

Many have told me the same thing about my valvesprings but after the power falls off @ 5k, it comes right back around 5500 rpms. I thought if it were the springs it would keep falling off.

orion4096

iTrader: (5)

It does it on the track, too. He put it on the dyno to see if he could figure out why it's happening. The problem occured with two different heads on two different long blocks. The older heads are now on the newer block and it still happens. So basically there were two sets of different valve springs tried on the new block with the same problem.

dtfastbear

All true. And, this motor is completely stock except for the intake and exhaust. Is it really possible that brand-new-GM-crate-motor valve springs could float at 5200 rpms? As Rob Zombie (love it!) said, the power starts to come back after the short hesitation. Wouldn't valve float just get worse and worse with increasing RPMs?

joecar

Valve float has a resonant RPM... valve float can go away past the resonant RPM.

Frost

iTrader: (45)

aftermarket rockers?

dtfastbear

Stock rockers. It really does have the symptoms of valve float (from what I've read) but I am just having a hard time believing that a completely stock GM engine with just intake/exhaust changes could behave this way. Is there any definitive test to know if one is experiencing valve float?

Right now, we have two very divergent theories and I'm not sure what path to follow. One is that the problem is electrical - essentially the quality of the spark. Although we've read the voltage at a coil pack and are seeing unwavering 14v, we still have unexplained results with the other set of coil packs causing all sorts of misfire issues throughout the rev range.

The other theory is that the problem is mechanical. This certainly "feels" like valve float around a resonant frequency/rpm. Again, is there any test for this, or do I just have to order some aftermarket valve springs and try them out?

Thanks again for playing along, everyone!

Cheers,

Dean

orion4096

iTrader: (5)

What about the accusump? Maybe you can run the valve wiring completely unplugged (make sure it doesn't have all the oil!)? I remember when I first got my accusump installed it had all sorts of electrical havoc with the stereo that was in the car. Any other electrical devices that might be putting noise into the system?

Dean, I think you mentioned this Saturday, but the nice thing about my coil packs is that the problem is all over the rev range and you don't have to rev it up to find it. I'll see if I can find another set and loan you my current ones.

EDIT: I vote electrical problem, but that's just because I understand that stuff better.

dtfastbear

At this point, anything is possible. With the exception of valve float, all the "easy" causes seem to have been at least partially ruled out by one of the tests we've done.

The entire time we were running on the dyno there was absolutely no power going to the accusump. I have an override switch because when on the dyno, we need to have ignition power without the car running while messing around with HPTuners. Without the override switch, the accusump would see zero oil pressure and start pumping its oil out into the engine.

But, the car's wiring is not stock by any stretch of the imagination, so there is a reasonable chance that it's something in the wiring.

I would love to put your coil packs back on the engine just to rev and diagnose, as you mentioned, without the car on the dyno.

Cheers,

Dean

joecar

If you believe the problem might be electrical, then do this:

find someone who has (and knows how to use) a high resolution automotive DSO/oscilloscope (e.g. Picoscope) to capture kV waveforms of the secondary (using a capacitive probe) while running vehicle on dyno...

capture waveforms for all 8 cylinders;

the secondary kV waveform will show if the secondary is degrading during the dyno run.

The Alchemist

iTrader: (19)

Have you tried putting it into speed density mode and re-running the dyno?

What are the injectors doing during this time? Could it be a bad maf? Maybe a bad program. Have you tried reflashing the pcm completely. Not just a basic reflash, but reflash the entire program. I know that's an option with HPtuners.

dtfastbear

It happened with two different MAFs. We suspected an intake issue, so we completely removed the MAF and this is now an SD tune on top of a complete reflash.

We also have laid the tune on a completely separate PCM. And, we've tried alternate tunes.

In the initial data logs at the beginning of this thread, you can see that the injectors had very high duty cycles. Suspecting a fueling issue, we put in a brand new matched set of 36lb. injectors. No difference in the behavior at all. This afternoon I'll post a couple pictures of the HPTuners data logs from several pulls.

Cheers,

Dean

orion4096

iTrader: (5)

I can't remember off the top of my head if we logged the injector data ... I think we did. The tune was a little on the rich side for a NA car (low 12s up top in PE mode), but nothing that would cause that dip. This is also one of many tunes that have been on the car with the same problem. You won't see the correct AFR in the scans because we only had the tailpipe WB and there wasn't a convenient lead I could plug into the HPT box.

I definitely want to see an o-scope put on the coils. I'll be really surprised if they are all firing correctly with my coilpacks.