dtfastbear

I'm getting a very strange HUGE dip in power from around 4900 rpms to 5700 rpms, but only *sometimes*. When the behavior happens, it is extremely repeatable, but it doesn't happen on every pull. Here are a couple pulls that show the behavior:



Between these two runs, you can see that we richened up the AFR. But, the behavior was still pretty much identical.

Here are some clean runs that didn't show the behavior:



One thing that we noticed in the logging plots in HP Tuners was that on the runs that the dip occurred, the readings from the MAF seemed to get a little "jittery" and fluctuate by 10% or so just before and during the dip. So, we cleaned the MAF using some electronics cleaner. We got 5 good pulls in a row, then this:



It was the first time that we saw anything other than a perfectly smooth pull or the same dip over and over. So, cleaning the MAF definitely changed SOMETHING. Is it possible that the MAF is going bad?

Any other ideas? We can't find anything in the tune or in the logging that would explain what's going on here.

Other context: I experienced the EXACT same behavior with my previous engine. This is a new LS1 crate motor. The "dressing" on the engine, the PCM, the sensors, etc. were all carried over. So, I don't believe it is something mechanical in the engine. It has to be either a sensor or in the tune.

Also, I suspected that it could be some sort of resonance in the intake tract, so I removed the air cleaner and the elbow attached to it. Same results.

Would love any comments/ideas!

Cheers,

Dean

djfury05

iTrader: (30)

Are the grounds on the back of the head tight?

Darkman

iTrader: (2)

This may not be relevant, but this is where I've seen the double curves before.

https://ls1tech.com/forums/7248360-post7.html

Cold06

iTrader: (9)

Bad Dyno maybe J/K hope you figure this out buddy!

dtfastbear

I will double check the grounds. Something loose could definitely cause the sometimes-ok-sometimes-not behavior. But, since this happened with the previous motor and after a motor swap, I doubt we'd have left the same ground loose in the same way before and after the swap. If the ground was fundamentally bad in some way, I'd expect it to behave badly on every pull.

Bad dyno isn't a bad idea either, except this happens on the track, too. That's why I took it to the dyno. You can feel the acceleration "let up" from 5000-5700.

The double dip power curve shouldn't be in play, here, as this is just a stock LS1 (well, TWO stock LS1's) exhibiting the behavior. Not some weirdo cam.

More ideas?

Cheers,

Dean

thebulgd

iTrader: (6)

check the tune. Whats the timing, is any being pulled when this happens? if you have TCS is it tuned out? What are the other sensors doing during this dip?

are the lines on the bottom of the second graph the AFR?

fuel pressure good thru out run? are your injectors good?

dtfastbear

Haven't checked fuel pressure throughout the run, but I could do that easily enough next time I'm on the dyno.
I think the injectors are OK, but if you look at the screenshots from HP Tuners below, you can see that the inj
duty cycles go all over the place during the rev range (on the bad run, which exhibited the big power dip) when
the power dip occurs.

Here's a run when the power dip DID occur:



Here's a run when the power dip did NOT occur:



What stood out to me and leads me to believe it has something to do with the MAF is the MAF trace between the two runs.
On the bad run, the MAF trace is all over the place. This should be just a straight read from the MAF. Why would it
fluctuate wildly like that? On the good run, it barely fluctuates at all, which is what I'd expect. It's also reading much
lower on the bad run, and it looks like the Inj duty cycle is shortened correspondingly.

Thoughts with this additional data?

TORCHD 02 TA

iTrader: (39)

If you think it is the MAF.. Try a SD tune?? And maybe you need bigger injectors....

STS_Silverado

I agree with tryin an SD tune.. and maybe bigger injectors

bayer-z28

iTrader: (14)

What size MAF and TB? The MAF is used 100% over 4000rpm. If it's too big, the air flow can either be turbulent or too slow.

dtfastbear

I have a very "open" intake (short run with a large K&N cone filter), shorty Sandersen headers and a very open 2-1 exhaust. Otherwise, absolutely everything else is stock 99 LS1. The fuel pump is aftermarket, but it is sized adequately for this application. That doesn't mean it isn't malfunctioning, though.

Could I REALLY be maxing out the range on any of these components like injectors, MAF or TB? If so, I don't mind swapping out for better aftermarket components. But it would seem to me, and I am admittedly a total newbie on this stuff, that if it were injectors or something else getting "maxed out", then the behavior would be that it gets progressively worse at higher RPMs. This behavior is very, very reliably just between 4600 and 5700 RPMS and the engine comes back on strong above that up to redline.

Sorry to ask for help and then challenge the suggestions, but I'm having a really hard time getting my head around this problem.

Cheers,

Dean

Frost

iTrader: (45)

Your injector output is ROUGH! Looking at duty cycle or injector open-time, I'd say you could cut wood with that!

dtfastbear

So is that the tune or could it be a problem with the injectors themselves?

While they are stock, they have an unknown history. They're off a used motor I bought off Craigslist, so who knows how long they've sat and under what conditions.

LS1-450

iTrader: (33)

Upgrade injectors & triple check the scaling of your MAF table from frequencies associated w/ airflow @ 4000 RPM & above during WOT. Also, check your PE & OL fueling tables to ensure that they are not restricting fuel during conditions where the PCM needs to reference them.

I've got some barely used "new design" Accel 30# ers (36# @ LS1 fuel pressure) that I'll sell you cheap if you can come up w/ 2 more. I have 6 of them. Again, these are the new upgraded triple cone design, not the old red tops. Just send me a PM if you want 'em.

Frost

iTrader: (45)

It's in the calibration, the injectors are just doing what you are telling them to do.

I'm not saying that's the cause of the weird dip, just saying that's looking rough.

dtfastbear

Could this be the problem?



Took the car to another dyno and it broke up under high rpms with a completely different base tune on it. So, I'm pretty sure it is something other than the tune. Could a voltage dip or voltage irregularities cause such a thing?

Cheers,

Dean

Frost

iTrader: (45)

I agree; it looks like something more tangible than a calibration; I was just pointing out what was in the log/graph for commanded inj open time.

dtfastbear

Yeah - this post wasn't commentary on your suggestion, Steve It was just the next thing that I found. The tuner who worked on the car on Tuesday felt very sure that the issue was "electrical". I pulled the alternator on a hunch after seeing the voltage dip slightly when I revved the engine up above 5000 rpm. This looks like it is definitely an issue, but it isn't necessarily my only problem.

I'm putting a new alternator on tonite and will also have a different set of coil packs available to me this weekend to test if the problem still persists after changing the alternator.
Thanks again for the input!

Cheers,

Dean

dtfastbear

Alright, I'm taking the advice of many who have replied. New injectors (36lb) going in tomorrow. I'm going to remove the MAF, clean up the intake routing and go for a speed density tune. I'm going to Rick @ Synergy Motorsports on Thursday to get the car tuned on the dyno - he's supposed to be one of the best.

I'll report back how it goes.

Cheers,

Dean

The Alchemist

iTrader: (19)

If you beleive that it's an electrical gremlin due to flucuating voltage, monitor voltage in HPtuners as well. See if you have a dip in the voltage that corelates with the decrease in power.