The Alchemist

iTrader: (19)

The reason I was asking about the maf/injector pulse width was wondering if you had some kind of tune issue. Not blaming the tune, but thinking there was something weird going on with it.

It's weird that the same symptons are being duplicated with a different motor, but the same electronics.

dtfastbear

Sorry, don't mean to be defensive when you ask questions about the tune. It's just that I've been banging my head against the wall for a year now on this problem, and it's frustrating that all the "simple" explanations don't seem to be the cause!

Here's a log scan from one of the pulls. They all look pretty much exactly like this one. In other words, NOTHING seems to be going wrong, except for the 50HP drop on the dyno, of course.

joecar

I thought of something else: walking crank reluctor... but if this were the case, I think the motor wouldn't recover.

orion4096

iTrader: (5)

I was looking for a tooth reader/display in hp tuners output from the ECU, but couldn't find one. The hope was that if there was a problem with the wheel or sensors it would have shown up in a waveform.

dtfastbear

Yeah, we've been looking at and thinking about the reluctor, but across two motors??

My buddy's leading theory is now a mismatched or "bad" harmonic balancer. Once thing that I probably didn't mention earlier in the thread is that I've changed out the stock F-body balancer for a corvette-style balancer for front sway bar clearance. I'm not 100% sure that it was a GM part that I got - I might have bought this one on ebay...

Anyway, everyone else that's having this issue seems to have an underdrive pulley. We recently came across some information that says "if you have undamped vibrations in the crank, you can cause premature valve float". Does anyone buy that? Our symptoms are DEFINITELY just like valve float symptoms, but they are happening at an RPM where I wouldn't expect valve float.

I'm going to put the stock pulley back on and test it. If that solves it, I'll probably get an ATI damper or something like that...

Thoughts?

Dean

rob69zombie

iTrader: (7)

Sounds like a plan Dean! My pulley was switched out today but I haven't heard if they made any pulls or not.

rob69zombie

iTrader: (7)

It's fixed!

dtfastbear

Hey, that's great news!!!!! Orion is going to loan me his stock pulley and we're going to try it on my car. Don't know if I'll get it done this weekend, but I'll definitely report back.

Cheers,

Dean

The Alchemist

iTrader: (19)

It was the underdrive pulley? Who's? Definitely post this up because people need to know and be aware. There's lots of internet myth out there and this is certainly a case where everything was checked, double checked, and the end result was the pulley.

rob69zombie

iTrader: (7)

It was a ASP 25% U/D. Good luck Dean!

dtfastbear

Thanks, Brother. Orion is stopping by this afternoon to drop off his stock CTS-V pulley. Car people are great people!

I *might* be able to get this on the engine and try it on the dyno on Friday. I'm not seriously optimistic, as I don't currently have an aftermarket U/D pulley on there, but I'm still holding out some hope.

Cheers,

Dean

rob69zombie

iTrader: (7)

My buddy told me that even though it worked on his car, he had read about trouble with ASP's in the past. I lost a few hp but it revs again; plus I'm gonna get a ftra to compensate for it.

joecar

Are you saying your pulley/damper might be slipping...?

dtfastbear

Slipping, failed or defective. I got it "off ebay" so I'm not even 100% sure it is an OEM part. Yeah, it's a long shot but my problem has the characteristics of this failure.

I've also got a guy with an oscilloscope lined up to capture the waveforms you suggested. Same guy who loaned me the pulley. Small circle we run in, here.

rob69zombie

iTrader: (7)

I bet it's the problem. At first I didn't think so on your engine, but after learning you changed it I'm gonna go with the balancer.

joecar

What oscilloscope does he have...?

If he has a set of capacitive kV probes (proportional to secondary voltage) those would be preferable over inductive probes (proportional to secondary current)... the secondary voltage waveform will show things like firing kV and duration, and if the burn progression is lean or rich, and if those vary as engine revs up.

The CKP/CMP waveforms will show if the CKP signal moves relative to the CMP signal (i.e. CKP reluctor walk/spin).

The injector voltage/current waveforms show when the injector is commanded, and when it actually opens/closes (the pintle movement induces back voltage/current seen as small bumps in the waveforms).

orion4096

iTrader: (5)

Rigol 1052e with whatever it comes with - I think it's just a set of inductive probes. I'm not going to get my hands on it until Wed or Thurs and haven't used a scope in over 14 years (basic EE class). Should be interesting.

joecar

I'm not familiar with that brand, but I'm sure you'll do fine.

Just remember to connect the probe grounds to vehicle/battery ground and not across any sensors (i.e. the probe grounds must all be at the same potential otherwise the o-scope may fry itself)... i.e. measure all waveforms wrt to vehicle/battery ground.

dtfastbear

Breaking News!!!

Because the problem is really exhibiting the symptoms of early valve float and because these other guys have demonstrated that cheapo U/D pulleys can cause the issue, I went ahead and pulled my front pulley so I could replace it with Orion4096's pulley straight off his crate LS6. Well guess what????



Which of these is not like the other? That's right, I have either a:

- truck pulley or something designed for another application OR
- a cheapo Chinese knock-off

You can see the casting texture on the (current) pulley on the left. The one on the right, off the LS6, is significantly lighter and has a MUCH larger rubber ring between the inner and outer sections.

I'm still holding my breath, but I think we might have a winner! I won't get to test it on the track for almost two weeks, but I'm totally fired up to find something suspect with the pulley.

Cheers,

Dean

dtfastbear

It was the pulley! I drove the car on the track this weekend after installing the harmonic damper shown on the right, above, and the car pulled cleanly through all the way to 6200 rpms which it has never done before...

I'll post back once I get the car on the dyno to be absolutely sure that the problem is completely solved, but I'm pretty happy right now!

Thanks for all the help along the way, guys.

Cheers,

Dean