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Thread: Low Power 1996 LT4 Corvette

  1. #1
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    Low Power 1996 LT4 Corvette

    I'm working on Diagnosing a lower power issue with my 1996 LT4 Corvette. It seems to have ok power at very low rpm, but won't rev out beyond about 3500 rpm. It feels like it is out of fuel but my fuel pressure gauge says it good (it is a GlowShift Guage though). It reacts just like the Service Manual says it should although it may be a little high (KOEO should be 47psi, creeps up 50).

    When I log Narrow O2 voltage both banks stay above 900mv. Does this look like a failing fuel pump at all? Conversely, if the fuel pressure is running 3-4 psi, would that be enough to cause an overly rich condition, to the point of not running?

    full throttle.png
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  2. #2
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    Fuel system seems OK, but those engines have that optispark thing which is notoriously unreliable, so maybe check the quality of the ignition system?

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    It doesn?t seem to misfire at all but maybe when it fails it misfires all at once. Or maybe the cats are clogged.

    Thanks for the feedback.

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    I guess the next step is to see if I can get my vmx nano to pull anymore information.

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    Did a little digging and came up with what is supposed to be the main spark table for the LT4. It looks like I should be closer to 30 degrees in the above screen shot instead of 26 degrees. Is 5 degrees enough to cause it to be gutless?

    When I get a chance to get the Tech2 nano clone working, I'll see if I'm getting knock retard. There seems to be an "extended spark advance vs. RPM vs. Map that also may be adjusting the above value, but I haven't found out what this table does or looks like.

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    As a follow up and a bump. I was sure I would have a stopped up Cat Conv from the symptoms, but finally checked the back pressure this weekend and it isn't a clogged cat.

    Anyone have any thoughts on where to look next? I don't feel a misfire so not sure if this could be an ignition. Dirty injectors not able to atomize fuel? Check the overall health of the motor (compression/leakdown)?

  7. #7
    Is this a new issue that did not used to exist, or the way it's always been? Stock engine internals (cam, heads, etc)?

    The GenII LT's do seem to like their timing, but as far as the 5 degrees of timing, that wouldn't be the difference between continuing to pull vs. not pulling anymore, it would just be like pulling pretty good vs. pulling better.
    It was a long time ago, but I remember having some issues with optisparks/heat soaked coils just not giving good solid spark at higher RPM and upgraded to the billet MSD opti and relocated the coil somewhere cooler instead of on the cylinder head. Spark quality may be difficult to definitively diagnose but if you could get a wideband O2 sensor involved, that would diagnose weak spark (by showing overly rich when the spark quality takes a nosedive) and also any fueling issues if it goes lean (although you know youre at least somewhat enriched based on your narrowbands being at 900mV).

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    I bought the car in 2017 with about 75k on it. It's the first "fast" car I've ever had but in retrospect it probably didn't rev out like it should even then. I don't drive it a lot so still less than 85k on it. About 3 years ago, I noticed it didn't want to rev out, I think it has been one of those gradual things that can get bad before you notice it. Fuel pressure checked out low and replaced the fuel pump (and added fuel pressure gauge). It seemed to pick up some although still didn't pull in the upper RPM range. Now gradually it has dropped off again until 3k is all it will rev to.

    I guess I need to go ahead and confirm my fuel pressure with a mechanical gauge. And I had already been thinking about adding a WB (the only vehicle I have actually tuned on is LNF ecotec that has WB stock).

    If the coil/optispark completely gives stops firing at 3k, would it not indicate lean on the NB since it would be passing that air through? But I guess maybe it is still firing enough to burn some o2?

  9. #9
    Tuning Addict blindsquirrel's Avatar
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    Manual trans or auto?

    Does it rev normally in P/N?

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    My old LT1 had a wierd trans issue (auto) that made it feel like it bogged way down or started laboring hard but didn't pull. is this an automatic?
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    It's a manual.

    But either it's self corrected or maybe I just have to much mechanical sympathy. I think the real problem is that it has a torque dip at about that 3k rpm and I shift because it stops pulling. If I keep my foot down, it will pull through it to at least 5k+, although it is probably more sound than fury at that point.

    I used my autel scanner on it, and the only thing that seemed wrong in the data it will record is the cam sensor is out a degree or two from what the service manual says it should be. So I probably have a timing chain starting to wear.

  12. #12
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    LT1's generally pull pretty consistantly.. You typically don't feel the torque peaks like a LS. In fact most of the torque graphs on a LT1 dyno are like a big shelf.
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    Quote Originally Posted by Alvin View Post
    LT1's generally pull pretty consistantly.. You typically don't feel the torque peaks like a LS. In fact most of the torque graphs on a LT1 dyno are like a big shelf.
    His is a LT4 though. It should rip past 6,000 rpm making power the whole way up the rev range. If it does not it has a mechanical problem.

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    Maybe I need to get mine to a dyno and see what it's really doing then.

    Any recommendation for something like getting the injectors cleaned? The next time I get a chance to play with it, I'm going to throw pretty much a new ignition system on it, except for the optispark. I think I replaced plugs when I first got it, but don't remember for sure. To many projects. I'll check compression and maybe leak down at that time.

    I keep hoping that Dr. Mike will get his convertor for the AEM 30-0310 on the market and then I can get a wideband to help see what is going on (hoping to make it easy to swap vehicles). The main vehicle I play with the tuning on is my LNF powered Goblin and it had a stock wideband.

  15. #15
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    Those injectors do go bad but typically show up as a misfire at idle.

    WE are just guessing without better data.
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  16. #16
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    Quote Originally Posted by Alvin View Post
    WE are just guessing without better data.
    I know. Just hard to know where to look to get more data. It's not showing any misfire in my Autel scanner and I don't feel anything. I'll proceed with pulling/changing plugs and see if that tells me something. I should have already checked them, but I was sure it was going to be a fuel issue first then went down the path of stopped up cat.

    It might also just be my imagination.

  17. #17
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    I'd like to see a log with you driving it specifically in a way that makes the problem show up and be obvious. Like, 2nd gear steady heavy throttle or WOT from down low until it does the bad thing. That log looks like you're just driving normal and shifting early.

  18. #18
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    Quote Originally Posted by blindsquirrel View Post
    I'd like to see a log with you driving it specifically in a way that makes the problem show up and be obvious. Like, 2nd gear steady heavy throttle or WOT from down low until it does the bad thing. That log looks like you're just driving normal and shifting early.
    I'll do that. I was running the Autel the last time since it would read a lot more PIDs than HPT would. Unfortunately the logging isn't as good. It would only log about 8 at a time, so I had to do multiple runs to compare everything.

    And the only problem may have been me shifting early. And not really knowing what to expect from this motor. My Supercharged Camaro and my 1600lb 300hp kit car may just have me spoiled.