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Please Help, 2000 Z28 won't go over 120mph?

This is a discussion on Please Help, 2000 Z28 won't go over 120mph? within the General Help forums, part of the LSx Technical Help Section category; I have 2000 z28 and it came with the Z-rated tires from the factory so from what I understand it ...

  1. #1
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    Mystic Teal
    2000 Z28

    Please Help, 2000 Z28 won't go over 120mph?

    I have 2000 z28 and it came with the Z-rated tires from the factory so from what I understand it was speed limited
    somewhere around 160mph as opposed to cars that came with V-rated tires that were limited to around 120mph.
    I've had the car over 120mph on numerous occasions without any issues until recently it started acting like it is
    speed limited at 120mph? I really have no idea what would cause this. The only mods I've done to the car are swapping
    out the 2.73's for 3.73's and installing a predator tune, but I've been over 120 many times after both of these mods so I can't
    imagine they are the independant variable. When I look at the predator tuner menu at speed limiter it is
    set at 175mph so I really can't understand why all of a sudden it refuses to go past 120mph? Aside from this issue
    the car runs 100% perfect. I hate to bother anyone or waste anyones time I was just really hoping it was something
    obvious that I was missing or that someone else had run into this same problem. Thanks in advance for any help,
    I sure appreciate it! ~Mark

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    Is it running out of fuel -- anything feel weird as it tops out? Could be that something is triggering "abuse mode" in the PCM and it thinks that it is protecting itself... not sure.

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    Former Mopar Man 2002ssslp's Avatar
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    There is also a slight chance that it could be something with the speedo. Can you give us more info to go by ? I think it is either fuel of ignition related. Have you checked your air filter and fuel filter lately ?
    My ride is a 2002 Camaro SS SLP #3296 with 30k, LTH, 3" Y, CME, Frost tune, K&N, ported TB, Blackwing lid, Bellows, MSD, Denso Iridium, and 85mm MAF, Bilsteins, Eibach springs, SLP strut brace, Adj. Panhard, TA Girdle, UMI, Pro 5.0, Nitto NT555
    My wife has a 2004 GTO with the rare SAP, 18" wheels, K&N Cold Air System, MSD, Ported TB, Frost tune, Denso Iridium, Flowmaster cat-back, 3200 Yank, 75k

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    Moderator 35th-ANV-SS's Avatar
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    You're going over 120mph on public roads? You're brave lol.
    It's on jackstands.

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    Electrical Engineer KMdef9's Avatar
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    If the car is still launching hard, it shouldn't be fuel/spark/intake related, otherwise it would be acting up then as well. I'm with Jeff, sounds PCM related.

    Can you use your predator to scan for hidden fault codes?

    Does it always happen at 120?

    I'd hate to recommend speeding that fast, but I'd try changing the setting in the predator from 175 to 165 or something.

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    Mystic Teal
    2000 Z28

    Thank you so much to everyone for replying. I too believe it has to be a PCM related issue as I can produce the same results going hard in 3rd gear or "Easing to 120mph" in 4th. Either way I get the same thing, the car refuses to go above this and it acts exactly like it's up against the speed limiter. I've taken your advice and I lower the top speed using the predator tuner to 150mph. Unfortunatley the weather has been rather crumby the past few days and I haven't been able to give it a try. I'll be sure to give it a shot the first nice day. I hope no one thinks I'm weaving in and out of cars like a maniac or anything lol. There is a stretch of Interstate near my house that is about 3 miles long and dead straight, with concrete barriers on either side so it makes an excellent test track. I'm thinking if it still won't go above 120 I may go back to the factory tune with the predator and see what happens. It's just odd to me that it seemed to happen without changing any components on the car. Once again, thank you so much to everyone for trying to help and I'll be sure to post the results as soon as I'm able. ~Mark

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    Electrical Engineer KMdef9's Avatar
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    Quote Originally Posted by mystictealZ28 View Post
    ...go back to the factory tune with the predator and see what happens. It's just odd to me that it seemed to happen without changing any components on the car.
    That's what I would do.

    I've seen electrical components have a mind of their own many times. Electrical gremlins are the worst.

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    Mystic Teal
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    Update: After a bit of trial and error it seems like the issue is the car wont shift into OD at Wide Open Throttle. If I let off the throttle for a bit it shifts into OD and then I'm able to exceed 120mph with no problem. I've done a bit of research on the 4L60E and I've found conflicting information regarding shifting into OD at WOT. Some say it will some say it will not. In thinking of past times I've gone over 120mph I've never consciously let off the gas to allow the car to shift into OD but it's possible that I "eased" into it and that's why I was never aware of this issue. From what I understand it's hard on a tranny to shift into OD at WOT anyways and I certainly don't plan on making a habit of it. I just want to make sure something isn't wrong with my car. If this is the status quo and some sort of safety feature built into the trans I can live with it but if it's indicative of a problem I would rather resolve it now. Once again, thanks so much to everyone who posted trying to help me out and please forgive my novel-long posts I have a tendency to ramble. ~Mark

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    Moderator 35th-ANV-SS's Avatar
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    I'm not sure if that's a safety precaution or not, but I will agree that it's extremely hard on a transmission to shift into an overdrive gear at high RPM.

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