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Thread: Need help tuning my supercharged 94 Z28!

  1. #1
    Electronic Ignition!
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    Need help tuning my supercharged 94 Z28!

    Ok, so I picked this Z up about 6 weeks ago and am having trouble getting it to run properly. I am having problems when I put car into gear. Idle's around 900 but when in gear drops to 300-500. I've been told its related to my cam. Looking at my data log I noticed that my spark advance at idle is in the lower 20's but when I shift into gear it drops to 9-12. IAC count is maxed out at 160, shouldn't that be at WOT, not when rpms are low? TPS voltage is at .57, I read in another forum that at idle it should be closer to .67, is this correct? Finally, car battery voltage sits at 13.3v at idle but when in gear drops into the upper 11's like 11.5-11.9 volts. I was getting dtc 33 map sensor but have deleted that code since. I am good at turning a wrench but my knowledge of efi's is very limited. I've been reading, reading, reading but its all still foreign language to me. Any help would be greatly appreciated.
    Last edited by gmcknny1; 2 Weeks Ago at 09:25 AM. Reason: add file
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  2. #2
    Electronic Ignition!
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    Forgot to mention I've been using Scan 94/95 for data logging and recently started using TunerPro RT.
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  3. #3
    Fuel Injected!
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    Battery voltage going low is probably just from lack of RPM's to the alternator, should fix itself when you fix the idle problem. The timing is also directly related to RPM, ie it's not that it's pulling timing due to knock or too high of idle, just that when the engine is turning that slowly there is more time for the mixture to burn at any given advance. For example the time between 20 degrees BTDC and TDC at 900 rpm is roughly the same as the time between 10 degrees BTDC and TDC at 450 rpm.

    I'm not particularly familiar with that setup but my guess is that you need to adjust the physical throttle stop (or start by cleaning the throttle body if it's dirty), that will allow a bit more airflow getting the IAC valve back into a range where it can be effective.

    My guess is that the bigger cam is pulling less vacuum which means at the same IAC opening the car is getting less air and producing less torque due to lower VE because of increased overlap. If that's the case and opening the throttle a bit restores a proper idle rpm then the next thing you can expect to do is decrease the VE in the lower rpms ranges and increase it in the higher rpm ranges to better match the cam. Of course you'll want to do this based on your data logs.

  4. #4
    Electronic Ignition!
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    Attached is my most current data log. It was just sitting in my driveway with no drive time. As you look through my log you will notice a few times where I rev the engine and then when the rpms drop is when I put car into gear.
    Attached Files Attached Files
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  5. #5
    Electronic Ignition!
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    Thanks for the reply. So if I adjust the throttle stop screw should I do the tps sensor adjustment by drilling out the 3 sensor screw holes and moving it accordingly?
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  6. #6
    Fuel Injected!
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    I don't think you'll need to. I think the amount of adjustment needed is so small that closed throttle tps voltage will still be within the normal range. There are certainly people more knowledgeable/experienced than me who could provide more specifics about where you want your IAC counts to be, and i'm sure you can find some forum posts discussing that, but I know you don't want it maxed out during idle. I would start by adjusting the throttle stop until it can idle at the target rpm with IAC counts below 140 and see if you can find better information on where it should be. Basically you want it to have enough adjustment both ways to be able to handle temperature and barometric pressure changes, along with extra load from accessories such as A/C, power steering, etc. When I get some more time i'll take a look at the xdf in tunerpro because there are probably some setting where the code predicts how many IAC counts need added when shifting from P/N into gear, and that might also need some adjustment to get a smooth transition, but that's more fine tuning and can come after...

  7. #7
    Fuel Injected! Terminal_Crazy's Avatar
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    Just some thoughts...

    Firstly i'd try cracking the throttle blades to see if idle improves and iac tries to close some. You then know you have some control.

    I'd strip check clean iac passages and throttle body.
    See where the throttle blades are.
    The blades are generally better being as closed as possible on an LT1 as this supposedly stops split BLM's (I've not directly witnessed that but altering the timing does)

    I drilled out the iac passage in the front of the throttle body which allows more air in through the iac and iac counts go lower.
    I would suggest iac counts around 40ish (1/4 - 1/3 of the range to allow some control) It will open up quite a bit whilst starting off cold. and will increase as driving speed increases.
    Switching heating fans on also increase the iac around 10 points which then will come down again.

    If you motor needs a lot of air i've read that people drill hols in the throttle blades like they do on carbs rather than cracking the blades.
    On carbs if the blades are cracked you move over the transfer slots affecting the mixture.
    I don't there is anything like that on the LT1 throttle body though.


    HTH some
    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  8. #8
    Electronic Ignition!
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    Ok so I cracked the throttle blades and adjusted tps sensor to .66v. IAC's went down into the 60's, so that helped. I did a data log using EEHack but when I try to attach to this post it says invalid file. How do I attach this file so I can have somebody look at my log?
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  9. #9
    Fuel Injected!
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    Did you zip the file?

  10. #10
    Electronic Ignition!
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    Nope, but I will, thanks.
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  11. #11
    Electronic Ignition!
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    Heres todays datalog using EEHack. If anybody can figure out what changes I need to make I will also add my bin file. Or just use the EEX.XDF file. Also should I use the Lock BLM patch and the Stoich AFR target under funtions?
    Attached Files Attached Files
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  12. #12
    Electronic Ignition!
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    Can anybody take a look at my data log and tell me if I should make any changes? Thanks
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  13. #13
    Fuel Injected!
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    I would have looked sooner but I hadn't used EEHack before. The software is much nicer than I expected.

    First of all the log looks like a major improvement!

    One of the first things I noticed is how the left bank BLM goes all the way to 153 after it enters closed loop, while the right bank stays quite close to 128. I'm not sure if that can be a result of your setup or if it's indicating that you need your injectors cleaned.

    Next I can see that the idle rpm still drops too low when you initially put it into drive, then it mostly recovers as IAC steps increase. I looked in the XDF to see if there is any kind of in gear adder and I don't see any. Maybe the operating system doesn't support that, or maybe it just hasn't been put into the xdf.

    Your idle rpm seems a bit low also. I downloaded the stock 95 z28 bin and the target idle speed is 800 in gear or P/N with a warm engine. You might try to duplicate that and see if it gives you a more stable idle.

  14. #14
    Electronic Ignition!
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    Do I keep making idle adjustments using my throttle stop screw or can I make changes to my tune via one of the tables listed?
    94 Z28 Auto/ Powerdyne BD11A Supercharger/ MSD 6A Ignition,Blaster 2 coil and MSD Opti/all forged rotating assembly/ .525"-duration 305 cam/ 24lb injectors/ Hooker long tube headers/ tuned/ NOS fuel pump

  15. #15
    Fuel Injected!
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    Adjust the desired idle speed using the "* Idle Speed Target RPM" table under the "Idle Air Control (IAC)" category. Your throttle stop is probably okay if not a touch too open. The IAC counts got down around 30 which is a bit lower than Terminal_Crazy suggested. I think the only potential "problem" would be if you were at a lower elevation with dense cool air the IAC could theoretically "lose control" if the IAC counts hit 0. In reality though the ECU can still control the idle speed by adjusting the spark timing. Notice that when the idle speed is hovering around the Target RPM the computer is constantly adjusting timing without changing IAC counts. Only when the idle is too high or low for a longer period does it adjust the IAC valve. Since you seem to be close to sea level judging by the Baro showing 99kpa you probably won't have any problems with your current throttle stop setting.

    I think that some stock calibrations for automatic transmissions have the P/N idle speed set a bit higher than the in gear idle speed. Doing that will likely help with a smooth transition when putting the trans in gear. Maybe try 800 P/N -> 700 in gear and play around with it until you like it.

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