Engine Gt40p Setup Won't Idle

Discussion in '1994 - 1995 Specific Tech' started by dstang01, Jul 11, 2012.


  1. dstang01

    dstang01 Member

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    Hey folks, I swapped on GT40p heads and a tubular GT40 intake (stock cam and stock throttle body) onto my '94 and am having trouble getting it to idle. It runs through the RPMs nicely and feels strong, but as soon as I pull up to a light the idle slowly drops to about 500 rpm and oscillates, eventually stalling. I played around with the throttle body stopper screw to raise the idle a bit thinking perhaps the higher flowing parts required more air at idle, this helped make the stalling less frequent but it still happens, especially when you load the engine with the brakes or power steering. I read this wasn't the best thing to do, and also read a bit about adjusting the bleed screw in the IAC, testing the voltage of the TPS, etc... but am confused and was hoping someone SN95 specific could point me in the right direction. I knew our computers were finicky, but didn't think a tame swap like this would cause so much grief! :confused:
  2. 94blackcobra

    94blackcobra Member

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    Try adjusting the air bypass screw on the TB and see if that helps.
  3. k00ksta

    k00ksta Member

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    Or replace your TPS (throttle positioning sensor)
  4. Muttstang5.0

    Muttstang5.0 New Member

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    make sure to check your timing! mine was way off after doing a similar swap (gt40p heads and explorer intake) and it wouldn't idle very well either. although that was just due to my ignorance of not being about to find the spout lol
  5. toyman

    toyman Active Member

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    I would follow the idle reset procedure. See this link. The only thing I would change in the procedure is the TPS voltage signal can be between .7v - 1.2v. The adaptive strategy will recognize the voltage in this range as being an idle signal.
  6. stanger9410

    stanger9410 Member

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    I had to re tune my car after installing only a cobra intake
  7. dstang01

    dstang01 Member

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    So I cleaned my IAC, followed the base idle reset, checked my TPS voltage (which was at 1.2, so to be safe I did some notching on the stock TPS and lowered it to 0.95, though it seems this is irrelevant on SN95s as they zero the sensor every start) and still I'm having idle issues. Timing was dead on 10, I advanced it to 12 to see if it would make a difference and no luck.

    When I start the car it idles fine for a few minutes (I'm guessing this is during open loop?) and after awhile begins to surge until it stalls. If it were a vaccuum leak causing the problem, wouldn't the idle surge right from the start? I'd like to test for vac leaks with the carb cleaner/water spraying test but since the idle is so unpredictable I can't really tell if there's a change in it or not. Are there any obvious symptoms that point to a vac leak vs. a poorly adjusted TB/IAC?

    I appreciate any advice, I know these threads are all over the place but I'm a rookie when it comes to tuning and whatnot so any points in the right direction would be great :)
  8. dstang01

    dstang01 Member

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    I've also got an exhaust leak at my passenger side header flange that I plan to work on when I get access to a lift, would that affect the idle at all?
  9. Stangstroke

    Stangstroke Member

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    You need to have your car dyno tuned when you change the aspects of how your motor breathes ie intake or exhaust you will need to recalibarte your motor when you make a motor breath better that usually means your moving more air and ultimately running a lean condition not good but the guy you choose to dyno tune your car he will first set up low idle and drivability and then make full pulls to maximize the changes you've made to your motor because we all want the motor to run to its full potencial.
  10. dstang01

    dstang01 Member

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    Hmmm, I was under the impression that the EEC would re-learn conditions for subtle mods like GT40p heads and a GT40 intake, I remember reading of folks getting by fine with the stock cam. That's unfortunate if I'll need a tune... I purposely left the stock cam for now until I had the time/money for my next stage of mods (quarterhorse, wideband, injectors, maf, TB, TFS1 cam, 1.7 rockers etc). Are you sure this mild combo needs a tune or could a vacuum leak be the culprit?

    I'll be pulling my codes tomorrow too so I'll keep you guys posted, thanks again for the input!
  11. 94blackcobra

    94blackcobra Member

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    Unfortunately, what you did really can't be considered a "mild" change.
  12. JJ95GTID

    JJ95GTID Active Member

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    Have you ruled out a vacuum leak.
    I had all the same issues you had and found I had forgotten to reconnect one of my vacuum lines.
    Also, I've had an intake manifold gasket slip on me after replacing the head gaskets.
    Air was bleeding in from between the lower intake and cylinder head.
    You can lightly spray some carb cleaner around the lower intake area while the engine is running to test for a vacuum leak.
    If the idle goes up after you spray you have a leak. If not something else is wrong.
  13. N8Dogg98

    N8Dogg98 Banned

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  14. fiveohwblow

    fiveohwblow Official Member

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    :lol:

    Better than e6 or e7 castings thats for sure.

    I have heard the 94-95 computers were finicky but DAMN. According to 2000xp8 or joe there is no need to have a professional tune with a hci and blower combo on foxes!
  15. toyman

    toyman Active Member

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    Yes. Air will be drawn into the exhaust stream which will result in erroneous O2 readings when in closed loop. The ECU will compensate by adding more fuel to bring the AFR back into spec.
  16. toyman

    toyman Active Member

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    FWIW I don't think a tune is necessary with a head and intake swap. Any change in air flow alters the O2 readings and the ECU will adjust the fuel load as needed.
    fiveohwblow likes this.
  17. Stangstroke

    Stangstroke Member

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    The guy that tuned my car told be that the 94-95's are the biggest pains in the butt:bang:to tune and very finickey. All i know is when i had nothing but bolt ons on my stock motor this guys tuning skills are awesome i went from not being impressed on the money i spent on bolt ons ie BBK cold air, BBK long tubes, BBK 75mm TB, BBK X-pipe none of these worked well at all until i went and dyno tuned the car. The car didn't like to idle it hesitated and never felt like it was running right after the dyno tune it felt like a whole new car it was like night and day it ran better and i even got better gas mileage everyone here can say what they want but in my opinion without a dyno tune after put a bunch of parts on your car your throughing your money away. This is also the way companys like BBK can advertise the HP gains for each product they sell because the DYNO tune!!!!:burnout:
  18. 2000xp8

    2000xp8 Mustang Master

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    Actually back in the day on the 94/95's i'm pretty sure that what the guys did originally was repin the harness to use the fox A9L then run a fan controller to run the fans, so the sn95's ran just like foxes.
    Although i don't remember at what point that was necessary, i know people now with modded 94/95's now that run fine with h/c/i swaps, kenne bell's, etc...
  19. dstang01

    dstang01 Member

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    My point exactly, that was my understanding. More air in with the same fuel as stock = lean condition = adjustment to fuel load due to O2 readings.

    I agree a tune is always going to be the best idea for optimization, but a GT40p/GT40 intake/stock cam setup should be able to at least idle on it's own. The car drives great aside from the idle and has a noticeable increase in power from 3500+, and I'll even go longer on a tank of gas, so I think it was a worthwhile investment for the little money I paid for the parts. Like I said, I'll be tuning once I add a cam in my next wave of mods but with tuition and whatnot I can't afford to do it all in one shot.

    I'll work on that exhaust leak, it makes sense that it would affect the O2 readings but I wasn't sure if the pressure at the flange would be too high to allow any air to pass in... either way I'll give it a shot and hope for the best!
  20. toyman

    toyman Active Member

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    Actually it draws air in as well. If the pipe wasn't hot you would feel suction along with the exhaust puff if your hand was on the pipe.

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