drperformance

iTrader: (5)

I have a 87 firebird that I put a 95 LT1 into. Last year the the car ran fine and since then I stroked the LT1 to a 12.5:1 compression 383 and put a direct port nitrous system on it. Now getting this thing running right is a head ache this year. First the car ran like crap and I got a code for the optispark so I replaced it with a MSD unit and the car runs good but I still have one problem.

When I start up the car the car "seems" to idle ok in open loop but when I change gears (freshly rebuilt 4l60e) the car seems like it wants to die with the idle dropping. When I am on the throttle the car runs great with no codes but when I get off the gas and begain to brake the engine surges until a few seconds after I stop and it steadys out? And its seems to be worse in closed loop stalling?

I replaced:
Optispark (this year MSD)
ICM (this year)
O2s (this year)
VSS (this year)
TPS (last year)
MAP (last year)

I also checked for a vacuum leak with map gas and had no responce? I am going to further check for vacuum tomorrow. Will map gas effect a 12.5:1 compression engine?

When I rebuilt the tranny I put in a vacuum modulator to control the line pressure, I have yet to see if it is leaking.

Any ideas?

Vader

Obviously, tis is a custom PCM program (or you wouldn't be running a vacuum modulator for the trans). Check the programming for the decel fuel cutoff MPH and threshold RPM, and see how they compare to your new gear ratios.

drperformance

iTrader: (5)

Probuilt transmisions told me to put in this vaccuum modulator with the rebuild, and when the computer was redone I did not tell them about this, should I have? It just makes the line preesure controlled from vaccuum instead of the computer, and it came with some sore of capacitor or resistor to plug in so there are no trouble codes.

I would agree with the deceleration fuel flow but way does it want to stall when I change gears also?

I ran a tank of leaded C12 race fuel in this car but since changed my o2s and have no cats, could I have clogged my fuel filter? Would a clogged fuel filter cause this?

I checked again today for a vacuum leak with map gas and found nothing, will map gas effect the idle of a 12.5:1 compression engine running on race fuel?

I have been building this car for 3 years and am so confussed and about to give up, any help will be great.

Thanks

Wishmaster's87IROC

iTrader: (4)

hmm? my new motor does the same thing.....surges when idleing and when I come to a redlight.....even cut off on me one time from the surging.

any idea on this?

Im fuel injected though..but I dont think that would make a difference.

drperformance

iTrader: (5)

I just thought of this and can not believe that I did not think of it sooner could it be the IAC valve?

biged007

don't know if this will help but my wifes bird was doing the same thing out of the blue with no codes drove me nuts. in my case vacum leak?? yep after tearing what little hair I have left out it dawned on me touch the brake and !!get were I'm going? The booster was shot confirmed this by plugging it off and tring it yes it stops like a truck but that was it.
good luck
Ed