3500 fiero idle surge

[merlot566jka]

well im still waiting on my aldl cable, so i dont have any ecm info to accompany the problem. no codes.

the last 3500 i had in here did the same thing, so i just adjusted the idle set screw and all was good, sort of. it idled high and would buck on me at different times, not a good fix.

the new 3500 has a new 4.3 70mm tb on it and i didnt adjust the set screw at all, the car will idle on its own till it comes up to temp, then it surges and eventually dies.
thinking it was related to the cts, i replaced it and the iac just to be sure. no changes.
if i barely hold my foot on the accelerator it will idle at 1100rpm and surge slightly, but as soon as i remove my foot it jumps up to about 1500rpm then down to below 500rpm and dies.

i tried reading posts every where about this problem, apparently the fiero had a big issue with this, but i dont even ahve the same ecm anymore, so i ruled that out.
some things that have happened to others i looked into, and couldnt find anything relivant. i had the batt checked and charged, alt is putting out 14.4v, coolant temp sensor ohms read right on the sensor and back at the ecm, the iac is working and i think its what the computer is using to surge the engine. im going to try a heated o2 on thurs and see if that helps.

Has anyone ever expierence anything like this? What was your fix

[1988GTU]

TPS? Is it working properly?

[merlot566jka]

at closed throttle its at .52v at wot its 4.87v

[bszopi]

Since you mention that it idles fine until it comes up to temp, then that means the problem has to do with when the engine goes into closed loop. My first thing to look at would be the O2 sensor, as that is what tries to control the A/F mixture once the engine is in closed loop. If that's sending bad signals to the ECM, then there is a good chance the ECM is trying to correct for something that isn't really there, and therefore killing the engine.

[merlot566jka]

brad, this is off topic, ill be headed your way on may 12th to tow the fiero to indiana for tuning and then ill be off to great lakes, il. then ill be off to florida! more navy mumbo jumbo. anyways, if your available i could make a pitstop in your area and show you the car....

and yes i think its the o2 causing a problem too. will confirm thursday

[bszopi]

May 12 is my bday, but I will still be around. Just let me know.

[merlot566jka]

ok, ill bring you a gift

[Joseph Upson]

It's in the chip dude, it's likely going into closed loop and the values available for tune are not in the range it needs to optimize idle A/F. It's probably getting to much fuel and surging because it's pulling fuel until the next swing in the opposite direction and adding too much and repeating the cycle.

Increasing the throttle position will treat the symptoms but not solve the problem. You want your TPS and MAP sensor readings to be at the lowest signal output possible at idle before you start tuning so that you have the highest resolution for the full range.

I say it's too much fuel causing the problem because I experienced it before with an aftermarket computer that allowed me to adjust engine parameters by intercepting and editing output signals to the engine. My idle fueling was to high and at stop lights I had to stand on the brakes because the surge that you speak of was strong enough to make the car move forward if I didn't do so (IROC Z28 ).

I also have a similar situation in the 3.9 but it actually does the opposite by getting better as it warms up. Once you get your hands on an emulator for tuning you'll be able to work the problems out better.

[merlot566jka]

thats what i was afraid of. im going to put the heated o2 in there anyways, cuase it needs it.

i made a post for ya on .nl

[Superdave]

my 3500 does the same thing with the N* TB.. I adjusted the throttle stop screw to get the IAC counts about right but it still dies in open loop.

Some datalogs will show what's going on though...

[MidnightriderZ24]

if youre goin to be up at GLD the 12th im gonna take off work and head up there man!

[merlot566jka]

gld?

[MidnightriderZ24]

great lakes dragaway.

[merlot566jka]

negative, the car will be in indiana at gmtuners.com's shop getting tuned the whole time im at great mistakes