I have a 96 GA with 226k, with the 3.1L it seems after the car warms up it has an erratic idle when in park, also while in drive as long as I'm not giving it gas sometimes its so erratic the car stalls....this does not happen everytime sometimes I can leave it running all day long but it does do it enough to where the car is undrivable.

What are the rpm ranges when this happens?

i would say between 200-1200rpms

it could be your iac (idle air control) or your tps (throttle postion control) and have you checked for a vaccume leak?

lift that cover under your shifter leader boot you will find this part (sorry don't have a better picture) the only picture of this part I could find was here

http://cgi.ebay.com/ebaymotors/CHEVROLET-GRAND-AM-2-3L-SHIFTER-MODULE-22582545-B135_W0QQitemZ280113914819QQcmdZViewItem

so disconnect this and see if it will start with this disconnected and if the same symptoms will continue? There is a microprocessor inside that gets shorted and causes similar issues. I know that it is not the same part. You have prnd123, but you got the idea.

It already has a brand new iac and a tps. But the shifter module.....never thought of that is there anyway to test it??? if not im sure i can get one for cheap from a junk yard...Thank you so much....will let you all know if this works

other then the IAC causing erratic idle;
- lower intake bolts loose or cracked vacum hoses
- leaking or clogged EGR or PCV valve(s)
- clogged dirty nijectors
- or faulty ignition system
- worse case, worn timing chain, low compression from leaking/bad valves, worn out cam


:2cents:

for now just see what happens if you disconnect prndl's connector

it acted the same way when i disconnected the PRND21 connector....on a different note i was driving through town today and it died at like 35mph. i restarted it and it would idle great but died every time i gave it gas. i had to hold it to the floor to keep it moving and it would rev up backfire and die..then barely save it self...and then do it all over again...after i limped it home it has not started since.

check your transaxle range switch on top of your trans (below your power booster you will see black swithc and and shifter cable coming to it.)


this are wires coming to your trans swithc check them with your dvom if you have one when you are in P blk/wht wire should have continuity with ground, yellow is open, grey is open, white should have continuity.
Make sure switch is not loose, also check connectors make sure they are not rusted, also open the hood and tell someone to shift from D to P while you are watching the movement of the shifter cable and switch arm. If you see that switch arm is moving in one direction and then stops, then you try to move that arm in the same direction bit more just to see if the shifter cable is loose and not pushing arm of the range swithc far enough to make a good contact inside of the swithc. At that point if idle improves then you will have to adjust the switch. Again Prn123 switch communicates with pcm and if pcm does not know in what gear is the transmission IAC, EVAP, EGR sensors will be confused.

still when you get chance to visit junk yard get that prnd123 module they will not charge you more than 3-5$ sometimes they will let you have it for free. Did you try reconnecting prnd123 and starting again?

Yes, the PRND21 was hooked back up when the car messed up this last time...My question is would the shift module cause the car to act like this and not restart? shouldn't the car at least start even if it doesn't know what gear its in?? I'm wondering maybe if the Crankshaft positioning sensor went bad(although it only has about 30k on a new sensor). I'm also wondering why it went from irratic idle and dieing to backfiring and dieing under acceleration....the only thing it would do right today was idle...it would start right back up everytime and idle while trying to get it home....then when it cooled down and i went out to try to get it into the driveway a little better is when it wouldnt start at all....i have also checked a bunch of wiring because the harnesses are starting to get a little brittle with 227k miles of engine heat...

- check main CKP sensor signal set your DVOM to read mili Volts in AC disconnect the two wire connector on the right side of the icm (you facing icm your right side) yellow and purple (twisted wire) is for your main CKP sensor connect your dvom leads to that connector and tell somebody to crank the engine for you, you should be getting around 200mV AC
- check your MAF sensor (clean if necessary)

the prnd123 connector has 3 wires: pink ckt1020 that is 12+, black ckt 150 that is ground and tan ckt 800 that is for UART

tan wire is transmitting important coded data to pcm so if that module is bad pcm will get wrong data and will confuse EVAP, IAC and EGR sensors
so in your case it is obvious that something is confusing your IAC when your transmission is in P

tan wire splices in two places s277 and s278 and connects to c207 connector, then continues and connects to connector 1 pin 14 or 15 one of them should be tan wire. I will find this splice and connector locations so you can check them also.

I checked the 3x CKP sensor (yellow and purple) and it tested bad...after i replaced it the car will start right up and idle all day....but when i accelerate the rpm gauge goes berserk and does match engine rpm also the car will not accelerate and will only do about 35 to the floor popping and carrying on.....also it doesnt want to shift right which i am sure has to do with whatever is causing the possessed rpm gauge

how is your vacuum modulator, remove the rubber hose from the modulator and see if any ATF oil is coming out,
- if your vacuum line to the modulator is bad or disconnected you can have similar problems
-do you have check engine light on?
- test your fuel pressure, It could be low. For now to test, you can unplug fuel pressure regulator vacuum line this will increase your fuel pressure test drive it with FPR line removed.
- check your trans oil level

-The vacuum modulator seems fine, all vacuum lines going to it seem -fine....engine light bulb is blown(ironic isnt it) so i don't know if check engine light is on
-trans fluid level is good


What would cause the rpm gauge to go berserk??? it definitely has never acted like this before and is not showing what the true engine rpm's are

rpm is icm/pcm controlled, what exactly is rpm gauge showing at what speed?
do you feel up shifts or not? You need to scan it for codes - go to autozone write your codes and we will see what we can do.

what happens when you remove that vacuum line form the top of the fuel pressure regulator? Do you have vacuum pump? if you do connect it to vacuum modulator and apply 15 inHG of vacuum it should hold vacuum.

also test your TPS sensor, when sensor is connected engine runing, you have to probe at the back of the signal wire, it should read about .59 V DC at idle and should smoothly increase as you increase throttle if you see any gaps in voltage replace tps.
wait 5 min I will tell you what color is your tps signal wire.

I dont have a vacuum pump.....

The gauge is accurate when im at idle but if i accelerate it will jump lets say if i have it floor the car only revs to maybe at the most 4k and pops and bogs, and cant get over that point but the rpm gauge will be sweeping from 2-6k or steady out at some random rpm.....but the engine remains generally in the same place

it has a new TPS sensor in it.....maybe at the most 30k on the new one....I also can no longer wrench and clunk around outside as im worried about making to much noise at 2 in the morning......gotta love living in town

ok it is pin "C" DK Blue wire, don't have diagram for 96 but 98 should be the same.. Get some sleep now and in the morning go to autozone ask them to scan your pcm

in PARK you cant rev it more than 4000 rpm it is engine protection feature.

we will see tomorrow maybe ICM?

switched ICM today. still no difference...I want to take it over to autozone and get the codes read but I don't want to drive it clear across town to get to it...I don't know if it would make it...it still does the same thing Rpm gauge bounces all over(not the engine rpm just gauge)...and it still pops through intake, and has no power, also whatever is affecting the engine and rpm gauge is also making the car shift funny.

If its backfiring in the intake, the timing is off. :2cents:

did it run better before the crank sensor was replaced ?
Might have to do a CKP relearn... :dunno:

how do i do a CKP sensor relearn??

you need a good scan tool for that,

- you must not have any codes
-engine temp. must be at the normal engine operating temp.
-go to ckp relearn procedure
-scan tool will tell you what to do
(most likely it will tell you this) - start engine, in Park press brake pedal with left foot, press gas pedal up to 4000 rpm (fast at once) if everything was done correctly it would show you that the relearn procedure was successful

Well you need to know if its throwing a DTC for the CKP.
And you need a good scan tool that can performe the CKP relearn, if it even does need done. Doesnt always happen to throw the trouble code or cause problems.

But your popping in the intake sure sounds like its outta timing.

So here is something interesting i switched the ICM back to the one I had on it before because oviously there was nothing wrong with and now the car won't start...so i think i might have a bad wire in one of the connections to the ICM...I wiggled wires and tightened the prongs up within the plugs but so far it still won't start.... I have to go to work now so i guess this weekend i will be diggin out the volt meter and check wiring :crying:

When you engine is warm and you turn off the engine and key is out, does the coolant temp gauge go down to cold?

I have tonight off from work...I am going to try to get it running again and i will see what the temp gauge does...

to help you start it :) ICM has 3 connectors. 2 on the drivers side of the ICM and one on the passenger side of ICM. The one on the passenger side of the ICM I almost all of the time forget to plug in since it is hidden by all kinds of hoses and wires. So check if all 3 connectors at the ICM are connected.

when you switch the ICP back and forth its not going to start with out doing a relearn for the ICP.
Same as the others, key "ON" for 11mins (or more)
Off for at least 30secs, then repeat. The security light should come on and flash off after the 2-3 time.

This thread is gotten out of hand for just a simple irratic idle problem. :uhoh:

If you seen my previous post, #7 (http://www.automotiveforums.com/vbulletin/showpost.php?p=5938288&postcount=7) you'd have this narrowed down.