'86 GT 4spd


The brass fitting that goes over it is gone. Once the car warms up I get the hunting idle symdrome, goes to 1k then dips to 900 then back up. I've replaced my vac lines with Rodney's SS lines and replaced my questionable EGR tube with a braided steel one, added another ground wire from engine to chassis.

My question how can I put the set screw back in factory position or is this possible?

to set mine, I took the snorkel of the TB/Air cleaner, covered the IAC hole at the bottom of the inside of the TB with my thumb, and cranked the screw untill it idled at 500. but, make sure everything else is correct first, this is as important as the base timing.

quote Originally posted by Pyrthian: to set mine, I took the snorkel of the TB/Air cleaner, covered the IAC hole at the bottom of the inside of the TB with my thumb, and cranked the screw untill it idled at 500. but, make sure everything else is correct first, this is as important as the base timing.

Did you disconnect the IAC first? I'm guessing this was done on a warmed engine also?

Yes, the engine should be warmed up before doing this. You don't need to disconnect the IAC, putting a finger over the hole in the throttle body blocks off the air going through it. You do, however, need to make sure that the throttle body is clean, especially where the throttle plate seats when it's closed. There should be a very small gap around the throttle plate when it's closed, or you'll have a very hard time getting the idle set properly.

------------------
Rod Schneider, Woodstock, Ga.
"You can't have too many toys!"
1988 Fiero GT
1966 Porsche 911
Van's RV-6 airplane-under construction

I just turned mine all the way out then watched the butterfly and turned it back in till it started to move. You dont want it to move much but you dont want it to close all the way or it could bind shut. Then do the finger over the IAC hole. If it still idles high you have a vac leak.

I was thinking 900-1k is about right for at idle (warm engine). Am I wrong about this?

My thoughts are that, since the "idle set screw" has been tampered with, the ECM is fighting with this new position of the butterfly. So by putting the butterfly back as close as possible to being in the factory position, the ECM won't have to fight as hard to idle and thus smoothing out my idle. Am I close to right on this? I know providing no vac leaks, timing corrrect, fuel pressure good , oil pressure good, voltages good.

Or will adjusting the butterfly only lower or raise my rpms at idle?

The IAC is used by the ECM to meter extra air around the throttle plate in order to control idle speed. The IAC has a minimum and a maximum setting, any setting in between the minimum and maximum is considered normal. The ECM strives to keep the idle around 950 RPM plus or minus 50 RPM (on a standard, don't recall the automatic number off hand) and will open or close the IAC so that the sum of the air going past the throttle plate and the air going past the IAC pintle is enough to keep the engine at the desired RPM.

That's why using that screw to adjust idle is pointless, the idle won't change until the combined air flow exceeds the ability of the IAC to compensate, then the idle will be too high or too low since so many variables change idle speed during moment to moment driving.

My preferred method to adjust the stop screw if it's been tampered with is to look at the IAC reported steps from the ALDL data stream using WinALDL, with the preferred number being around 16-20 with the engine warmed up and cooling fan and A/C off.

JazzMan

Hunting between 1000rpms & 900rpms ain`t bad--I`d leave it alone, I had the 1500 to 500rpms loping like most have reported on a auto trans which made it even worse while in gear --at least with a clutch you can push it in & not tell that much . You can --But I would not recomend it , with your current rpms, is set your butter-fly by measuring from the bottom of the t.b. to the outer edge--it should be 2 & 1/8 th inches, there is a lip inside that the butter fly stops on, this is where you want to start the measurement, thats what I measured a factory one to be, then set mine that way--as mentioned above it won`t work , there is also a idle -iac set proceedure in the archives . After fooling with this for about a year--I finally bought a used t.b. with the cap still installed --and everything is fine..

quote Originally posted by JazzMan: My preferred method to adjust the stop screw if it's been tampered with is to look at the IAC reported steps from the ALDL data stream using WinALDL, with the preferred number being around 16-20 with the engine warmed up and cooling fan and A/C off. JazzMan

that does sound like a good way to go. I need to get WinALDL going, and see just how bad my 1/2 azz methods are.

I had to adjust the throttle stop on an 88GT that I bought on ebay. I backed the throttle stop screw until it was no longer touching the throttle plate. The car idled fine, but the throttle plate stuck in the bore. The cure for that was to advance the throttle adjustment screw until it JUST lifted the throttle plate from the bore. The car still idled correctly, albeit with the typical Fiero idle hunt.

Ok, had a little time to try an fix my idle problem. I found a thread about this that said to short the a-b terminal, turn key to run position leave there for about 30 secs ( to get the pintle to fully extend ), unplug the IAC connector, remove shorting pin, start car and adjust idle set screw to idle engine preferable at 500 but less than 900, turn car off reconnect connector, unplug ECM fuse so it will relearn, drive car to get up to 45 mph.

Well, when I went to start the car after removing the connector, the car wouldnt start, plugged connector back in car started.

Tweaked idle set screw, idle seems to be idling between 1k-1100. Now I'll let the car cool down and see what it is at cold start.

Can anyone tell me why when the car wouldn't start with the connector on the IAC unplugged and the car warm?

quote Originally posted by wanobi: Can anyone tell me why when the car wouldn't start with the connector on the IAC unplugged and the car warm?

'cause its not getting any air. give it a little gas, so the throttle will be slightly open

quote Originally posted by wanobi: Ok, had a little time to try an fix my idle problem. I found a thread about this that said to short the a-b terminal, turn key to run position leave there for about 30 secs ( to get the pintle to fully extend ), unplug the IAC connector, remove shorting pin, start car and adjust idle set screw to idle engine preferable at 500 but less than 900, turn car off reconnect connector, unplug ECM fuse so it will relearn, drive car to get up to 45 mph. Well, when I went to start the car after removing the connector, the car wouldnt start, plugged connector back in car started. Tweaked idle set screw, idle seems to be idling between 1k-1100. Now I'll let the car cool down and see what it is at cold start. Can anyone tell me why when the car wouldn't start with the connector on the IAC unplugged and the car warm?

Okay a few problems here.
1: you cant adjust the idle set screw just based on engine RPM, you need a scan tool, get the car running, then adjust the set screw until the IAC counts are within factory spec ( I think 30-40 )
2: The proper way to adjust (okay the most common and seemingly best) idle is to remove the IAC, spin the pintle until its completely seated (IE all the way open) then replace it.
3: I dont think pulling the ECM fuse will reset the ECM memory, you have to unhook the orange wire by the battery (1 wire with a connector in the middle) or unhook the battery its self. 30 seconds recomended, 10 should do it
4: to properly set the idle you need to start the car, but do not touch the gas. Then idle it for 2 minutes, turn it off for 1, then start it back up to idle for a bit, then drive it.

Heres the proceedure --how ever --as above mentioned I don`t think it will fix the problem, it will work the 1st time you use it, but after the engine cools, it will be back, hope not.

If everything is working properly, no exhaust leaks, no EGR leaks, and
the engine is properly tuned, you can go through the IAC system.

Idle is controled by the IAC valve (idle air control) via the computer and
is not adjustable.

You can go over the system if your not happy with where it is idleing.
Proper idle for the V6 is roughly 900 RPM.

The set screw in the throttle body is for minimum air flow through the
throttle plate which is often misunderstood as the idle screw. Any tweaking
of the throttle stop screw will give the ECM fits. The ECM wants to be in
control of the idle and is not happy when it can't control it. If the
normal
learn limits of the ECM are exceeded, they will be reset to nominal,
causing
an erratic idle.

The IAC and associated passages need to be clean to work right. Remove the
IAC carefully. You can clean it (the nipple) using carb cleaner and a small
brass brush or rag. At this time also clean the throttle palte. Once clean,
install the IAC back in the throttle body and reconnect the IAC wires.

For the ECM to properly control idle, the throttle stop screw must be set
for "minimum air". This is a process that sets the idle with the IAC fully
extended. To fully extend the IAC, jumper ALDL pins A and B together (just
like when you check codes) and turn the key on, but do not start the car.
With the key on, not running, and in diags mode, the ECM will keep
trying to
fully extend the IAC. After 30 seconds or so, pull the IAC connector
off the
IAC *before* doing anything else. This will capture the IAC fully extended.

Now pull out the jumper in the ALDL, and start the car. Typically the
"minimum air" idle speed is in the 500 RPM range. I find the car can bearly
run at 550. So as long as you can get it to idle on its own between 600 &
700 your good. Set the idle using the throttle stop screw. (The engine
should be fully warm to do this.) Now shut the engine off and reconnect the
IAC wires. The ECM does not know where the IAC present position is, so pull
the ECM fuse (or disconnect the battery) for 20-30 seconds. (This will
cause
a complete ECM reset of all learned parameters, including the learned IAC
ones. Then reinstall the ECM fuse.

Turn the key on, wait 10 seconds or so, and turn the key back off. This
will
now reset the IAC to a known key-off "park" position. Now start the
car. The
engine should idle properly under control of the ECM. There are some
learned
values, such as an IAC offset for A/C, etc that need to be learned, but
this
will happen under normal driving conditions. I suggest driving the car
right
away under all conditions. Stop & go, steady cruising over 45 mph, full
throttle, and so on. Pull over a few times and turn the car off, then
restart it. The IAC can only learn X amount of counts with each run
position. If everything else on the engine is in good condition and
operating properly it should be around 900 RPM after coming to a complete
stop with slight variations and improve over time.

Yeah I tried this procedure two days ago. couldn't get it to idle below 1100. So I assume there must be some vac leak

your didnot state how many miles are on the engine... if it has high miles...the timing chain is lose, and from there your engine has trouble finding its position for timing... (replace timing chain) MHO

For a "hunting" idle, Just replace the MAP sensor with a new one. The new ones from GM are a different part number than original and are able to read more accurately. Of course this is as long as there are no vacuum leaks and the throttle body is clean.

As for the idle screw, They should be factory set at 800 rpm if I recall. It is in the service manual. I'll try and find the exact page. I think the reason the seals get broken is because if the IAC ports get dirty and the idle drops off, some people break the seal and adjust it.

If you do clean out the throttle body and IAC ports, Remember the car will idle incorrectly until you drive it over 40 mph. This is when the ECM rests/adjusts the IAC.

There are only 51k miles on it.

I had the car serviced. the mechanic cleaned the tb, so I assume that I have a leak somewhere there. before this the idle was good and now after the service the idle is steady at 1200... *grr*

quote Originally posted by Oreif: For a "hunting" idle, Just replace the MAP sensor with a new one. The new ones from GM are a different part number than original and are able to read more accurately.

Have you got the new part number handy that we can use at the auto parts store or perhaps at GM online?

quote As for the idle screw, They should be factory set at 800 rpm if I recall. It is in the service manual. I'll try and find the exact page.

That would be some very important information to post for us. It would be appreciated if you could look up the accurate factory RPM number and the IAC step counts. The piece of mind knowing that we had our Fiero's set to exact factory specs would be priceless.

Thanks in advance!

Interestingly, I have noticed that my idle set screw moves. When I drive it around a bit, and it gets up to temperature and then I shut it off and come back the next day, it will sometimes not start or run without me giving it gas. I noticed that if I turn the screw clockwise about 1/2 to 3/4 of a turn, it will start and idle properly for me for a while. I wonder if I need to pull the screw out, coat it with threadlock compound and then put it back in?

low alternator output will also cause hunting idle.

//www.fiero.nl/forum/Forum2/HTML/077235.html Allmost there.....

The TPS is supposed to read .45V at facotry idle set.

quote Originally posted by pbd: Interestingly, I have noticed that my idle set screw moves. When I drive it around a bit, and it gets up to temperature and then I shut it off and come back the next day, it will sometimes not start or run without me giving it gas. I noticed that if I turn the screw clockwise about 1/2 to 3/4 of a turn, it will start and idle properly for me for a while. I wonder if I need to pull the screw out, coat it with threadlock compound and then put it back in?

You may want to put a temp patch on it until its idling the way you want it to for several days, then try some sort of sealant on the threads..(something that will come back out again if needed)

Or drop a small drip of lock-tite on it. Even a drop of finger nail polish would probably do. Plus you could get it in matching colors!

[This message has been edited by Dodgerunner (edited 10-13-2006).]