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Old 05-04-2009, 01:16 PM   #1
wolfehunter
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Default Throttle stop screw adjustment.

I messed with this screw back in December (I know...I shouldn't have) and want to be sure I get it back where it belongs. I received these instructions from Old Nuc but want to make sure I understand them correctly:

Take the air inlet tube off and let the car warm up till the fan cycles on and back off at least once. Just let it sit and run till it does this. this adjustment sets the mid stroke of the IAC valve. So now look into the throttle body inlet, se the hole in the bottom? Good, slowly slide your finger over it. You are going to completely close it off while tweaking the screw to maintain something like 500 RPM. The objective is to close off as much as yo can and still have 450 to 500 RPM. As you can guess this takes 2 people and a bit of time. Depending on the mechanical condition of the engine it may not run at 500 RPM, this is ok as long as the screw is close and you have the hole almost completely blocked. If you get the hole blocked and you are at say 700 RPM just back it out till you get close to 500. The book test is 700 to stall for just blocking the port but the design setpoint is 500. When you get it adjusted and if the engine is still running do not just pull your finger off. take it back slow so the IAC can do its thing. It should start trying to control as the idle comes up over 800 RPM or so.

I tried this over the weekend but the car will not stay running after I slowly close off the inlet hole. Which way should I turn the screw if I'm experiencing this? I noticed that if I quickly take my finger off the hole the rpm's immediately climb to just over 3000 rpm. After a few moments it will go back down to 800-900. Just an observation. someone mentioned that to get this adjustment close would be contact with the screw then open 1/4 turn. I'm assuming this is clockwise until the screw bottoms out then 1/4 turn counter clockwise. that is where I set it for now. I initially had to turn the screw in at least a full two turns before it made contact. Suppose that was bad!?
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Old 05-04-2009, 01:22 PM   #2
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Default Re: Throttle stop screw adjustment.

have you cleaned your idle air control valve and passage? If the IAC is working properly, you shouldn't be able to kill the engine with the throttle plate.
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Old 05-04-2009, 01:44 PM   #3
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Default Re: Throttle stop screw adjustment.

Unless someone messes with the throttle stop screw, it was set correctly from the factory.
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Old 05-04-2009, 01:57 PM   #4
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Default Re: Throttle stop screw adjustment.

Quote:
Originally Posted by wishihadacamaro View Post
Unless someone messes with the throttle stop screw, it was set correctly from the factory.
Correct, but he already said he messed with it, and knows he shouldn't have. No need to beat the dead horse.
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Old 05-04-2009, 02:02 PM   #5
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Default Re: Throttle stop screw adjustment.

Quote:
Originally Posted by wishihadacamaro View Post
Unless someone messes with the throttle stop screw, it was set correctly from the factory.
Quote:
Originally Posted by wolfehunter View Post
I messed with this screw back in December (I know...I shouldn't have)
OP:

The directions are sound. You need to turn the screw in more (clockwise) untill the throttle plate opens enough so that when you close over the IAC inlet with your finger, the car STAYS RUNNING but at 500 rpm.

The reason for SLOWLY removing your finger is that the IAC needs time to adjust, as it is NOT an instantaneous device.
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Old 05-04-2009, 02:11 PM   #6
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Default Re: Throttle stop screw adjustment.

I cleaned the throttle body according to the Richpin video so I'm assuming I cleaned the IACV and passage but maybe not? I can kill the engine by slowly sliding my thumb over the hole in the bottom of the throttle body and you are saying I should not be able to do that....right? What else might I be looking for or trying? I am only having the hesitation /stuble problem when the car is up to temp.
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Old 05-04-2009, 03:11 PM   #7
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Default Re: Throttle stop screw adjustment.

Why anyone thinks that the idle air control valve takes time to move is beyond me. If this were so, then when anyone turns on their a/c (the ones that work) and notices an immediate bump in the throttle and the same when turning off their a/c, doesn't realize that the rpm jumping is directly the result of the iacv making the small rpm adjustment and nothing else.

What you can do is preset the throttle stop screw to physically open the throttle plate a small amount, enough to see light pass through. This is simply a ballpark adjustment. As to adjusting the throttle stop screw; usually when I view something like this, the throttle stop screw is either opening or closing the throttle plate and I view the stop screw in relation to throttle plate movement. Opening the throttle plate would mean adjusting the stop screw clockwise (CW) or turning the screw inwards, towards the movable throttle pivot bracket. Closing the throttle plate would then mean adjusting the throttle stop screw counterclockwise (CCW) or turning the screw outwards, away from the throttle pivot bracket. Perhaps removing the throttle body and correlating the stop screw and throttle plate movement can make things clear. The whole idea is to never have the throttle plate completely closed off; if the throttle plate is closed completely and you cover the bypass hole for the iacv, the result is completely shutting off airflow through the throttle body. The throttle plate has always been adjusted to open enough to allow airflow through in case the iacv should fail in the extended position (closing off the bypass air port). The issue you have is attempting to bring this misadjusted idle speed back and blocking off the iacv bypass air port is simply to block it off and have the idle drop down to 500-600rpm. The engine must be be fully warmed up and running a smooth idle before making idle speed adjustments to the stop screw.

If for some reason the idle speed is way out from misadjusting the iacv, you'll have to make another guesstimate on the iacv adjustment or refer to specs on initial adjustment of the iacv from either the factory or OEM sheet that may accompany replacements. Someone should have the recommended setting for how far the valve is extended from its housing. Both iacv and throttle stop screw are intertwined to have the EFI system control idle speed. Covering the small bypass hole does not require slowly covering and slowly uncovering it, simply stick your finger over the hole. The rpm should drop immediately as you're only blocking off one hole while the throttle plate is supposed to open just enough to allow the slower idle speed, 500-600rpm. Unblocking the hole upsets the EFI system - the iacv already retracted into its housing so when the hole is uncovered a sudden rush of more air immediately raises the rpm but the speed up is detected and the PCM immediately extends the iacv to lower the rpm but now to 800-900rpm. The rpm rise shouldn't race and stay high at all as this indicates a sticking/sluggish iacv and not because its slow acting. If the iacv raises the rpm very high beyond 2500rpm and slowly brings this rpm down, the iacv is faulty. Clean it or replace it and start the idle speed adjustment again with the replacement iacv. Others have done this successfully and have seen the rpm drop, jump up and return immediately back to a normal idle when covering and uncovering the bypass air port.
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Old 05-04-2009, 03:17 PM   #8
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Default Re: Throttle stop screw adjustment.

Stop screw testing/setting.

Slide finger over IAC port until RPM drops noticeably (greater than 100RPM), remove your finger quickly. The engine should rev up to greater than the RPM before you slid your finger over the IAC port and settle back down. The IAC is fast but not instantaneous.. If it does not you have an IAC problem, fix this first.

Assuming that you have successfully demonstrated that the IAC is functional then proceed to adjusting the stop screw. Slowly slide your finger over the port and adjust the screw to hold 450 to 500 RPM. You need help to do this. You want about 500 RPM when the port is completely covered. Slowly remove your finger and observe the car idles at the nominal 800 RPM idle. You have to do this when the engine is at normal operating temperature. The temp gauge minimum reading is 3/8 for 96 and newer or 1/2 for 95 and older cars. Don't make this adjustment unless the car is at normal temperature and the IAC functions.
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Old 05-04-2009, 03:52 PM   #9
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Default Re: Throttle stop screw adjustment.

OK, so I don't have to totally cover the hole with my finger? Just enough to drop the rpm's a few 100 and then pull off. Got it.

Saturday when messing with this the rpm's would climb to 3000 after pulling finger off of the hole. It would hold at 3000 rpm's for 6 to 8 seconds before slowly dropping back down to 800 rpm. Is the IAC functioning properly?


Next, the screw adjustment. It was mentioned that "contact and a 1/4 turn will be close". Is that meaning contact with the throttle plate? And then the 1/4 turn clockwise to bump it open just a little? I think I'm starting to understand now. We're a little slow here sometimes in Pa!! Thanks for the help.

I'm sure this isn't the cause of my miss/hesitation under load but want to get the screw where it needs to be. Should have left my grubby fingers off of it!
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Old 05-04-2009, 04:40 PM   #10
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Default Re: Throttle stop screw adjustment.

Quote:
Originally Posted by wolfehunter View Post
OK, so I don't have to totally cover the hole with my finger? Just enough to drop the rpm's a few 100 and then pull off. Got it.
Yes, thats the idea. make the PCM crank the IAC open and then snatch yiur finger out and catch it napping.
Quote:
Originally Posted by wolfehunter View Post
Saturday when messing with this the rpm's would climb to 3000 after pulling finger off of the hole. It would hold at 3000 rpm's for 6 to 8 seconds before slowly dropping back down to 800 rpm. Is the IAC functioning properly?
That sounds like it. The port will restrict flow more than the IAC so when you totally block the port the IAC winds all the way back, much farther than the port will pass so you get a delay before the RPM starts back down.
Quote:
Originally Posted by wolfehunter View Post
Next, the screw adjustment. It was mentioned that "contact and a 1/4 turn will be close". Is that meaning contact with the throttle plate? And then the 1/4 turn clockwise to bump it open just a little? I think I'm starting to understand now. We're a little slow here sometimes in Pa!! Thanks for the help.
That will work fine for a place to start. Drive the screw in until it just contacts the throttle stop and then 1/4 of a turn or so. The PCM positions the IAC 80 to 100 steps open from full closed everytime you turn the car off. Once started it will run to the RPM command position and eventually end at the hot idle point. If the throttle stop is not set right you end up with a low or high RPM start and that will cause problems. Also the start point of the TPS (throttle position sensor) is set by this screw.
Quote:
Originally Posted by wolfehunter View Post
I'm sure this isn't the cause of my miss/hesitation under load but want to get the screw where it needs to be. Should have left my grubby fingers off of it!
Miss and/or hesitation can be caused by many things, but not the stop screw. As to the miss/hesitation look at plugs, wires, firing order, MAP port plugged, EGR leaking, or mechanical problems.
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Old 05-04-2009, 04:54 PM   #11
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Default Re: Throttle stop screw adjustment.

Is the contact with the throttle plate something I can feel (screw bumps against) or is it something I should be looking for with my eyes?

New plugs and wires, cleaned the EGR but used the old gasket so I'm going to replace it, plan to do the nickel test on the EGR when I replace the gasket. Checked my firing order but need to do a search on MAP port being plugged. I didn't check that yet.

Thanks again.
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Old 05-04-2009, 06:30 PM   #12
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Default Re: Throttle stop screw adjustment.

Unscrew the MAP from the manifold and lift it straight up. Capture the rubber coupler and then clean out the port under the MAP into the manifold. Then look at the little hole in the bottom of the MAP.

You both feel and observe the contact of the screw with the stop. If you place a finger tip against the stop and drive in the screw you will feel the motion.

Probably redundant but what brand of plugs?
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Old 05-05-2009, 10:03 AM   #13
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Default Re: Throttle stop screw adjustment.

NGK's. I will do some more searching on the best way to clean the Manifold Absolute Pressure Sensor. Found a few posts that say it is easy to break and to be careful. Didn't find a picture yet of what it looks like so not sure what I'm looking for. Can I use brake cleaner or another cleaner on it?
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Old 05-05-2009, 10:25 AM   #14
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Default Re: Throttle stop screw adjustment.

As you are standing in front of the car there is a rectangular black box screwed to the cars right end. It has an electrical connector on the end also. Unplug and unscrew, lift straight up. Do not loose the small rubber seal, it may stay on the nipple on the MAP or it may stay in the manifold. Clean out the port in the manifold with a round brush or a q-tip, its not a long passage. Look at the small hole in the bottom of the MAP and see if its clean and not plugged. The MAP will not survive and spraying of cleaners into it.

You can squirt carb or brake cleaner into the manifold port but try and keep it to a minimum as you will be burning it later.
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Old 05-05-2009, 10:50 AM   #15
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Default Re: Throttle stop screw adjustment.

Thank you for all your help.
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Old 05-05-2009, 11:01 AM   #16
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Default Re: Throttle stop screw adjustment.

Post back how this turns out.
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