What Is Correct Idle Speed???? - TCCoA Forums
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post #1 of 38 (permalink) Old 12-26-2002, 05:57 PM Thread Starter
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What Is Correct Idle Speed????

What's up guys...Hope your enjoying the Holiday's .......I was wondering what the correct idle speed (when car is in PARK) is for my 94 3.8L N/A????? Can anyone help me out on this one???
Also...what is the correct Idle speed when the car is in DRIVE but standing still (at a traffic light or something)????


All help is greatly appreciated
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post #2 of 38 (permalink) Old 12-26-2002, 06:46 PM
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The idle speed is set in Park or Neutral. The speed is slower in drive due to transmition gearing. My cars' idle is 550. Yours is different because it is a california car. It will say on the emissions tag in the engine compartment. A rule of thumb; the throttle butterfly should be closed when idleing.

See if that works to solve idle problems, if not, post again I have a few other ideas.

Jim

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post #3 of 38 (permalink) Old 12-26-2002, 07:12 PM Thread Starter
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Jim

I checked my emissions tag....It says that Idle speed (along with a few other things) is controled by the Electronic Engine Control...and it is not adjustable????

But, today when i went back to the mechanic we adjusted that screw to lower the idle....and when i got back home...I realized it was too low...so i adjusted it to what It used to be before the HG job.......if it's not adjustable....then how come that screw is there?...and why were we able to adjust it?

Sorry for all the ??'s

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post #4 of 38 (permalink) Old 12-26-2002, 07:28 PM
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That screw doen't adjust the idle speed, but it does. I will try to explain.

The idle speed is computer controled by the IAC. Which allowes air to bypass the buterfly and allow air into the engine. This is how idle is controled and it isnt adjustable unless you add an aftermarket chip. The screw on the throttlebody will open up the buterfly and also allow air into the engine, thus speeding up or lowering engine speed.

I have no idea why the screw is there, I guess it may have somthing to do with using the car at different hights above sea level where the air is thinner and the car needs more air. Or maybe it is a consperacy

When you adjust this you want to do it after the car warms up, and if it is warm and you suddenly start it wait a minute, other wise the timming is advanced and the engine is running faster then the usual idle.

I hope that answered the question, if not post somthing else and I will try to reword.

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post #5 of 38 (permalink) Old 12-26-2002, 07:51 PM Thread Starter
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Jim

It makes sense...Thanks (though we still don't know why the screw is there).....

So this means we can adjust the idle with the screw....but if we adjust it wrong...then the comp will fix it right?

Also....I did adjust the idle when the car was warm and after the idle was down to normal (timing was normal).......is your idle computer controlled too?

The problem I'm having is that once in a while my idle will drop a little too low when I'm at a light in DRIVE.

Also twice when i first started the car.....It started up...but it felt like it barely was going to stay on...It was almost going to stall and shut off.

None of this ever happened before my HG job.....I mentioned all of this to the mechanic and he said that (he told me this while he was doing the HG too) that my idle motor was no good...It wasn't working properly)....Though It's funny how it was working fine until I drove it into his garage...then all of the sudden it's no good.

He said he found a hole in one of the hoses causing a vaccuum leak...so he cut the hose and reinstalled it...he showed me which one....hose looks fine??

What do you think about all of this...and the questions above?

Thanks alot for your help
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post #6 of 38 (permalink) Old 12-26-2002, 08:04 PM
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BeirutsFinest, whats up dude, i seriously think he messed up your IAC, i mean he had to take it off, in order to take off the upper inake. Try adjusting the screw while the car is on, until the car wont try to shut off, maybe that will work, or you'll need a new IAC.

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post #7 of 38 (permalink) Old 12-26-2002, 08:11 PM Thread Starter
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Marvin

I already did adjust it....It's not acting like it will shut off right now....once in a while....it will just drop when I'm at a light

Here's what i figure:

The computer has not relearned everything yet...I mean it has been disconnected from the battery for 5 days...and it has had some new parts put in....so it just needs some driving around to adjust itself.

I adjusted my idle is the way it used to be before the HG....which is about 700 Rpm's when in PARK.....it was idling at about 900 RPM's , sometimes 800 RPM's after i got it back from the mechanic

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post #8 of 38 (permalink) Old 12-26-2002, 08:13 PM
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If you adjust using that screw the computer wont fix it. Yo are adding more air to the system, think of it as an over ride.

Both my cars have the computer controlled timming. (pretty much all new cars do)

Set the correct idle at 550 and go for a drive, if it is wanting to stall at a light turn it up a little. If you do this and it still wants to stall, or gets to low, and the idle speed is to high you may have a bad IAC.

It doesn't suprise me that you have a few bugs to work out. A HG is a big job, and when things are touched they tend to brake. It just happons.

A vac line could be bad with one pin hole, or even if it is soft and colapsing under load.

Also wanted to say you got a killer deal on this HG job. Where did you have it done?

I have a few spare IACs laying around we could try some on one of my cars to see if they are good, then put them on your car and see what happons.

Another thought. Do you have a timming light?

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post #9 of 38 (permalink) Old 12-26-2002, 08:15 PM
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Quote:
BeirutsFinest, whats up dude, i seriously think he messed up your IAC, i mean he had to take it off, in order to take off the upper inake. Try adjusting the screw while the car is on, until the car wont try to shut off, maybe that will work, or you'll need a new IAC.
You do not have to take off the IAC to take off the upper intake, but we have simmilar ideas.


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post #10 of 38 (permalink) Old 12-26-2002, 08:18 PM Thread Starter
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Jim

I did this HG Job near my house (West Orange)...It was done by an older guy about 55...he works alone and he knew what he was doing and talking about.....Yeah he gave me a good dea, he checked and did everything he could to make sure I wouldn't end up going back to him because of a problem.

No timming light.....also 550 seems too low for me.....It used to be at around 700 before...and i set it back that way....I have to see what happens after some driving around and starting it up a few times.

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post #11 of 38 (permalink) Old 12-27-2002, 12:55 AM
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I have a CA Emissions car and checked the idle tonight. 700 in gear (cold,) 600 in gear (warm.) Idle in park was 750 when warm (A/C not running.)

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post #12 of 38 (permalink) Old 12-27-2002, 07:46 AM
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Josh does your car have a distributer?

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post #13 of 38 (permalink) Old 12-27-2002, 01:04 PM
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Yeah.

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post #14 of 38 (permalink) Old 12-27-2002, 01:59 PM
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you should check the timing with a timing light...pull out the plug on passenger side strut tower to check timing...too much timing may make it stumble at idle in drive...factory timing is 10 degrees...
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post #15 of 38 (permalink) Old 12-27-2002, 04:19 PM
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beruitsfinest...by the way

i pm'ed you...(about the radator)
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post #16 of 38 (permalink) Old 12-27-2002, 04:30 PM Thread Starter
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Josh

how did you find out your idle speed?? was it on the emissions tag, or did you simply start the car up and watch the idle??

Thanks
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post #17 of 38 (permalink) Old 12-27-2002, 04:32 PM
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eBay has some great deals.

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post #18 of 38 (permalink) Old 12-27-2002, 04:34 PM Thread Starter
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Talked to Dealer

I went to the dealer today and talked to a mechanic over there......He said he did something similar with his cougar....he told me to just pull the IAC plug and adjust the idle to about 500-600 rpm's and then shut the car off.....then plug the IAC plug back in.......he said the computer shoud adjust the idle anyway.

What do you guys think??

Also My owners manual says:

"if the battery is ever diconnected, replaced, or experience a dead battery....once the battery is hooked up again....you must allow the computer controlled engine to relearn it's idle...to do this you must shut off all accessories and turn the car on and let it idle for 1 minute (car must be warm for the computer to learn)...also you must allow about 10 miles of stop and go traffic for computer to completely relearn it's idle..............If this procedure is not followed the engines idle speed may be adversely affected....The computer will eventually learn the correct idle speed...but it will take much longer than the previous procedure mentioned above"

What do you guys think?

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post #19 of 38 (permalink) Old 12-27-2002, 04:52 PM
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You need to do what the mechanic said, but find a friend or a relative that has a precision tachometer. That'll make it a lot easier. Set the idle to like 550, then plug the IAC back in.

What's happening is the computer IS trying to adjust the idle, but if your base idle is messed up (it has to assume some kind of constant in order to adjust anything) then everything is going to be proportionally 'messed up.'

Good luck.

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post #20 of 38 (permalink) Old 12-27-2002, 06:46 PM
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it will idle without the iac plugged in???
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post #21 of 38 (permalink) Old 12-27-2002, 08:03 PM Thread Starter
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OK Josh

what your saying makes alot of sense now....This is what I have been thinking since the dealer told me what to do "that the computer needs some sort of base idle speed to go by" and no matter how much i play with it...I need to kind of reset everything by unplugging the IAC plug and adjust the idle then plug it in....so the computer will have a base to start off with...then it can adjust from there.

I'm going to try this tomorrow

....................my only real problem is when i start the car after sitting over night....and when starting it anytime during the day........what happens is when i start it up.....well you know how you have to wait a minute for the rpm's to drop down....well mine doesn't even go up high enough to have to wait...it's already down at 700 rpm's....basically there is no advance timing.

In the morning after sitting overnight...when i start it up.....during the start it sounds like it will start up normally....but once it starts up.....the idle is very low....almost like it's going to shut off......idle starts off at about 400 rpm's...then as the car warms up the idle gets higher and higher till it reaches normal idle of 700.....this should happen the other way around....the idle should start off high...then drop gradually as the car warms up.

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post #22 of 38 (permalink) Old 12-27-2002, 08:47 PM
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the iac controls the warm up when you start the car up: rpms reving to about 1500 then slowly dropping down...the coolant temp sensor also tells the computer how much the iac should let the car warm up...but if it has been sitting over night it should go through a full warm up mode...i think its the iac or coolant temp sensor...
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post #23 of 38 (permalink) Old 12-27-2002, 09:15 PM
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The IAC has NOTHING to do with the warm up of the engine. The coolant temperature sensor advances the timming of the engine until the coolant reaches the proper temperature, then backs off the timming to the normal idle. If the coolent is already at the desired temperature it will advance the timming for 30 seconds, then retard the timming to the proper timming.


If there is no advance at idle it is a bad IAC.

Jim

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post #24 of 38 (permalink) Old 12-27-2002, 09:26 PM
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well i guess if your right than your right...but i dont see a timing advance that could make the motor idle at about 1500 rpm like it does when you start it up cold...fooling around with my distributor one time...my motor would bog with timing anything above 15 degrees...idle wouldnt come to anything near 1500 rpm...let alone 1000...you need more air and fuel as timing advances...where would this air be coming from when you start the car...IAC maybe??? i'm no pro tech at all but i think it makes sense i think...
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post #25 of 38 (permalink) Old 12-27-2002, 09:56 PM Thread Starter
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Before

Before I figure a bad IAC...I'm going to try the method that the dealer told me....unplugging the IAC and adjusting the idle.......

I'll try this tomorrow and see what happens.....then I'll work my way from there.........cause The car didn't do this the day i got it back from mechanic...I drove it around all day that day and started it like 3 or 4 times.....everything was normal except for the high rpm's when in PARK....it wasn't until one whole snow day of letting the car sit...and then starting it the morning after that i experienced this problem

What is the cost of an IAC anyway???

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post #26 of 38 (permalink) Old 12-27-2002, 10:05 PM
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The car is tuned to run at 10 degrees (base timming). Once you fool with the timming the fuel system has to be altered to these changes. This is what a chip would do. Try retarding the timming and see what happons. The engine speed will slow down, and if you push it far enough, stop. The opposite (advancing) causes the engine to speed up based on where the engine fires as the piston is traveling in the cylinder. If the car fires while the piston is traveling up the engine will slow down because it will have to push aginst the force of the firing. If the plug fires at TDC or on the way down, the engine will speed up. The closer to TDC on the way down will produce maximum power. By moving the distributer you change when the plug fires during the cycle.

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post #27 of 38 (permalink) Old 12-27-2002, 10:05 PM
 
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JSGAuto, are you sure about the coolant temp sensor advancing the timing? I had thought that it helps the EEC control the A/F mixture to choke a cold engine by closing the throttlebody more than when warm. I'm not an expert on the IAC, but it would seem that the IAC would have something to do with controlling the TB opening.

The choke on a carbed engine does the same thing. If I'm wrong, then I'm as misguided as my ex thinks I am. Am I misguided? And if so, don't tell my ex. ;P
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post #28 of 38 (permalink) Old 12-27-2002, 10:11 PM
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The IAC doesn't control the butterfly in the TB. The IAC contols the idle of the bypass. At idle the butterfly should be closed all the way.

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post #29 of 38 (permalink) Old 12-27-2002, 10:19 PM
 
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Okay, cool, so the IAC controls the airflow bypass. So the IAC is controlled by the EEC to adjust A/F mixture? Where does the EEC obtain the info to control the airflow? I had thought that the coolant temp sensor had something to do with this. If I'm wrong, I'd really like to understand the actual dependencies and how the EEC controls operation.
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post #30 of 38 (permalink) Old 12-28-2002, 09:48 AM
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the iac doesnt control air/fuel mixture...iac is really just giving a steady airflow so the car can run at idle...maf is seeing that air to adjust for a/f...along with o2 sensor's, cts, tps, etc...sending signals to eec...

the tb butterfly should be closed at idle...there is a hole on the inlet side of the tb butterfly and the outlet side to the intake manifold...the iac is like a tunnel connecting those two holes with a valve to control the amount of air passing through it...if you open the tb butterfly a little at idle by tightening the screw, idle will just be higher...maf will see the airflow...its just like giving it a bit of gas...the iac should still function correctly...(?)

but wait a minute...

i could be wrong because i'm actually not sure if the iac is closed under throttle...if the tps sees that the tb butterfly is opened by YOU turning the throttle stop screw will it signal the eec to close the iac???
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