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Old 10-28-2010, 09:31 AM   #1 (permalink)
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Still has fluctuating idle

When I got the truck about 3 months ago, it had the notorious fluctuating idle problem once it warmed up and on the brake. It also idled way to fast when the brake was not depressed. So I attempted to adjust the idle speed with the screw on the throttle body. It helped and allowed it to idle down lower. I hooked up my multi meter to the tach relay in the fuse box and tried to adjust the idle down to spec(750rpm) I think. Even with the screw turned all the way in, the hot idle will get lower than 850 or so. At that time I called it good because it was better and caused the fluctuating idle problem go away.

About a week ago, it started to do the fluctuating idle thing again. Checked the idle speed adjustment and it at the same place it was when I made the initial adjustment. The idle only fluctuates when I step on the brake. Just idles high when not on the brake.

What am I missing? Is there something else I can test, adjust or clean to cause this problem to go away?
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Old 10-28-2010, 09:46 AM   #2 (permalink)
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Check to see if the TPS is in spec. you have a multimeter so you can do it
google, "93 FSM" and thatll take you to the factory service manual and itll tell you how to do it in there.
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Old 11-01-2010, 09:39 AM   #3 (permalink)
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Quote:
Originally Posted by Ajones1165 View Post
Check to see if the TPS is in spec. you have a multimeter so you can do it
google, "93 FSM" and thatll take you to the factory service manual and itll tell you how to do it in there.
So I performed the “on vehicle” test for the TPS. I only got one good read out of the 4 that are listed in the FSM. So I am thinking the TPS is shot.

So I am looking at replacements. Do I have to go with factory? I found some Beck Arnley , Airtex and Standard Motor Products on Rockauto for around $65. My local O’Reilys has a Brog Warner for $85. I have heard that on certain Air/Fuel censors like these, you want to go with OEM, but would rather stay away from the stealership if I can.
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Old 11-01-2010, 01:23 PM   #4 (permalink)
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The TPS may or may not be bad on your truck

But it is not the CAUSE of your idle woes.

The fluctuating idle is caused by your idle being too high which allows it to bump against the overrun fuel cut.

If the engine speed is above a set point (which is lower when the brake is on) then the injectors are cut off till the speed drops below that value. then the injectors come back on speed goes up injectors cut off......

you get the picture.


Anyhow you need to find the cause of the fast idle. A good place to look is the auxillary air valve. When idling hot there should bee no air going through it. get your engine hot and block off the passage to the auxillary air valve just in front of the throttle plate. If your Idle drops real low then you have found the culprit.

Other possible causes are the a/c idle up solenoid or the a/c idle valve. or a miss adjusted throttle plate. If the throttle plate needs to be adjusted make sure you adjust the TPS afterwards.

Hope this helps
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Old 11-01-2010, 04:38 PM   #5 (permalink)
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Quote:
Originally Posted by Vicoor View Post
The TPS may or may not be bad on your truck

But it is not the CAUSE of your idle woes.

The fluctuating idle is caused by your idle being too high which allows it to bump against the overrun fuel cut.

If the engine speed is above a set point (which is lower when the brake is on) then the injectors are cut off till the speed drops below that value. then the injectors come back on speed goes up injectors cut off......

you get the picture.


Anyhow you need to find the cause of the fast idle. A good place to look is the auxillary air valve. When idling hot there should bee no air going through it. get your engine hot and block off the passage to the auxillary air valve just in front of the throttle plate. If your Idle drops real low then you have found the culprit.

Other possible causes are the a/c idle up solenoid or the a/c idle valve. or a miss adjusted throttle plate. If the throttle plate needs to be adjusted make sure you adjust the TPS afterwards.

Hope this helps
Victor
Victor..thanks for the tip. i am going to check that out tonight.

So just to be clear. You are talking about the hole that is inside the throttle body, rite in front of the throttle plate. I believe it’s on the bottom? It is not the two tube looking things that are on the outside of the auxiliary air control valve.?
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Old 11-01-2010, 04:42 PM   #6 (permalink)
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YES! i wasnt even thinking about that. thanks Vicoor.

the tube looking things outside are what the coolant goes through, and when it gets hot enough, it closes the valve, not letting extra air get through, therefore dropping the idle.
its pretty much like a choke on a motorcycle.

but yes, the little hole inside the throttle body is what you want to plug. when you plug it, turn the truck off, pull the intake tube, plug it really well so it wont get sucked off, then put the intake tube back on, then start it, and like Vicoor said, if the idle is a lot lower, then your AAV is staying stuck open. i was able to clean mine out, but im not sure if youre supposed to.
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Last edited by Ajones1165; 11-01-2010 at 04:48 PM.
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Old 11-02-2010, 04:33 PM   #7 (permalink)
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A truck without A/c or Power steering with not have a A/C idle valve correct?
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Old 11-02-2010, 07:01 PM   #8 (permalink)
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Truck without AC will not have an AC idle up, just an empty connector for it to plug in to.
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Old 11-04-2010, 09:06 AM   #9 (permalink)
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Also check that the throttle cable isn't holding the throttle plate open too. there should be just a little bit of slack in the cable when the throttle is closed. also make sure the stops haven't been adjusted to hold the throttle open.
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Old 11-05-2010, 04:05 PM   #10 (permalink)
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Ok, so I tried to plug up the idle air control valve as suggested. I think I did a decent job of plugging up the hole with a rag and reattached the air intake. I made sure air could still get by to the throttle body.

It did drop the RPM, from around 950 to around to 750. Does this mean that it’s the Idle air control valve you think?

Ajones, you said you were able to clean your IACV. What process did you use to clean it out? Did you just take off the throttle body and then remove the IACV and clean that?
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Old 11-06-2010, 01:05 AM   #11 (permalink)
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Yes, i pulled the throttle body and then the valve.
Then i pulled it all apart. Be care full not to strip the screws, use an impact driver if you have one, if not, just have someone hold it really tight and use a long screw driver with both hands pushing down really hard.
To clean it when apart, i just used about two cans of brake clean and a lot of water flushing through it as well as blowing air through it with the air compressor. my theory was that the brake clean would loosen the stuff up, but idk if it was any more effective than using the water and air. just didnt want to get it back together and it not have done anything. The part that you pull out that has a spring in it(the cylinder looking part of it), if i remember right, in there is the actually valve itself. it works just like a thermostat, when the wax inside gets hot, it moves(closing the valve in this case). id just leave that part off to the side, and wouldnt chance getting any chemicals on it, i have no idea if it would hurt it or not, i just didnt mess with it when i did mine.
if you have any other questions, pm me for the fastest reply. idk if i left anything out.
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Old 11-06-2010, 06:04 PM   #12 (permalink)
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Yes, If the RPMs dropped when you plugged off the air passasge (when the engine is hot) then the Idle vlave is open when it should not be.

You can try cleaning it, But this type of valve is not as likely to be fixed by cleaning.

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Old 11-09-2010, 08:57 AM   #13 (permalink)
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Don’t mean to beat a dead horse but……….

I performed the test again for the IACV. Wanted to be 100% sure before tearing into it. Got the car good and hot and then plugged up the hole in the throttle body. So what I found out was that it did lower the hot idle from around 900-950 to around 680-700 per the multimeter. But if I rev the motor up for 15 seconds and then let it return to idle, it will only idle back down to where it would before, around 900-950rpm. Does this point to something else or should I go ahead and try and clean/replace the IACV?
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Old 11-09-2010, 11:51 AM   #14 (permalink)
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Is it possible that after revving the engine you lost your seal on the plugged passage?

Also, while the AAV is the most likely culprit, it could also be the A/C idle soenoid, or the A/C idlle valve.

try pinching off the vaccum hoses to these and really all of the vacuum hoses one at a time to see if anything causes a drop in rpm.

And make sure that when the idle is high, you can't get it to go lower by manually helping the throttle to close

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Old 11-10-2010, 01:22 PM   #15 (permalink)
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Quote:
Originally Posted by Vicoor View Post
Is it possible that after revving the engine you lost your seal on the plugged passage?

Also, while the AAV is the most likely culprit, it could also be the A/C idle soenoid, or the A/C idlle valve.

try pinching off the vaccum hoses to these and really all of the vacuum hoses one at a time to see if anything causes a drop in rpm.

And make sure that when the idle is high, you can't get it to go lower by manually helping the throttle to close

Victor
I thought of that as well, but it seemed to be in place even after I rev'd the motor.

The truck does not have A/C, so it would not have the A/C idle valve i think.

All I can think is that it is the idle control valve and plan to replace it this weekend. Hope it works
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