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Operation: Smooth Idle

98EdBomber

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THe idle screw shouldn't be touched on EEC-V cars(96+). The service manuals are adamant about that. Even newer EEC-IV says to leave that screw alone. The IAC is a closed feedback system that adjusts airflow and idle speed. If you are idling too fast something else is amiss in the system.

I've seen that screw set procedure as stated for older CFI cars without IACs.


Ok. I had a looksy and the stop screw is not pushing on the plate. Anyhow, I tested the TPS and with key on engine off the voltage is about 1v and smoothly increases to about 5v at WOT. Within spec so i think my tps is good.

Besides that my CEL came on today!


IAC VALVE shorting...LOL

Out of curiosity I tested the resistance and from pin to pin spec is like 6-13. I got a big fat 0. From pin to IAC case i also got a big fat 0 and the spec on this is 10K ohms or greater. Must be fried or else I did not set my multimeter correctly. I believe if you want to test for ohms in the range of 6-13 you would set it at 200? Meaning there should not be a decimal if the value does not exceed 200. But to get infinite from pin to pin and pin to case and still have an idle is interesting.

Harness voltage coming from pcm is present and within spec. 11.3v or something. New one should be here in a few days. Hopefully this will do the trick.

I think the only part that baffles me is that by presuming the IAC is not functioning at all, the engine still idles. But when I disconnect the connector the engine stalls.

Can the IAC have a short but still be partially working?
 


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96eb96

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Ok. I had a looksy and the stop screw is not pushing on the plate. Anyhow, I tested the TPS and with key on engine off the voltage is about 1v and smoothly increases to about 5v at WOT. Within spec so i think my tps is good.

Besides that my CEL came on today! Ran a scan and low and behold...drum roll please.........................

IAC VALVE shorting...LOL

Out of curiosity I tested the resistance and from pin to pin spec is like 6-13. I got a big fat 0. From pin to IAC case i also got a big fat 0 and the spec on this is 10K ohms or greater. Must be fried or else I did not set my multimeter correctly. I believe if you want to test for ohms in the range of 6-13 you would set it at 200? Meaning there should not be a decimal if the value does not exceed 200. But to get infinite from pin to pin and pin to case and still have an idle is interesting.

Harness voltage coming from pcm is present and within spec. 11.3v or something. New one should be here in a few days. Hopefully this will do the trick.

I think the only part that baffles me is that by presuming the IAC is not functioning at all, the engine still idles. But when I disconnect the connector the engine stalls.

Can the IAC have a short but still be partially working?

You set the meter to the lowest range that matches what you want to measure. 200 ohms sounds correct. Measure a lamp if you want to check the meter.

If you get a 0 reading for the IAC its shorted. RIP IAC.

Did you ever test the IAC with 12 volts straight? That will kill it.
 




98EdBomber

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You set the meter to the lowest range that matches what you want to measure. 200 ohms sounds correct. Measure a lamp if you want to check the meter.

If you get a 0 reading for the IAC its shorted. RIP IAC.

Did you ever test the IAC with 12 volts straight? That will kill it.

Dont know what you mean? I tested the harness for voltage coming from the pcm with the key on. Got 11.3v. I tested for resistance with the connector removed by putting the leads on the IAC pins.

Did I have the meter on 'volts' when i tested for resistance? NO
 




2000StreetRod

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Idle adjustment procedure purpose

. . . I did, however, adjust the idle screw in case the IAC craps out. I don't believe this should have any affect on the idle because it had the vibration before the adjustment. I just have the peace of mind knowing if the IAC dies, the engine won't stall. Thanks to the info. on this site for the tutorial on adjusting idle screw.

Idle speed is not adjustable to my knowledge.

98EdBomber has correctly grasped the purpose of my idle adjustment procedure. The IAC valve is a spring loaded solenoid mechanically linked to an air valve. I suspect the PCM utilizes pulse width modulation (variable on/off duty cycle) to displace the valve actuator from the default spring loaded position. The electro-mechanical device is prone to displacement range reduction due to friction. This can often be corrected by cleaning. The device is also prone to failure due to opening or shorting of the solenoid windings or a stuck valve actuator or valve. Adjusting the idle stop screw to approximately 200 rpm less than the PCM commanded idle speed for a warmed up, properly operating engine opens the throttle plate enough to keep the engine from stalling in the event the IAC valve fails.

waskly is correct that the PCM establishes a TPS idle voltage at every ignition key on cycle and periodically. That is why opening the throttle plate slightly with the stop screw (which increases the TPS voltage) has no detrimental effect. Since both the IAC valve airflow and throttle plate airflow is metered air (measured by the MAF sensor) there is no impact on the air/fuel ratio. No lean codes will be triggered.

The important thing to note is that the throttle plate stop screw should never be adjusted for an idle speed greater than the PCM commanded idle speed.

I hope that your new IAC valve results in you achieving a smooth idle!
 




96eb96

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Dont know what you mean? I tested the harness for voltage coming from the pcm with the key on. Got 11.3v. I tested for resistance with the connector removed by putting the leads on the IAC pins.

Did I have the meter on 'volts' when i tested for resistance? NO

I mean directly connecting the battery to the IAC to see if it works (at any time in the past). Like 2000streetrod said, the IAC runs off a pulse cycle, some time people will just apply 12 Volts to test it. That will cause it to short out.
 




98EdBomber

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I mean directly connecting the battery to the IAC to see if it works (at any time in the past). Like 2000streetrod said, the IAC runs off a pulse cycle, some time people will just apply 12 Volts to test it. That will cause it to short out.

I did nothing of the sort. I checked for primary voltage received from the pcm and also the internal resistance. I'm still curious as to how I may have shorted it out. All I did was unplug the connector a few times to adjust the stop screw and engine stall. I unplugged it maybe three times and after the third time the CEL came on. I've done the procedure before without any ill effects so I guess it could just be coincidence. Odd nonetheless.

At any rate, at least I can check one more item off my list to achieving a smooth idle.
 




96eb96

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I did nothing of the sort. I checked for primary voltage received from the pcm and also the internal resistance. I'm still curious as to how I may have shorted it out. All I did was unplug the connector a few times to adjust the stop screw and engine stall. I unplugged it maybe three times and after the third time the CEL came on. I've done the procedure before without any ill effects so I guess it could just be coincidence. Odd nonetheless.

At any rate, at least I can check one more item off my list to achieving a smooth idle.

Thats fine, hopefully the new IAC will help your issue. A long time ago I tested an IAC that way because of an idle issue, burnt it out, and the new one made it idle like new. I know there is a diode in there too, at least in the old one I took apart.

I've cleaned IACs in the past only to have them fail again in a year. A new IAC after 13yrs is a good idea anyway.
 




96eb96

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Actually I just checked motor mounts yesterday. It was just a visual inspection. No cracking, etc. Even took a wrench to them and they don't seem loose. Is there a proper way to diagnose a bad one? If it were bad I am wondering how this would cause the idle to become intermittently rough? Wouldn't it cause a continuous shake? Thanks for the advice.

Put in in park, set the parking brake..stand on the side of the truck and have someone goose the throttle with their foot on the brake. Then watch for any engine movement. Sometimes things warm/up cool down, reach a critical RPM, etc and cause vibration. Again, I don't know if its an issue with these engines but something to think about.
 




98EdBomber

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Thats fine, hopefully the new IAC will help your issue. A long time ago I tested an IAC that way because of an idle issue, burnt it out, and the new one made it idle like new. I know there is a diode in there too, at least in the old one I took apart.

I've cleaned IACs in the past only to have them fail again in a year. A new IAC after 13yrs is a good idea anyway.

Yes. At 151,000 miles. It is absolutely due for a major tune up. I just replaced the entire front end suspension and it rides as good as new!

All things aside, best truck I ever owned.
 




98EdBomber

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Put in in park, set the parking brake..stand on the side of the truck and have someone goose the throttle with their foot on the brake. Then watch for any engine movement. Sometimes things warm/up cool down, reach a critical RPM, etc and cause vibration. Again, I don't know if its an issue with these engines but something to think about.

Thanks
 




98EdBomber

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IAC Valve replaced

IAC Replaced.

Idle is better. Engine still vibrates a little so I don't think my problem is solved yet.

There is also an engine humming going on. Dont know if that is causing the roughness but it might be. You can hear it inside the cabin mostly as its tought to determine the exact area outside with hood open. Definitely not the IAC.

Next step is to take the belt off and run the engine and see if the hum goes away.

1. Idler pulley?

2. Belt tensioner?

Could these two cause engine humming? And I listened carefully to these two and it really doesnt seem to be coming from the fron. More so from the firewall side of the engine up and around the intake. Its not a hissing vacuum leak but almost like a guitar string plucked that resonates. Hard to describe. It also is steady and doesnt seem to fluctuate with speed. Mostly heard at idle though. This is definitely not something that was there a while ago. Like I said ever since i got back from my Cali trip i started to notice it.

If not those two, does anyone have any other clues what may cause engine hum and vibration???

Thanks.
 








waskly

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if you think its a pulley, take the drive belt off, and start the engine, if the noise is still present then its in the engine, else its an accessories.

just a quick note, the alternator, water pump and ps will not work so dont do this for extended period of time.
 




silentchris

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the hum you here inside combined with an idle surge could be the fuel pump making noise sometimes as they begin to wear out they do that usually the hum will match the surge hum rough no hum smooth
 




98EdBomber

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Thanks. Ill check fuel pressure and update.

I took it to a local shop around the corner and the guy popped the hood and said he didnt notice anything wrong.

Can exhaust parts (Cats, etc) cause anything to resonate throughout the cabin and cause that humming?
 




98EdBomber

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if you think its a pulley, take the drive belt off, and start the engine, if the noise is still present then its in the engine, else its an accessories.

just a quick note, the alternator, water pump and ps will not work so dont do this for extended period of time.

Yep. I am aware of this. Ill check it tonight.

Thanks guys.
 




98EdBomber

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Update

A big thanks to everyones help and advice as I seem to have achieved an idle I am satisfied with.

My fuel pressure was in spec.

I removed the belt and checked the pulleys and all checks out. Nothing seems to be worn out.

I noticed the belt tensioner was rattling and somewhat moving more than i thought it should so i replaced it. Seems to have improved the idle.

I believe to have also solved the humming noise by replacing the belt tensioner.

Thanks again to this wonderful site!

Peace.
 




Paraphoe

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I believe to have also solved the humming noise by replacing the belt tensioner.

Thanks again to this wonderful site!

Peace.

The vibration you cold feel through the cab? Tensioner seemed to solve this?
 




98EdBomber

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The vibration you cold feel through the cab? Tensioner seemed to solve this?

Yes. I did notice a reduction in vibration although its not 100% completely gone. Try it out. Cheap and easy replacement. 10 min. job.

Mine was rattling which you could hear from the pass side wheel well. When I removed it the pulley, the bearing was obviously worn out as I could simply hear it grinding. And visually while engine was running it seemed to be moving or "vibrating" a bit too much. The newer one seems to not move around as much. I do believe its purpose is to not only keep belt tension but to reduce vibration as well.

Good luck.
 


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98EdBomber

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Ok its been a few days after the last parts were replaced and I am STILL getting an abnormal vibration while it is warming up.

I checked the resistance and voltage of the EGR regulator today and its within spec.

I checked the DPFE connector and back probed while engine warming. Its within spec.

Im at a complete loss at what could be causing my engine to vibrate when warming up.

Its seems to have smoothed out while at stop lights and now the major issue is while warming up.

And its not the intake gasket shake or vibration problem. That has been fixed.

Im going crazy !!!!! ANYONE????

Thanks
 




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