Low, Low idle: got an idea?

[Creager]

Heres the deal,

In the past i had issues with my engine's RPM surging downward towards 500-600 rpm, sometimes equating into a stall. Replacing the IAC with a new motorcraft unit seemed to fix the surging part, but the RPM at idle still has a tendacey to get low once the truck has warmed up.

Following iginition, rpms raise as high as 1400 (This engine has lots of mods, i figured this was somewhat normal, although it never went this high with the old IAC (1200 most). But the old one caused the RPM to surge like crazy, it just may have never given the engine a chance to idle in open loop at 1400rpm)

After approx. 15-30 seconds, RPM slowly decreases to 1200, then 1000, then finally 750 where it stays until warm. Once its warm, it wants to idle (smoothly) at 600. Which is a little low for me...

MAF is clean, ive tried swapping between a PRO-M maf and the stock ford unit, both dont seem to make a difference.

Fuelpressure (at the rail) rises to 35-40psi when turning the key before ignition, drops to 28psi at WOT. Idles around 30 psi

Compression is an even 195-205lbs between all 6 cylinders

No obvious vaccume leaks, PCV system may be a little suspect. But i've never had a vaccume leak that caused RPM to drop?

The comp cam definatly idles a little rough, but it still shouldnt drop low past 750rpm. Using the a little throttle levels out the RPM.

ideas? New IAC junk?

 

[Turdle]

You might need to drill a small hole in your throttle plate--cams usually require this because of the low idle vacuum

I'd start with a 1/16" hole--work up from there until you achieve the correct idle, with the tps adjusted to between .86v, and .96v

I ended up with a 5/32" hole in mine--

 

[happyjack87]

when you say to drill a hole in the plate, do you mean in the center of the plate or are you talking about making the holes for the screws to go through bigger?

 

[Turdle]

when you say to drill a hole in the plate, do you mean in the center of the plate or are you talking about making the holes for the screws to go through bigger?

A new hole in the plate for air flow--

 

[manaen]

Do you have a reader to see what the computer thinks it is idling at?

did you replace the throttle body? maybe the stop screw was set improperly

do you have a programmer? you could always reprogram the idle.

 

[manaen]

If I remember correctly the procedure for setting the stop screw is to disconnect the IAC and set the idle to 500 RPM's using the TB stop screw and then reconnect the IAC and the computer should bring idle up to 750.

if you unplug your IAC does the engine stall?

 

[Turdle]

If I remember correctly the procedure for setting the stop screw is to disconnect the IAC and set the idle to 500 RPM's using the TB stop screw and then reconnect the IAC and the computer should bring idle up to 750.

if you unplug your IAC does the engine stall?

And then set the TPS to .9v or so--

sometimes with an aftermarket cam-the idle needs to be raised a bit, and adjusting the throttle plate doesn't allow enough TPS adjustment-so a hole can be added to the throttl plate.
The original poster has a modified camshaft profile--

I myself have a 5/32" hole in my throttle plate to idle at 950 RPM's with 16" vacuum and a TPS voltage of .89v

 

[97greenmonster]

If you unplug it, and it stalls, is that a bad thing. Mine does that? But it idles smoothly, even with the AC on!

 

[manaen]

The original poster has a modified camshaft profile

Ahh my bad, I didn't give much thought to the effect on vacum with the aftermarket cam.

 

[manaen]

If you unplug it, and it stalls, is that a bad thing. Mine does that? But it idles smoothly, even with the AC on!

I think if everything is working properly and all accessories are off when the IAC is unplugged it is supposed to idle at or around 500rpms

The computer is good as making up the difference on the throttle stop adjustment, and additional load (AC) on the engine. You may find that the engine could be more responsive if the throttle stop is set properly.

I am trying to find the Ford recommendations for these years, pouring through a few books now. I will post it when I find it.

 

[97greenmonster]

Thanks. This concerns me. My response from the throttle is AMAZING, so i wonder what it would be like with it corrdctly adjusted! How do you do that, throught the TPS?

 

[manaen]

Thanks. This concerns me. My response from the throttle is AMAZING, so i wonder what it would be like with it corrdctly adjusted! How do you do that, throught the TPS?

well I wouldn't expect much difference, it would only make a huge difference if it is way out of adjustment.

Still looking I couldn't find the reference on the ford CD's hmmm :scratch:

 

[manaen]

I just checked my engine, and when I disconnect my IAC it went to 300rpms, I contributed this to the change in TB when I replaced my engine, so I set it to 500rpm and plugged the IAC back in. The computer picked up where it left off and went right back to 750. my computer must not be running windows, otherwise I would have had to reboot it

As expected it really made little difference in throttle response.


Sorry for the semi-hijack here, but this is basically relevant

 

[happyjack87]

where is the screw to adjust the throttle? i couldn't find this in the service manual.

 

[manaen]

where is the screw to adjust the throttle? i couldn't find this in the service manual.

it is on the throttle body, it is the stop screw that sets the stopping point for the closed butterfly inside the throttle body. The screw is usually located on the outside of the TB close to where the throttle cable attach's. The reason you don't see it in any book is because it is supposed to be a set and forget thing. Ford doesn't want people to be turning at that screw, much like the TPS which is normally not adjustable. Yet we all know the benefit of a properly adjusted TPS.


This is not to be confused with an idle screw, adjusting this screw to set your idle is not the goal here, as the computer will set the actual idle speed. The point is that the IAC has a certain working range and if the throttle body is out of range it cannot function properly. So setting the "static" idle to 500rpms puts the IAC right into it's working range.


I must say after setting mine this afternoon, and having more time to drive it around, my truck does seem to pull much better off of idle, but it could just be the cool night we are having tonight. I never thought to check it before this thread came out, and when I replaced my engine I was more concerned with getting it running before a fishing trip to Maine over the following weekend. Then when I got back life got in the way and I have not had a chance to go over all the sensors and settings. That is gonna be my goal before whitetails take over my spare time in the next week or two .

 

[happyjack87]

so then so that i do this right, i just start the car, unplug the IAC, set the screw to 500rpm and then plug the IAC back in? and if everything goes right my idle should be at 750rpm in park?

 

[manaen]

so then so that i do this right, i just start the car, unplug the IAC, set the screw to 500rpm and then plug the IAC back in? and if everything goes right my idle should be at 750rpm in park?

exactly..

 

[97greenmonster]

Yea. i think mine is way off bacause when i unplug the IAC, it dies, it doesnt even hesitate. It dies!what kind of screw are we looking for. do you have a pix. im sure i could find it, but i dont want to play around an destroy it before i sell it!

 

[manaen]

Just look at your throttle body, it is the only screw that used for a stop for the butterfly on the TB. On a SOHC it is on the same side as the throttle cable toward the bottom of the TB. It has an 8mm hex head for adjustment.

The more you turn it in, the more it opens the TB, and the higher the static idle is going to be. Keep in mind that this will also have an effect on the TPS so you should reset your .95v TPS setting after the adustments are made.

 

[happyjack87]

well then could you explain how to do that part as well.