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IAC restrictor plate

7K views 11 replies 3 participants last post by  squekster 
#1 ·
I am having a problem with a hanging idle sometimes. I have
been searching some cures for this and found a IAC restrictor plate.
I am going to fabricate a couple of plates today with some of the
bypass hole demensions that some have recommended and give that
a try. Any suggestions what to do instead?.
 
#7 ·
It is up and running. There is a real difference . It pulls alot harder than before. Ive only brought it
up to around 5,000 so far. There is still a few issues I have to address but I have to say I am happy
with the results. I made those restrictor plates. I made one with a restrictor hole dia of .28 in and
another one with a .400 in hole diameter. After installing one at a time they didn't seem to do much
but that doesn't mean that it won't work for you. If you are intersted in giving them a try then PM
me with your address and I will mail them out to you. I didn't make any EGR restrictor plates. After
researching that there is no real benefit to be gained from doing that.
 
#6 ·
I was reading a thread on a different site earlier and one of the members had made 20 of the homeade plates and gave them away to the members who had sent him their name & address.All but one of them had total success ridding their surging issues with the plate.It was just a plate with two holes for passages and two mounting holes which is slightly different than the plate you buy from online supplers,but if it works who cares.Ive got one of the Ford Motorcraft plates that I bought online years ago and it definitely works.If anybody decides to buy one from Ford or another source,instead of making one,heres the Ford part #F2PZ-9F939-A.
I believe cheaper versions are available too vs the Ford plate.
BTW- this plate was installed on some Ford vehicles,but I'll have to search for a list and put it in my next post,that way the plate can be found at Pull-A-Part,if anybodys interested.

The plate was added to cut down on warranty work because Fords service techs had to do a ton of throttle body cleanings due to carbon buildup,which was causing a restriction in airflow between the throttle plate and the inside diameter of the throttle body (caused stalling,low idle) plus it was causing the throttle plate to stick.
The plate was added under TSB #91-25-07.One vehicle it was installed on was
pre-91 Broncos.

The difference between his and the Ford plate is: the Ford plate has one passage the same size as the oem iac valve and one much larger passage plus adjustable allen screws with a pointed end,which allows you to adjust the exact amount of air you want to bypass from one side of the throttle plate to the other side of it.His plate has one hole the same size as the oem iac passage & another hole 3/16" in diameter.1/4", 5/16", 3/16" are a few of the hole sizes you can drill into the homemade plate.
 

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#8 ·
My idle is rock solid once it comes down which is what I am trying to get rid of.
It hangs high longer than it should. The thing that puzzles me the most is that it
is not consistent. Some times it idles back down quickly and sometimes it doesn't.
I already have the IAC adjustable plate but that differs from the restrictor plate.
I tried to adjust that plate and found that the set screws were frozen and kind of stripped
so I am going to have to drill and retap the holes and replace the set screws. That will give
me something to do this weekend also I read that maybe I should do a base idle reset
some have said that it cured there hanging idle problems, it really should have been done
after the head change anyway.
 
#9 ·
Some idle hanging is to be expected because Ford done that to prevent stalling when slowing down to a stop.It was also done during shifts to prevent a drop off in rpm,that way you wouldn't have to concentrate on keeping the pedal in an exact position to maintain the desired rpm while pushing the clutch pedal down and shifting too.However the idle shouldn't hang too an excessive point.Probably just 2-3 seconds.A major cause of surging or an excessive high idle (time wise) is having the idle set above 625-650rpm.Its supposed to be set as close as possible to 600rpm without going any higher than 650rpm,if at all possible.The ecm will try to bring idle down as much as it can with other means,if its set too high at the throttle body,but it can only drop idle so much,especially if youve got the screw adjusted to a point where its putting the iac out of its normal duty cycle.Its alot easier for the iac to bring idle up than it is to bring it down,so setting the mechanical idle above its target rpm will make it harder for the iac to do its job correctly which will cause surging,high idle or rough idle.The target value is 672rpm,so staying at/below 650rpm (iac disconnected) is imperative.
Issues with the clutch safety switch and vss will also cause surging and high idle because these sensors affect idle strategy and the vss is used to trigger the idle dashpot strategy.Its used when slowing down to a stop and during shifts.So be sure theres no issue with these sensors and no code 67 & 29.
A suspect tps/wiring will also cause the idle to hang high and a verification of this is: if the idle hangs high and you shut the engine off then upon a restart the idle drops to normal,thats a verification something is up within the tps circuit.
 
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