This doesn't sound like a problem. The IAC allows a certain amount of air past the throttle blade so that rpm comes down gradually to prevent stalling.
Also it keeps the engine speed more closely matched to transmission speed.

 

You obviously havn't driven a car with this issue before it is a pita.
I don't even know where to start, when you shift gears when accelerating shifting into a higher gear equals less engine RPM for the same road speed. So when the rpm's hang or even climb a few 100 rpm it is going THE WRONG WAY to match transmission speed.

To the original poster, have the car tuned. The tuner will modify the IAC tables and all will be well. My car that is currently untuned has the same issue and I almost don't want to drive the car it is so annoying. When I purchased the car it had an unchiped A9L in it and my car did not have this issue until I changed to a T4M0. I finally got all the electrical wiring nightmares sorted out on my car and was going to get it tuned yesterday but it blew a head gasket last week, so I won't have it done in time for NMRA in new jersy. I have a friend that tunes cars for dayton performance and he assured me that he can adjust the IAC tables to solve the problem. After the tune is done I'll report in.

 

rpms

yes i understand that. but in my 93 when i'm driving and i push the clutch in to shift to the next gear the rpms fall not to 0 but they fall a few hundred rpm when i'm shifting normaly. in the 94s case they almost go up 100 to 200 rpm everytime i push the clutch in between shifts.

 

yes i understand that. but in my 93 when i'm driving and i push the clutch in to shift to the next gear the rpms fall not to 0 but they fall a few hundred rpm when i'm shifting normaly. in the 94s case they almost go up 100 to 200 rpm everytime i push the clutch in between shifts.

I guess it's a matter of preference but that wouldn't really bother me. Of course, I don't really know how severe this hanging idle really is. It does have to do with the IAC strategy in the PCM. Unless your IAC is responding slow (sticking). You're using fox throttle body and IAC with SN95 computer?

 

I had a problem like that before in my coupe, but I don't remember what the problem was. I think it was lack of horsepower!:idunno: So, I fixed it by doing a H/C/I swap. That seemed to fix it. But, I remember, it was a PITA before the swap when the RPMs would hang or even go up between shifts!:shakehead Anyone else know? Maybe it was the throttle body reacting slowly...(I swapped that too!)...

 

i already did the heads cam intake swap and i also swaped the throttle body from the 94 to a fox body set up. someone once told me that the only way to get rid of that is to get like a pms or some kind of managemeny system to change stuff. i don't know though.

 

Remove the IAC and spray the heck out of it with 1 CAN of break cleaner.. Also, check your tps and adj. if needed.. Let the IAC dryout and plug it back it.. Remove the negative batter cable for about 15 minutes and take her for a spin!

 

i replaced the IAC not to long alog and i replaced it with a ford one not an autozone one. i thought that was the problem also.
and i set the tps to .096 on the meter. and i also replaced the tps with a brand new ford one.

 

......and i set the tps to .096 on the meter.

theres your problem!!!......its supposed to be set to .96v not .096

 

1. pull codes.
2. The problem you are describing sounds like an IAC problem. Do the rpms drop down to idle when you slow to zero mph?
3. Why in God's name did you switch to a pre 94 tb??
4. If an IAC is malfunctioning, spraying cleaner inside it amounts to "delaying the inevitable".

 

I have the same problem with my 94 5.0 (hanging or jumping btw shifts). Mine started right after I swapped the IAC with a Borg Warner unit from Advance Auto part (the original one had a bad idle surge). I replaced the new IAC under warranty since I thought it was a bad part, but the second one had zero change from the first. I noticed with mine if I let off the gas for a second or two before I push the clutch in it doesn't jump in RPMs, just falls like it's supposed to. That lead me to believe that it was a TPS problem, but I have not had a chance to swap mine out (i did check voltage and it was at 1 volt, which is within range). What I want to check next is make sure the voltage between closed and WOT is smooth. I think if the voltage isn't smooth (ie: hangs at certain voltages for too long) it could confuse the computer and cause funny business.. This would make some sense since I can get the car to stop jumping RPMs between shifts 100% of the time if I let my foot off the gas a second or two before I push the clutch in. PLEASE let me know if that logic doesn't work.

I would love to get to the bottom of this because at times it doesn't look/sound like I know how to drive my car since the RPMs jump between shifts and my mufflers are not quite so it sounds dumb. I will post any updates that I come up with.

 

jteresi, why did you replace your IAC in the first place... I want to see how similar our problems are.

 

ryno, have you pulled codes? In my case, the code said something like "unable to control idle motor".

The classic IAC problem is:
1. hanging revs between shifts.
2. revs return to near idle once your car stops (i.e. 0 mph).

Make sure your elec connections are clean and tight btw.

 

I pulled my codes, I got a few smog codes that I knew I was going to get. The only other code was 33-EGR valve opening not detected. I took a video of the CEL blinking so I was sure I read the codes right. I am going to pull the codes again over the weekend and see if anything changes.

 

Do a Google search for an IAC restrictor plate. It will still allow it to move air for when fans, a/c, and other acc's come on, but solve the idle hang. I did it, and it works like a charm!

 

What year car, engine, and cam were you running? Did it solve RPM hang/jump between shifts or the infamous 5.0 hanging idle at while the car isn't moving?

 

im having the same problem.. 302 tfs heads and stage 2 cam and gt40 intake.. it can be adjusted on the motor side of the throttle body though right?? guess ill be working on that now..

 

I fixed it with the help of M1Tanker! Well, at least my hanging/jumping RPM problem between shifts. Also, my idle seems to be a little smother. My car is a 94 GT 5.0, but the concept is the same for most other years.

Here's what I did: I took the old gasket and traced it out with pen on gasket paper, then cut the perimeter with scissors. I used a piece of cut off brake line, a hammer, and a piece of bar stock to make the holes for the bolts to go through. Lay the gasket paper over the bar stock and use the hammer and brake line to punch out the holes. For the other two holes (the air in and air out) I used two different size sockets, a hammer, and the piece of bar stock. Make sure to clean off all the debris from the hole to make them as clean as possible. The hole on the air cleaner side I made the original size (5/8" in my case), and the hole on the engine side I made close to 5/16". My first trial I used a 1/4" hole for the engine side but I think the RPM dropped too much between shifts.

I think if I had to do it again I would make two identical gaskets, stick them together with gasket maker, and use them together for added strength. My logic for that is almost all the air the engine gets while engine braking is through the IAC ports. I don't know if it is necessary but I am going to check the gasket on there now in about 200 miles to see if it is torn.

In total, it cost $5 in Fel-pro gasket paper from Advance Auto Parts and 1/2 hour.

To answer a possible question that may come up: My idle speed decreased about 75 RPM. This doesn't bother me, but at least with the 94-95 throttle bodies, you can adjust the idle with the allen-head set screw on the top right of the throttle body (most of the time it's under the rubber cap). All you do is adjust the screw and rev it a few times, then the engine will settle down to the new idle speed. I don't think you can do this with the Fox bodies, and yes, I do have a factory throttle body.

I hope that helps and I will post any further updates.

 

Yeah, I couldn't remember the hole sizes. I did this a few months ago and it works like a charm.

 

You could just put a carb on it. 8)
-P.

 

how lucritive is this iac restrictor plate??

ryno/m1tanker can you get some pics up of what you did? all i see is what tools you used but you didnt explain what you did very well

 

ryno/m1tanker can you get some pics up of what you did? all i see is what tools you used but you didnt explain what you did very well

I thought I explained it alright. I guess I could have made it more clear is that I used the sockets and hammer like a cookie cutter.

Here is a pic of everything I used to make the gaskets besides the pen to trace out the new gasket. I switched from using a piece of cut off brake line, for the holes the bolts go through, to using a 4mm socket.

The gasket paper is Fel-pro 3157.

OEM IAC gasket - used as a template (the black gasket in the pic)

Sockets: (used like a cookie cutter with the hex side down)
4mm (bolt holes) - made a 1/4" hole
5mm (IAC air out -> to motor) - made a 5/16" hole
11mm (IAC air in -> from air cleaner) - made a 5/8" hole

Caliper - double check hole sizes

Piece of metal - put under gasket while punching holes

Make sure to clean the loose gasket paper out of the holes.

I have not noticed any other changes in driveability/performance besides losing the hanging/jumping RPM between shifts, a slight decrease in idle RPM, and a little smoother idle.

 

I'll try and post pics later today/tomorrow.

 

I made mine out of sheet metal. Ductwork does the trick. I traced the outline of the gasket and the holes, found center of the holes and drilled a 5/8" hole in one big gasket circle, 5/16" in the other and made the mount holes 1/4". I drilled them out on a drill press (backed with a 2x4 under the holes), then cleaned the burrs off with a dremel. I trimmed the outline last with a pair of tin snips, so I had the extra material to grab onto while working with it.

 

I made mine out of sheet metal.

Good idea, thanks.

I think eventually I am going to make one out of sheet metal and sandwich it between two gaskets. That way I don't have to worry about gasket paper tearing from the air passing through.

 

I sandwiched mine between 2 gaskets. I was going to use 1/8" alum. plate, but I didn't wanna buy longer bolts.

 

I have hanign rpm's as well and it's killing me. Sometimes the rpms just drop and the car will stall.

I have replaced, MAF ,IACV, fuel filter Put on an IACV restrictor plate, set base timing, and pulled codes
only codes I get are for smog and EGR in which I Have deleted.
All in all, I guess with heads and the B cam, the car just need a good tune on it.

I have sorted though any vac leaks I could find at all. I dont know what else to do exept a tune.

95 Cobra 5.0L
Another thing too the long tubes probable dont help witht he o2's way far back.

 

When I swapped in an E cam over the winter, my original IAC restrictor plate's holes were too small and stalled with the cam.

I had a chip burned for it which took care of all my drive-ability problems except hanging RPMs between shifts. I took a piece of duct work, drill bits, and tin-snips, created another restrictor plate, and fixed my problem.

I think I ended up using 7/16" hole for the smaller hole and 1/2" for the larger. I can't remember for sure, but it worked. It took me 4 tries with different hole sizes to get the car to behave how I wanted, but it's fixed for now.

 

94/95 are a nightmare. Before the tune mine would only idle without surging with the tp set at .91 volts exactly. It is auto, but give it a try.

 

just drive the car about a half mile from a cold start.

pop the hood. and unplug it. It will run just fine.
It will not idle hunt. it will not stall on deceleration.

although saying this gets my posts deleted for some reason.


try it before you buy it!.

 

Mine just started hanging today. It had happened before, but I don't remember what "cured" it, so on to this restrictor plate I guess. Thanks for the help guys.