Hanging/Slow dropping RPM FIX inside.
 

Difficulty (1-10): 3
Time Required: 1 hour
Equipment: Socket set, .5square feet of 1/32" aluminum sheetmetal, tin snips



This is for anyone that doesn't like when the RPM's that are slow to drop or even rise slightly when you push the clutch in to shift. Another symptom, is that when cruising in a higher gear and you let your foot off the gas, the car almost cruises on its own for a bit.

First, understand the problem. The engine is recieving too much air through the IAC valve. From my knowledge Ford did this for emissions reasons.

As a side effect of this fix, when you downshift, the car burbles a lot more, sounds like a carburated engine. You get a little more engine braking effect.

The FIX:
Most older mustangs have an external IAC valve. Meaning that it is easy to find, follow a small air hose (1/2" ID) to a silver cylinder thing on your intake (the IAC solenoid). Once you found it, the older fix was to buy a 1/2" copper pipe cap, drill out a small hole, insert this into the line the restrict air flow through the IAC, and your done.
The Mach has a slightly different setup.
We have an INTERNAL IAC air supply. Meaning that there is no hose leading to it.

To find your IAC valve, first remove your entire shaker assembly from your car.

Next, look between your intake and the firewall, there should be a silver cylinder shaped object with 2 bolts leading to your intake.

Remove the two bolts and move this assembly.

You should see the following, the IAC assembly has two "ports" with the solenoid controlling the air flow between these two ports, what we want to accomplish is to restrict the airflow leading through this.

To do this, there should be a small gasket on your intake, or it could be stuck to the IAC valve.

Remove this (carefully)

Trace out the shape of this gasket as well as the holes for mounting bolts, and the two airholes, onto a piece of 1/32" aluminum or something similar.

Cut out the outline of this "restrictor plate" from the aluminum.

Cut out the holes for your bolts to go through.

Cut out a single one of the Air holes.
Now the tricky part. You have to decide how big to make the other hole because this determines the amount of air you are restricting.

The smallest you should go would be around 5/32", the worst case if you go to small is your car will stall when fix is complete, if this happens just take the plate out, make the hole bigger, and check that.
The hole I used is around 9/32" which is about the medium you would want to go.

After your "restricter plate" is cut out, mount it with the small hole on the passenger side of the IAC, and make sure to replace the gasket against the intake.
Start your car, you should notice the RPMs drop when you start back to normal very quickly.
If it idles smoothly, try putting turning on the AC, this will load the engine down.
If your car stalls out, remove the plate, make the hole bigger, try again.
If still idling smoothly, bolt your shaker back on, take it for a spin. You might need to adjust your driving/clutching style after being used to the crappy RPM hang, so don't be discouraged.
Also, you might want to disconnect your battery during the procedure to make your computer relearn it tweaks after you finish.

If, after you start the car you notice a "whistling" sound, then take a look at your plate, make sure the BIG hole is at least as big as the hole on the IAC valve. ALso make sure edges of holes are rounded (sharp edges are bad), and the holes are round. This happened on mine, and I fixed it.


Any questions, please post here.



Please don't yell at me if you break your car, you accept that you are trying this at your own risk. It works on mine, but your mileage may vary.

Re: Hanging/Slow dropping RPM FIX inside.
 

Mach1Canuck,

Works like a charm. Thanks for the info.

Made a nice, short and easy project for a Saturday morning.

I have an automatic but never the less, really hated the hanging RPMs when starting up. All fixed now. :)

David <><

Interesting. I'll keep this in mind. I really haven't noticed any hanging RPM's with my car.:THUMBSUP:

Where and what type of aluminum are you buying to make the template? What tools are you using as well?

Question: The dealership fixed my hanging RPM issue and mu Mach runs according to spec's. However, by doing this mod you've detailed above, will it cause a normal running engine's RPM's to drop quicker than they do today? I might consider this mod if it will make the throttle more responsive by dropping the RPM's quicker after revving it up.

awesome buddy! as a professional race car driver www.enjracing.com i hate the slow rev drop. this will be great! how fast does it actually come down now?

hamradioextra: Thanks! Glad it worked out for you!

SVTSnake: The aluminum I used was 1/32" sheet, you can find it at Pepboys or an autoshop as sheetmetal repair kit.
The tools I used was a socket set to remove the two bolts on the IAC valve, a marker to mark out the gasket on the aluminum, a pair of tin snips to cut out the outline, and a dremel to cut out the holes. As for the template, I use the gasket that is between the valve and intake as the template.

Dawg: Yes it should cause the RPM's to drop a LOT faster, depending on what size hole you cut for the second hole. Like, the RPM's on my car on startup drop to idle immediately, like, start, then boom, they're at 800.

enjracing1: As fast as you want it to, depending on the hole you cut, as I responded to Dawg.

Any more questions guys?

Fast Drop in RPMs
 

The smaller the hole, the faster the drop. Like Canuck said, get it too small and the engine will not idle properly.

In my case, 9/32 is perfect because with the auto in gear and the A.C. on, the idle is already taxed. Turn the fan on high and the alternator will decrease your RPMs even more.

The manual trannies can probably go one size smaller, depending on the condition of your engine tune up. I tried the smaller size first but didn't like the extremely low RPM on a cold engine. After increasing the size to 9/32, it was perfect, for me.

IMHO, 9/32 is probably the optimum for most of us.

Many people on this site who weren't raised with carburated engines like I was, don't even realize that there is an RPM hang. The earlier EFI engines didn't have this problem either. This has been bugging me regarding the newer, computer controlled cars for a long time.

Glad it's finally over.

Apparently the purpose is to burn all the fuel from the system after letting up on the gas.

Like Canuck said, there is more "Burble" now after letting off on the gas pedal, primarily because the unburned fuel is now leaving small back fire sounds, hence, the Burble sound.

It now sounds like my 70s and 80s cars.

Gotta love it. :)

BTW, I used a piece of sheet steel for my mod. It really doesn't matter what material you use, as long as it is perfectly flat and strong. Any metal will do.

Again, I want to say, Great Piece of Info from Canuck!!

http://www.mach1registry.org/photopo...500&ppuser=140

David <><

Thank you for that much-awaited mod for the hanging RPM's.
I WILL DEFINITELY TRY IT!!

Tomorrow morning I will be in Pep Boys looking for that 1/32" aluminum plate.

But I can't install it till the end of the next week. Here's why:

Yesterday I took my car to the dealer because of the Hanging and Stuck RPM's issue. They told me they couldn't find anything and that I already had the latest PC calibration. So they decided to leave a Data Recorder in my car for a week to then analyze the data.

So I can't disconnect the battery until they remove the data recorder.

But I know they will still not find anything.
A few months ago, they replaced my TPS, then the IAC, and then some TP wiring harness. No luck! RPM's still hang.

I'm sure this Restrictor Plate will work! It makes a lot of sense.


Thank you very much Mach1Canuck!!!
You're the man! :worship:

:THUMBSUP:

:worship: :HOPPY: :worship:

I just saved this information onto a word document... Priceless! :COOL:

I already made the Aluminum Restictor Plate.

I bought the aluminum sheet metal at Home Depot.
Also bought a Grinding Point to make those holes with a perfect shape.

It looks like factory made lol

Once again, thank you very much Canuck!

As soon as I install it next week, I will let you know the results.

:HOPPY:

Sweet, hopefully it works out for everyone.
Maybe someone can mass produce these and make some dough? :)

For the automatic guys, is it just the RPM drop at startup that you noticed after installing?
Or do you get a fair bit more burble when you let off the throttle?

Hey Canuck,

I'm just curious? What do you mean by "It works on mine, but your mileage may vary"?

Can we actually expect the car to spend more gas?

I prepared 2 Restrictor Plates. One with a 9/32 hole and the other plate with a little bit smaller than 9/32. Let's see which one works better for me.

By 'Your mileage may vary'. I just mean that you could have different results than me..
I haven't figured out the mileage though, but I am speculating that it will increase slightly, but not enough to really measure.

If you think about it, if it idles lower/returns to idle faster, than it uses less gas right?

Also, you use less gas when you don't want to (letting off throttle, braking)

This is all assuming the engine computer will compensate for the richer condition caused by less bypass air.
Hopefully, it looks at the O2 sensors and lessens the amount of fuel being admitted.

I would be interested in seeing the A/F change on a dyno with this mod. Am thinking with the computer adjusting for the lesser air (by removing fuel), you may end up lean.

I would be interested in this also.
If someone has access to a dyno, would like to see someone who has freshly reset their computer and installed the plate. (computer has not adjusted)
And someone who has driven a while with it on and their computer has made the tweaks.

so does teh plate replace teh gasket or go between it. I am interested in trying this.

This is what I believe:

First the IAC, then the Restrictor Plate, then the gasket, and then the Intake.

But Canuck can tell us for sure.

cgg074 has it right. Or at least what *I* did. I think as long as your plate it totally flat, and free from debris, it should make a good seal by itself, I added the gasket on the itake side just to make sure.

If you want to be totally bulletproof, you would want a gasket on either side of the plate, but I think it's overkill...

is there not an easier way than fabbing a plate? i am not in shop anymore and do not have access to some tools anymore ( sheetmetal cutter)

The problem is, I believe under engine braking or decelleration, a fairly large vaccuum is created in this area. Use anything flimsy and you risk it getting torn apart/sucked into your intake into your engine.

1/32" Aluminum or Tin, is VERY easy to cut, buy a $5 pair of tin snips.. you might not even need those, its just what I had handy.
You could probably whittle the whole thing out with a dremel if you had the time.

With my automatic, I notice a fair amount of burble. Really sounds great going through the underpasses.

On the way home from Church today, with over 30 miles on the mod, I really got on the accelerator. I live way out in the country with plenty of open road. I think it is running better. I not only got a chirp going from first to second, which is usual, I got an honest tire squeal that caused me to go sideways a bit.

Ever since putting on after market mufflers several months ago, my car has been running a little rich. Perhaps the computer is leaning out my A/F ratio due to the mod or perhaps it's my imagination. Never the less, it's my story and I'm sticking to it. :)

David <><

mach1canuck.......... if you are standing in front of the car looking at the motor, is it the left or right side port on the intake that has the small hole cut in the restictor plate? thanks.

If you are standing in front of the car looking at the motor, the small hole goes to your left.

About cutting the sheet-metal, go to Home Depot and buy a Aluminum Sheet-Metal Roll. It's cheap and you can cut it with normal scissors.

I took photos of my plates. When I get home from work, I will post them so everyone can see them.

this would be an excellent opportunity for someone to make some money.

patent pending....

;)

This wouldn't adversely affect the cats after a period of time would it? You know, the unburnt fuel/burble? THX

Oh yeah, what's the advantage to the rpm's dropping more rapidly than they already do on my Mach? I've owned and tuned some carb cars before and yes the efi does seem to hang a bit more, but not annoyingly on my Mach at least. thanks for the info.

dank yours might not b bad

mach1canuck, if you are willing to make me one I would be willing to pay you. lmk

My RPM jumps to 1900/2000 RPM and slowly return to 1200 and then steady at 900 in a minute in the mornings. Now with the Borla Stingers on the way, I'm definitely going to do this trick. My wife thinks I'm revin deliberately in mornings. :SMASH:

My question is do I need to remove it prior to the emissions test? Probably should. :frown:

Any warranty issues with this mod?

DanK: Cats, yes it could affect them, but they really only handle a lot of unburnt fuel at startup anyways right? Before things have warmed up. And isn't there a pair of pre-cat oxygen sensors to help the computer tune the A/F ratio? Interesting to see if they compensate.
The advantage the having the RPM's drop more quickly, is that, I *HATE* how they stay where they are, or even increase when you go to shift gears. I did this to make the car act more *carburated*, less computer control y'know? I like to call the shots and let the car know when to add air/fuel. As a plus, the car sounds wayyy cooler when you let off the throttle (at least on a stick).

03ZincMach1: I could make you one as a direct copy of mine, the only thing I would be concerned with is that you would like the amount of *restriction* I use, like how fast the RPM's drop. I think it's like a personal preference thing. Also, the amount of mods you have on your car, will probably affect how much air the car needs (how big the hole might be), so what's great on my car, might make no difference, or just stall your car out. See what I mean? If I get ambitious soon, I'll just scan in a template that you could print and use to make the plate. By the way, it's not so much a *plate*, like you can bend this aluminum betwen two fingers, it's just stiff enough to work. But, let me know if you still want me to try.

Mach1Speed: Yeah, I'd probably remove it before the emissions test,you probably wouldn't fail a visual(if you did they're looking wayyy too close), but the smog test, unless your computer has learned to tweak the fuel (the jury is still out on whether it does), you might give off too much smog.

Burl: Warranty Issues, there could be, I'm not a Ford Engineer.. this could cause your airbags to deploy and your tires to deflate for all I know.. :) The only thing I know is, this fixed my hanging RPM problem, made the car sound cool, and ate up a Saturday afternoon(gave me an excuse not to mow the lawn).
Honestly though, As long as you are clean, don't get metal shavings in your intake, and don't break anything, I don't really see any problems.
But, as with anything else, I would remove this before running to your dealer with a blown engine and they cancel your warranty.

:LAUGH: :LAUGH: :LAUGH:

Just for the H*ll of it
 

I did this mod to see the effects.
Before:
No current issues with hanging or increasing RPMs during idle or initial startup, but I do see the hanging RPM's during shifting. Although it was not that big of a deal to me, I wanted to try this anyway.
After:
The small hole size I went for was 1/4". First thing I noticed when I crunk her up was the strong smell of unburned fuel. First trip out it definately brought your RPM's back to level very quickly. During coasts with the clutch still engaged, it does give more braking effect to the engine. And definately gave more gurgle. Throttle felt a bit more responsive during the first few minutes of running. But after a quick ride around the block, I returned and let the car idle for a moment. I gave it a quick pat on the throttle to about 2500 and let off, the rpms stayed at 1100 for 10 to 15 seconds, then took a dive to 300 and slowly came back up to 800. Did this scenario several times with the exact same outcome.

That was enough for me. Maybe I needed to increase the hole diameter to get a smoother idle. I'll stick with the way things were. I had no problem with it, just wanted to tinker a bit.

Good idea for someone with chronic symptoms. But if just want to tinker, not worth the hassle.

PS, got my metal from Lowes, in the rain gutter section. I bought a rain gutter end cap, had just enough metal to do the job, $1.31.

Anyone in Atlanta want an IAC valve restrictor plate?? 10 bucks and its yours.

Can anyone host a photo that I took of the plates that I made?

PM me your email and I will send it to you so you can host it somewhere.

Thanks!

Here you go guys.

Here's the photo of the two plates that I made, the original gasket, and the IAC.
The one to the left has a 9/32 hole, and the one to the right has a little bit smaller hole; I will try this one first. If it stalls, I will use the 9/32.

Hope this helps!

There you go, thats the gear all right.

Should make it a little easier for people, better than my half *** description at least.. :)

Mach1Canuck,

Have you experienced a low idle or a dive to low rpms then slowly back to normal just like dmjasmith experienced??

Maybe the 1/4" hole was too small.

I did experience that when the hole I made was too small, around 7/32 or so. Also, the engine came *very* close to stalling out when I turned on A/C and the compressor came on. Thats when I enlarged the hole to where it is now.. 9/32 or just a *touch* bigger...(from smoothing out the sharp edges with a file)

The way mine is now, there is no drastic RPM dive, no erratic operation.

Look at this link
 

Someone did a very good job of taking pictures of the whole operation:


http://forums.stangnet.com/showthread.php?t=470711

I went to a dealer and did a test-drive on a 2004 Cobra.

It feels awesome!!

And the RPM's don't hang. They come down very fast.

So my question is, WHY OUR MACH'S EXPERIENCE HANGING RPM'S??

We don't share the same exact engines, but we both have 4.6L Modular V8 DOHC 32-Valves.

As soon as I get rid of the Data Recorder on my car, I will install my restrictor plate.

:THUMBSUP:

I really hate those hanging RPM's and the car driving for itself for a few seconds when releasing the gas pedal.

I would suspect it has to do with the car being supercharged. Being that it probably has a different intake setup than the mach... can anyone check this out if they have a cobra? I'm prety sure it has an internal IAC supply fromt he intake.

I did my little IAC mod Monday night. Picked up a piece of sheet metal from Pep Boys for $3. Used my dremel to make the holes, and a pair of tin snips to cut the outline. 9/32 worked like a charm, and I have no more hanging RPM's! Shifting now is sweet, and you can get through the gears faster.

Thanks again Mach1Canuck! Awesome little mod...

Could this help those of us with the K & N Aircharger stalling problem? I notice when my car stalls, the idle fluctuates and then dies. I read somewhere that the Aircharger makes the car run too lean. I'm getting to the point where I don't want to drive the car anymore. I thought I had it cured, and then it started stalling again.

Any insight would be greatly appreciated.

Quite possibly...its only 1 hour wasted if it *doesn't* help.....
Not much to lose to try it out.

Good luck with it!

Manufacture
 

As a suggestion, can someone make a standard size plate and is at the smallest setting, which can then be modified by the owner to the correct size?

This would be worth $20-$30 bucks to me for someone to come up with the proper quality plate and a minimum (starting) hole size so that it can be fine tuned.

Any takers?

I have no tooling for such a project, but I know that this car has a problem (not according to the dealership, as of yesterday). Mine takes 10 seconds or better to reach 900 rpm after accelerating in first and second gears - it sucks!

Those symptoms are...
 

classic symptoms of an intake leak - most probably post MAF. You should double/triple/quadruple check your K/N install.

Installed mine yesterday and this morning was the quietest startup (quick return to 1000 rpm). Can't wait to install the Borlas when it arrives.

Shifting is just like my 71 mustang and loving it!!.

Thanks again Mach1Canuck. :THUMBSUP:

Re: Manufacture
 

Cono,

I would be happy to make you one free of charge. I'm currently working on an article for my mustang club on how to do this. I need to make pictures anyway and I will just ship you the end product. Send me a PM of your address.

Mach1Canuck,

Thanks again for this wonderful tip. I plan on giving you the credit for figuring this one out to my club members.:CHEERS: :THUMBSUP:

No problemo guys. The thread on Stangnet another member made has some realy good pics of the install. Pretty much foolproof, you should ask him for the use of the pics if you want to.