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This 03 Mach 1 of mine came with a ACCUFAB TB. The RPM's do hang on occasion. I tried the normal cleaners and now it looks nice and shiny. But the RPM's still hang. Cable and foot pedal look good. I have read that some went back to the factory TB for better results? Looking for some ideas.
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Sorry I don't have an answer for this one. I wanted to bump it so maybe someone else sees it.
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Not sure about the Accufab TB hanging up, but the 03/04 Mach's are slow to return to normal idle. A popular mod involves the Idle Air Control. Here is a write up I did a couple of years ago. Very simple and quick.
A popular mod on the 03/04 Machs is to put a Restrictor Plate inside the IAC. This mod makes the engine slow down quicker when you let off the throttle, more like the old carbureted cars. I did this mod to my white 04 Mach and had to experiment with the hole size in the plate. On my first attempt the restrictor plate gave me the slight hesitation like you mentioned. I made the small hole a little bigger and it went away. Maybe the previous owner installed the restrictor plate and did not get it right.
Sorry, no pics of mine, but the IAC is just above and behind the intake. Here are instructions on how to do the mod, these instructions are from another site, but have been copied around the net. I did not need to remove the shaker when I did mine. I used an aluminum Orange Crush soda can to make my plate, been working fine for over 50,000 miles.
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 air holes, 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.
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Sounds like a pretty easy project. I'll trying anything at least one time.
Thanks