Could it be a mis-fire?
#1
I have noticed lately my Mach I seems to have a slight “hiccup” at times. I think it started after I replaced the air filter with a drop-in K&N filter (don’t know if that has anything to do with it). The hiccup doesn’t seem to happen when the car is under load or at cursing speed. The sensation I am feeling is not like a miss it is just something I don’t think is right. It does seem to go away after driving it a while. The car has 32K and has never been tuned up as far as I know. It had 30K when I bought it. One issue is when shifting from 1st to 2nd gear the throttle seems to lag a bit causing the shift to be rough, which does not happen all the time. Any suggestions??
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#2
How oily is the KN filter? I had a 78 Trans Am one time that did the exact same thing like it was bogging down and it turned out the filter was to moist and was sucking the excess oil from the filter in.
I removed the filter went back to stock and problem solved. Not sure that is your issue just sharing my experience.
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#3
That is quite possible, I've seen it happen on many K&N filters. Especially after some one cleans and re/oil's one. I've also seen them out of the box extremely oily. So this is a good possibility. simply swap back to the stocker and see if it solves the problem. JTS 71 Mach1
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#4
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 carborated 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 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.
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#5
Check the mass air flow sensor they dont like the oil in K&n air filters at GM when I worked there they had a service bulletin about it.
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#6
Have not had time to do anything to the Mach I but it seems to be less noticeable each time I drive it which isn't but once or twice a week. The throttle lag is gone and it seldom has the "hick-up". Maybe the K&N filter oil on the air sensor is evaporating. (if that was my problem). I wish I could run this thing on the 1/4 mile to see how it performs. Even if I had a place to go I don't think I could make a quick shift from 2nd to 3rd. My reflexes are not what they used to be. I still enjoy driving it with the manual transmission. Hard not to stomp the firewall out when I get in my truck or SUV.
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