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On mine, the throttle screw has a sticking saying "do not adjsut". I think its used for initial factory adjustment of the throttle position sensor.
You can also use a rod ( or hose )to listen to each injector. Put one end on the each and the other on your ear while the motor is running. The bad injector will likely sound different ot the rest.
I know early Ford injectors have been troublesome. I've been down this road.
Well I hope this post makes it through. the last one got lost due to the server being all tied up.
I have shaken and looked for bad connections more than once. If it is a wire I am going to need the truck to die and not restart before I can find it. I have checked the grounds several times as well.
The dizzy module is the module on the side of the distributor or the brains I some times call it.
I have listened to the injectors on the drivers side but the pass. side is a little harder to get to. But I must say I didn't give it my best shot either.
If one cylinder is not acting up right on a 6 or 8 cylinder, you would not notice a difference.
More likely you could have a vacuum leak. Spray a trottle body cleaner, carb cleaner around the intake area and vacuum hoses and if the RPM or idle changes then you just found where the vacuum leak.
the up and down idling does sound like a vacuum leak more than anything else (or IAC, but you have a new one). Have you checked the upper intake gasket.
One cylinder not firing will be very noticeable...just try pulling a spark plug wire off of one cylinder and see how rough it runs.
The ign. module on the dist has been known to go bad on earlier models...which is why they began mounting it on the fender/firewall on later models.
I'm starting to think it might be a faulty injector as well. let us know.
I have had a similar problem, or at least it appeared to be similar. I did in fact make a small adjustment at the idle stop screw. It was very small indeed. it raised the idle to about 700 rpm and has not been a problem since. I proudly shared this with a Ford mechanic thinking that I had invented something new. He deflated my ego by saying that they did this kind of adjustment quite frequently on older trucks.
I have sprayed carb cleaner every where I could think of and more than once. I have replaced the upper intake gasket already as well.
Tomorrow I may try the other injectors and I will make a visit to the junk yard and grab a couple of modules off of some trucks and see that helps. That way I don't throw away another $30 at this thing.
I will keep everyone updated and thanks for the help. This is one of those things where all the input and experience of others is needed.
I know it's a dumb question but what trans are you running ? I had a problem with my trunk idle when I first swapped in my used c-6 and it turned out to be a bad vacuum modulator was allowing the trans fluid to be sucked into intake and burned thru the vacuum line. pulling the hose off revealed the oil trail to headaches.... changing it was a relief.
I have unplug both the trans vacuum line and the power brake vacuum line with no change it the way it idles so I have ruled those out as a possible cause. Of course I plug the intake vacuum port when I unplugged them. My trans is also a C6.