Hello Everyone. I sure hope someone can help me with my truck. I'm 78 soon to be 79 in Nov. I have been trying to fix my truck for over a year due to me being a DA...
Here is what I have done so far.
Replaced MAF
Replaced Positioner
Replaced IAC
New radiator not involved. Will check ignition coil next, but don't think that's it.

Because I could not get the idle down I have backed the idle screw all the way out. Still my rpms are over 1000.
I took the power plug out of the IAC and rpm's was still high unaffected. Yet I hear no vacuum leak. Engine running extremely rich..
I think there is a modulator valve off side of transmission ??? Not sure. It is vacuum controlled. Will check this next as when driving and it starts acting up (engine light comes on, a decrease in power, downshifts) light goes off and acts normal, but does not last. Keeps going through this cycle. I am wore out with trying to find the issue. Any suggestions would be so much appreciated. Thanks all.
Bill.

 

I'm not on the Welcoming Committee but, Welcome to the forums.
Give engine type and mileage so we have a clearer picture of the "patient".

Your '94 was the last year for the vacuum modulator. Check the hose condition as well as both its ends for fit / splitting.
Re the new IAC: Did you buy a Mororcraft or Hitachi? Those two are the ones which work.
See if the PCV hose is cracked / split. Make sure the PVC valve's seat is intact. They're plastic and can break and be a huge vacuum leak.

WHICH positioner did you replace?
Do you have a code reader for when the engine light comes on? It could be a big help in diagnosing even if it goes off.

A compression check (done at each spark plug hole) could reveal a burned exhaust valve.

 

Thanks so much very responding. I am 78 and a bit slow these days but I really need my
truck please bare with me as I have also health issues. I will do my best to work with you guys.
4.0 L XLT with over 276000 don't have the money get another vehicle
I have a obd1 reader and will check again
The position sensor is the TPS (Throttle)
I have not touched the plugs , wires, or coil yet.
I did not know about Hitachi and Motorcraft being the only ones that work. I will try and find this out
I banged on the IAC with not change.
PCV is new was check awhile ago and checked ok.
In addition to all these running problems my dash lights and RPM gauge is out
I don't drive much due to the rpms being high.
I have asked my son to check the hose from the modulator valve.
will get back as soon as I can
Thanks

 

Went to the Doc today. My truck started out same old problems high rpm, shifting during high rpm, kicking in and out of gear, at time engine stopping.
The later coming home my truck started running normal. normal rpms, and shifting normal. ??????? this is driving me Bananas.
Engine light came on with issues. Then went off when normal.