gasforblood

I have replaced the iac tps and map sensors I have put msd cap and button new hoter wires and hoter plugs it idels high and will drop down and go back up also bogs when I mat the pedal or give more than half throttle

OMG It Runs

EGR.

Had exact same issue with mine. I ended up having to replumb all the vacumn hoses since they had holes and replaced the EGR valve and line.

Somewhere you have a leak, or its your ECU which is less common.

A few of these trucks have issues with water running down the firewall and into the ECU. Check your ECU for moisture, might not hurt to unplug it and spray some electrical dryer in the contacts then plug back up. See if it improves anything.

rocknrod

Or a vacuum leak. I had the same issue and found four vacuum leaks in old worn out hoses. I would have never believed it.

CliffH

Mine idles up and down. When in drive it tries to take off when it "revs" itself up. This goes up to 900 rpm then drops quickly to 500 or less rpms. Previous issues were randomly dying while at speed. I seafoamed vacuum lines and cleaned off tsp, iac, and some other electronics. Recently it was having trouble starting and I had to pump the fuel pedal to get it to start. Seafoam to the fuel tank. Now the new idling issue but no issue starting.
1991 5.0 EFI

CliffH

So I pulled the vacuum hose at the brake booster and no issues. Of course brakes are impossibly stiff. Now I will test a directional valve, but I suspect a bad brake booster.

KevFord1993

Try TPS.

Was an IAC valve when it happened to me not too long ago!