On my 1990 E150 5.8L, I can start it up cold and it goes to high high idle for about 2 seconds, then it drop to the 700 rpm range and just idles as if it's already warmed up. The check engine light is on as soon as it drops to low idle, I can't seem to fine my code reader at this time...any idea what may cause it to immediately drop the rpm? Left to warm up to operating temp, the engine runs fine and has plenty of power.

 

The first thing to do would be read the codes. If you can't find your code reader then you can just use a jumper wire and record the flashing lights from the check engine light. Warm the engine up to operating temperature and then cut it off. locate the diagnostic connector over by the battery on the passenger side and install a jumper wire between the Signal Return and Self Test input connector. In this video the guy skips the "key on -engine off" test and goes straight to the "key on - engine running" test.
To do the KOEO test just connect the jumper and then go sit in the van and turn the switch to the run position and wait for the flashes. If I remember correectly, the first set of codes flashed will be for any immediate issues as the ECM checks the sensors. After the ECM reports those codes, it will flash for any codes stored in memory. All of that info as well as how to read the flashes is in the Actron manual below.




If you need any help with reading the codes or what they mean, here is the manual for the Actron CP9015. I have two of them because I could not find mine one day when i needed it and didn't know how to use the jumper.
https://actron.com/sites/default/fil...lish_16153.pdf

I had a similar problem last year with my van in the winter when it was cold but I never saw a CEL come on. Sadly I cant tell you what fixed the problem because a few months ago I replaced the ECT, ACT, EGR solenoid, MAP sensor, and IAC all in the same day. I replaced a sensor and then cranked the engine and let it run for a few moments to make sure there were no codes and that the van was running okay before moving on to the next sensor. So I am not sure which one actually fixed the problem but the van runs much better now then it had been. I was also having issues where after the van was up to temperature and I would say go in the grocery store for 15 minutes or more, the van idle would serge up and down sometimes the engine would die. That was also fixed. I replaced the sensors simply as part of the maintenance on the van because they were all 33 years old.. I saved the old ones for an emergency.
One thing you could try is cleaning the IAC valve and the throttle body.