the water level for the ecm temp sensor would be well underneath the water level at the top of your radiator, so if you have a full radiator when the engine is cold, I doubt that you'll have a problem with the ecm temp sensor not being submerged in coolant.

it's possible, but i'd try the other things first.

 

I checked the coolant, and tried burping it. It was full to the top. I got no drop in coolant. So I pulled the hose off the throttle body that comes from the IAC. I plugged the throttle body, and then put my finger over the hose. It was pulling vacuum as if the IAC wasnt closed. So I waited awhile. The truck was toasty hot. Still pulling a vacuum. So I removed it to take a look inside. First off the plug in the back isnt plugged in. I have no idea where the connector is. I see a hole in the valve that if moved enough will seal off. So I cooled the IAC off and it moved a little. I then heated the IAC and it moved but didnt seal the little small hole. This is where I have to be pulling a vacuum from. Its clean inside. No residue or gunk. So I plugged the nipple on the TB, and then the hole where the hose for the IAC comes from. This didnt change anything. Again I have no engine codes. How sensitive is the screw on the throttle body to adjust idle? If I close it the engine will almost die. If you back it almost out, it will run the engine up to about 1200 rpm. Man this idle issue is whooping my butt.

 

So I played around more this weekend. If the port from the IAC to the TB is plugged off at the throttle body, and also at the IAC. Meaning no vacuum leak. The truck will be idling close to 750-800rpm. If you rev it a few times, then let the TB close down, no touching the idle set screw, and fully closing. It will idle around 1000 rpm for a min or two. Then all of the sudden will drop idle back down to 750ish. Sometimes a little lower, sometimes a little higher. If you open up the screw for the idle adjustment to set it at 800. Let it idle, then rack the throttle a few times, it will rev up to 1000-1200 ish, and hold for a min or two then fall back down. When you first start the truck cold or hot, it will sound like crap for a few seconds, then smooth idle out to a prrrrr and speed up to like 900-1000. It will then hold this until you rack the throttle and then let it set. I burped the coolant numerous times. I have yet to pull the temp sending unit out of the thermostat elbow. I do know I have a temp problem. The dash will read cold sometimes, and perfect normal temp others, even when you know the motor is cold. I was told that sending unit is not connected to the ECM though. What coolant sending units are connected to the ECM, and what OHM's should they read.

Thanks in advance

 

I checked the coolant temp sending unit located under the upper radiator hose is good. It checked out withing specs. I am still having a varying idle. Sometimes it will be 650, sometimes 1000. With what seems like not a very high idle when cold. The idle will also fall if the truck is left to idle for a min or two.

So known good items:
VAFM
TPS
Coolant temp sender
Cold start injector
O2 sensor
fuel pressure regulator
plugs, rotor, cap
new fuel filter
good vacuum
137-142psi compression rebuilt motor 15-20k miles ago.
Why is the computer holding idle up, then dropping it after a min?
Almost like a vacuum leak is closing after a min or two.

 

we don't know for sure if that's what is really causing it.

if it's not the throttle cable hanging it up, it could also be something like a bent throttle shaft, or leaky throttle shaft bushings.

did you completely remove the dashpot from the equation, for test purposes?