I have noticed that my truck on a cold start up it will warm up fine With a high idle right before the thermostat opens up the idle gets really low for 20 seconds or so
why does it do that ?
it’s around 200 degrees and then after about 20 seconds or so the idle smooth out this happen when the truck sits and idles for the warm up period.

 

We need more information so we can help you.
What year, exactly, is your vehicle? What engine, exactly do you have? IE: 22R or 22RE? Have you, or a previous owner, done any modifications to the engine, and if so, what? Have you done any testing? If so, what, exactly? What were the results you got?
Do you have a copy of the FSM for your vehicle? They are available for free on the net, at least a PDF version is probably out there you can download and keep on a hard drive, or zip drive, to use when you need it. You may need to google it to find one, but they are out there. Just takes some searching around. Well worth the effort, if for nothing else than the diagrams, and trouble shooting flowcharts.

Most of us have put this information into our signatures, so we don't need to keep typing it in. It might be a good idea for you.
Again, help us help you
Pat☺

 

Lol oh sorry it’s 1990 22re 2wd base model no air-conditioning no power steering totally stock

 

That's odd, most of these trucks have power assisted steering. You can steer with the engine off, it's just hard to do. Are you sure you don't have this system in your truck?

No AC I can believe. I moved out of Yuma, Az, and just took the belt off my AC compressor. It's cool enough up here, I don't need to waste engine power on AC any more.

Anywho, as to your original problem...
It could well be the Aux Air Input valve not closing as the engine temp rises. The valve is attached to the bottom of the TPS, and it has a large air hose going up to the Throttle body. It also has two small water hoses feeding it, to allow the valve to close as the engine coolant temp comes up. These valves are known to get sticky as they age. It may be yours isn't closing until the thermostat fully opens. The ECM ASSUMES the Aux Air valve closes at the correct temp, and adjusts the mixture accordingly. If the ECM is changing the mixture, but the valve isn't fully closing, the mixture will be off during those few seconds the temp is doing the last changes.

Hope this is some help.
Pat☺

 

Ya my 1990 22re did not come with power steering no pump One belt alternator do you think the truck is running too hot at 215 degrees The temperature of a truck goes from 185 to 215 And it has a 190 OEM thermostat
I took the temp measurements on the thermostat housing on top with a temp gun

 

Yes, I think 215 is too hot. You may need to replace the radiator, or the water pump.

If the outlet hose of the radiator is nearly as hot as the input hose, bad radiator. Ensure the fins on the outside of the radiator are clean and not clogged up, preventing air flow through the radiator. Also, ensure the down tubes aren't clogged up, preventing flow behind the fins, allowing the exchange of heat out to fins, and thus overboard. You can look into the radiator cap's hole and see a few down tubes. Use a flashlight, and verify at least those few. If they're clogged, or restricted, chances are the rest are too. Replace the radiator. Some shops can/will rod out the downtubes, but the price is nearly the same, or even more than, the cost of a new radiator.
Make sure that if you need to replace the radiator, you get a new cap for it. One that actually fit's correctly, and is rated at 13.9 PSI. That's what your system is designed to take.
Make sure the thermostat is opening all the way. You can put it into a pot of water on the stove, and heat the water up until it's nearly at the boil. You should be able to see the thermostat open when the water has reached 190°, approximately, it's ok. If not, bad thermostat.
If you can, leave the radiator cap off, and watch the flow across the radiator under it, IE the speed of the flow through the radiator, you can tell if the water pump is bad. Very slow, or no, flow across the radiator, bad water pump. Be careful! As the truck heats up, the level of the water in the radiator will rise, and overflow the radiator cap's hole.
Also, look under the shaft the water pumps pulley is mounted on. There's a "weep" hole there. If there's water coming out, unless is just a drop, bad water pump.

Hope something in there helps a bit...
Pat☺

 

just a comment in support of your test, when I have used my HF temp gun right on the Tstat housing, the reading is just about exactly the thermostat rating, 190*