1989 318 coolant issue

I checked the.thermostat. It’s good. Rad is full. New pump. I don’t understand why it only does is when u turn it off and it sits for about 10 minutes. Then it pukes. Aj as usual I’ll have to read your post a couple of times. That’s a lot of info. My wife is about to drive it to town. We will see what happens. It seems to me that when it gets shut of it heats up (which is natural) and builds pressure. It never runs hot even when just setting and idling. I let it idle for over an hour the other day and it didn’t even try to get warm. As far as the lean burn system goes. I was really wanting to keep it as factory as possible. It is really a clean truck. But it does suck gas and is a sure enough dog as far as pose goes. So I may have to delete it. I’ll post some pics today of the engine compartment and I need to read the thread on deleting it. I saw it the other day but didn’t look at it.