I went through the whole drain/fill/run/drain routine on my 88 GL (SPFI with what I presume to be a single row rad) today, did a straight distilled water flush cycle twice and the Prestone cleaner in between. Drained from radiator (duh) and pulled both head drain bolts each time, and filled it per the method shown by Miles Fox in his video series. Strangely though, I've never gotten it to take more than one gallon of coolant or water each time I filled it and ran it? Right now after finishing the job and refilling with 50/50 mix, squeezing all the air out of it I could, running it til the thermostat opened, the cooling fan kicked on AND then back off, and heater valve open/heat blasting, it still only took exactly a gallon of coolant.......

 

I'm confused, because it is  supposed to take 1.5+ gallons of fluid?!?

Heater core and hoses will hold some coolant. And the engine block does not fully drain when the head plugs are pulled. May still be some in the bottom of radiator. Whether all of that fluid adds up to 1/2 gallon, I can't say.

 

Do know the engine has coolant left in it after pulling the drain plugs. Had one on the engine stand to pull heads, rotated it to one side and coolant hit the floor. Same thing when I rolled it the other way to pull opposite head.

It took some driving for mine to "burp", then it took another quart.