1988 3.0 gets pretty warm on grades on the highway. Needle goes to around 3/4 of the way up when climbing long steep grades (especially in warm weather). I’ve pulled real steep grades in 30-40 deg temps and my gauge do much. Anyways, as soon as I flatten out or go back downhill, the guage will drop back down to around the 1/4 mark. On a fairly flat stretch at around 65 mph it will stay around 1/3-1/2 mark which is normal operating temp I assume (only toyota Ive owned). When driving around town and on trails it stays at the 1/4 mark. It’s never gotten up to the red caution area on my cluster. After these heating episodes grades I’ll notice the reservoir gets drained to almost empty.

Backstory: When I first got the truck I replaced the thermostat and main coolant hoses with all OEM parts and purchased a new csf radiator and cap. Truck had slipped a timing belt shortly after and I ended up just dropping a new engine in (0 miles). Out of curiosity I opened up the original engine since the coolant I had drained out when I got it was nasty. As expected, the coolant passages were awful. Brown sludge and completely blocked.

Been putting this off for way too long now. From what I’ve read, the best route is pressure testing the coolant system and flush it. Im hoping junk from the original motor didnt get into the new radiator and clog it. Is there anything else I’m missing that would be causing these issues?

 

It probably did. Did you turn the heater all the way to hot when you went to flush it? Both front AND back (if you have a rear heater)?
Did you "burp" the system well after you flushed it? Again, with the heater turned up all the way?
Have you checked the operation of the fan clutch well? In other words, if the temperature gauge rises above about 1/2, does the fan start making a "whooshing" sound?
Is the fan shroud installed properly? That includes the little strip along the bottom.

Are you using a 50/50 mix of the Toyota Red coolant, with distilled water? If you use the Toyota pink coolant, obviously it comes pre-mixed, so you won't need to mix it with water.
When you flushed it, did you use any kind of commercially available flush chemical? There are several that work surprizingly well. Did you flush it with distilled water once the first flush was done with and drained out? Until it ran clear when you drained it? It could take 3-5 fill/run/drain cycles to clean out the ick that the flush breaks loose and then drains out. You should run it for about 20-30 minutes AFTER the thermostat opens. You can tell when the thermostat has opened by a sudden change in the temperature of the upper radiator hose.

Did you check the Temperature of the radiator during operation? Was it real hot where the upper hose enters it, but much cooler where the bottom hose comes out? Was the air flow through the radiator strong when the fan was operating? In other words, when you could hear a "whooshing" sound from the fan? Just like the first 30 seconds to one minute after you first start the truck. Do you feel a lot of heat coming out the radiator after the thermostat opens? All the way across the entire back of the radiator?

Ok, ok, I'll shut up now. I wish you all the best of luck!
Pat☺