That right there sounds like an air flow issue. While going down the highway, I switch my fan off. Rpm's are at about 3500 (overdrive sure would be nice) and the temp hangs between 180~190. I know, I have an open grill car. Oh yeah, and heat isn't boxed in "under" the hood either, but still. I think you need more ventilation. The fans being able to overpower the effect of incoming 75 mph winds isn't right.

 

that was my thought. the air dam is in place, i closed off the open space on the sides between the grille and core support, the rad is sealed to the core support, no a/c condensor for the time being, and removed the rubber strip at the rear of the hood to let the heat out. headers are also wrapped. i was thinking of diluting the coolant to 70/30 water/antifreeze and adding water wetter.

i was also thinking of diverting the trans cooler to its own stand alone cooler as well as adding an oil cooler. think that will help?

 

Darn, you already took off the rear weatherstrip, you're a step ahead of me! I was going to suggest an old school air flow trick, where you "adjust" the hood hinges so the hood is jacked up on the windshield end. I know you desensitized the k.s., but have you tried with it unplugged....just for fun? A stand alone trans cooler can help since the tranny fluid wouldn't be heating up the coolant, but I'm not sure that's going to cure all your ills. Hmmm, lemme see what, if anything else, the gray matter can come up with.

 

nah, the jdm hood prop is for ricers. i usually remove that strip on most my vehicles. helps in winter to keep the wipers from freezing too. i havent tried it unplugged. the engine maker said for timing to be at least 36* at 4k or else damage can occur. i figure a trans cooler added will take a good bit off the cool side of the rad as i know my trans runs hot right now. 2400 stall converter, 2.73 gears, and no lock up. not hot enough to cause any issues, but may be effecting the coolant enough. didnt check the the cooler lines. how about the oil cooler?

 

IF the only problem you have is it running HOT when going down the hiway with fans OFF and it runs cooler with fans ON then the fans are an obstruction in the air flow. The air coming thru the radiator is spinning the fans and making them an obstruction and building pressure between them and the radiator. At 75 you could remove the fans and never run hot, only need fans for stop and go and non hiway speeds.
Running stand alone trans cooler is so when engine runs hot it does not heat trans fluid and is better than cooler in rad. I'll bet if you removed fans and drove down hiway at 75 engine would run cool at 75 mph

 

So the air coming in at 75 is spinning the fan blades backwards preventing further air from coming in?

It might be time for a mechanical back up gauge to see exactly what it is in the head. I know even after I installed my be cool I ran hot for a little bit until all the air evacuated. Interesting note on the timing though, I was told NO MORE than 36* @ full advance or damage could occur...

 

Could you post a picture of the radiator and fan setup, I'm curious what it looks like. The fans themselves (turned off) would not be an obstruction, enough to make the engine run too hot anyway, as the incoming air would make them free-spin. The surrounding area beyond the fan openings (obstructive shrouding) could be though, which is where a pic may help.

 

In original post it seems the ONLY PROBLEM he has is with engine running HOT when driving down hiway at 75 with fans OFF, turning fans ON cools engine down while still at same speed.
Why not just hook fans to thermostatic switch to automaticly come on at like 180??

Fans are still spinning in right direction, just not as fast as air coming thru radiator.
Could be the fans are dragging a little and not freewheeling as fast as they should be.
I'm only going off of what he described in O P.

 

the car ran hot with my old setup, a 1600 cfm fan and a 900 cfm fan with no shroud. couldnt keep under 220 when 90* out, but would go down to 195-200 at highway speed with fan on. i am considering rechecking the timing. the numbers came off my balancer when i sprayed the intake with brake clean to clean up a mess, and cant find a new decal to put on. i may back some timing out of it. anywho, heres some pics. also, this is the fan i am running. Derale Cooling Products 16927 - Derale Performance High Output Rad Electric Fans - Overview - SummitRacing.com

 

Looking at the fan setup you have, I can understand why you have air flow issues; the incoming air gets stone-walled when it hits that flat shroud that's encasing the radiator. That doesn't explain why you were still having problems with the old fan setup though, but that's what I see with what you have now. I had someome in another forum having the same heating up problem as yours, and he too had a flat shroud surrounding the fan. Per my suggestion, he removed the shroud and mounted the fans directly to the radiator, and just like that his overheating problem was gone. If you look at yours logically, free-flowing incoming air can't keep moving if there's something in front of it stopping movement. That shroud isn't exactly a good aerodynamic shape. Now if it was tapered to where the fan was in essence mounted on the apex of a cone, that would be a different story.