Hello all. I have 1999 ranger 4.0l 4x4 that sucks the upper radiator hose flat at operating temp when I accelerate. Done a bit of research on the subject but so far I'm still clueless to what the actual problem is.
I just replaced all the parts associated with the cooling system (radiator, cap, thermostat, pump) so I'm sceptical that anything is malfunctioning. The heater is blowing very hot air as well so I don't think I have any air in the system either. Any thoughts?

 

Welcome to the forum

May need a new upper hose

But problem started with failing rad cap and maybe blocked overflow hose or tank.

When engine heats up the coolant inside it expands, that's what creates the pressure inside the engine/rad cooling system
The rad cap is rated for 14psi, at 15psi it is pushed open and warm coolant flows out to the overflow tank via the overflow hose, this happens anytime you drive more than 15minutes
Thats the HOT and COLD lines on the overflow tank

After you shut off the engine, coolant will cool down and shrink in volume, so pressure in the system will go down
If ANY coolant went over to the overflow tank while driving then pressure in the engine/rad will drop to -1psi, negative pressure
In the center of the rad cap is a smaller valve, this negative pressure will pull that valve down and SUCK coolant back in from the overflow tank, via the overflow hose
This keeps the rad always topped up with coolant

If the smaller valve should fail or overflow hose or tank is clogged up then negative pressure will go up, sucking upper rad hose flat

After this upper hose has been sucked flat a few times it won't hold its shape when heated up, which is why you need to replace it now
The reason it goes flatter with increased RPMs is because the water pump is pulling coolant out faster from lower hose which creates a lower pressure in upper hose as it tries to pull more coolant thru the thermostat, because its lost its shape from being sucked flat many times before, it will flatten out again, new one shouldn't

Lower rad hose going flat could mean clogged radiator

 

I've checked everything you mentioned. My overflow tank and hose is free of any clogs and the current radiator hoses both lower and upper are no more than 2 weeks old since I replaced them along with everything else mentioned in my first post along with a new cap as well. I have not seen the lower hose flatten out at all.
For slightly more detail the upper hose flattens during acceleration and also returns to shape after going back to idle.
Is it possible for a faulty thermostat to cause this since this only occurs when the engine is warm? And should I go ahead and get a new hose even though the current one is only 2 weeks old?

 

Its possible but would cause higher running temp
New hose should be fine


Have to ask this so don't get mad, lol.
Fan belt routing
You can, on most engines, install fan belt so water pump spins in opposite direction
This would flatten upper hose as you described

If fan is blowing air onto the engine then this is not the problem

 

No worries I've checked this as well and according to what I've found online and the sticker in the engine bay the belt is routed properly.

 

Just an update I think I solved the problem. Turns out there were two options for the thermostat housing a 35mm and 40mm. I had the smaller one and swapped it out for the larger. Not only did the upper radiator hose fit better on it but now the hose is no longer collapsing. Only theory I can think of is that the smaller housing was causing a choke point but that's just a guess on my part.

 

Thanks for the update

That was an odd one, never been asked for a size on my 1994 4.0l OHV

 

Ya it was pretty odd considering none of the hoses or thermostat had multiple options. just the housing did.

 

Hum... new one for me,