79 318 over heating at 50 mph fine at idle new water pump tried chemical test no fumes in radiator been vacuum flushed 3 new thermostats new hoses and cap no leaks please help

I do not think this is thermostat , in city driving if it was thermostat the issue would be worse , if you have something clogging the radiator fins , mud , leaves etc... this could cause an overheat

Flush cooling system maybe you need to Replace the Thermostat or radiator cap.

Don't remove radiator cap when engine is hot.

Is your reservoir full to the right level? Check hoses for a leak. I would have the system flushed if there are no hose leaks in case air is in the system which can cause overheating. This can be done for about $50-$100 or less at many places. Sometimes when a thermostat is replaced the system isn't bled correctly. I hope its not your water pump, but it may be.

Good luck.

Try removing the thermostat and reverse flushing the motor, Actually try flushing in both directions, this will tell you if you have a circulation problem but 1 thing you should know is that the water goes in the water pump and out the thermostat, try running it without the thermostat and check your water flow

thet moment stop your engine and grab top radiator hose, feel the heat then grab lower radiator hose they have to be hot same heat. Top hose very hot is radiator blockage, Or lower hose very hot thermostat damage ,for test thermostat tide whith a string or wire have one glass cold water and juck boiling water deep thermostat in and you have to see the opening if is not then you have to replace it. Don ever youse radiator flush with the thermostat on. Blown head gasket means continual air bubble when you remove radiator cap special white froth,check for loose or damage hose and heter leaks when cooling system is under pressure.check low and high rpm exhaust fumes.

engine overheating could have low coolant level or water pump failing or radiator clogged need flushing.if i have over heating problems i will replace thermostat and replace radiator pressure cap.flush out radiator replace hoses and add 50/50 coolant and water.

Have a coolant pressure test done looking for a head gasket problem. as well as the fan

Flushing a radiator at home is most often a waste of time. The chemicals used by a radiator shop are far stronger than anything you get at the parts supply house. What happens is that the flow you see is deceptive as the water tends to flow around a partial restriction and does nothing to remove it.
Though your garden hose has a lot of pressure it doesn't have the volume necessary. A garden hose will usually be sufficient to flush out a heater core though as it's smaller.
Have a radiator shop flush your radiator core and have a hydrocarbon test done on the cooling system to eliminate the possibility of head gasket problems. When re-filling the system, turn the engine off just before it reaches 210 degrees and let it sit for a few minutes so the thermostat can open, then continue filling. Always fill with the heat on high. Thermostat goes in with the spring end into the engine.Forget about the water pump...unless you got one made for a twin engine marine (CCW) application it has nothing to do with the problem though I've heard but never actually seen that the belt can be installed so it turns backwards.
When the engine is at operating temp cover the grille and force it to nearly go into overheat...the electric fan should come on and the mechanical fan should lock up to try to counteract that.
Don't open the system when it's hot (you found that out already). And, don't allow the engine to actually overheat.
My bet without seeing it? Likely a partially obstructed radiator. 4.0 engines make a lot of rust residue even when maintained. If that's what you have in there, or even the 4cyl. It is more than likely.

This could be a multitude of things:

Stuck closed thermostat.

Cooling fan not coming on.

Clogged radiator.

Low on coolant.

Water pump failing/leaking.

Head gasket blown, leaking exhaust into antifreeze.

Clogged catalytic converter.

Thermostat: If the Van heats up, then feel the upper radiator hose. If it's cold, then your thermostat is closed. Replace thermostat.

Coolant: If low, simply fill with distilled water and coolant. Low levels will let engine over heat.

Radiator: If there are cold-to-the-touch spots on the radiator after it should be warmed up, then a clogged is probably occuring. Have radiator flushed.

Head Gasket: If you smell exhaust fumes in your coolant reservois tank, then you got a blown head gasket.

Cat Converter: Only can be tested if off the car. Or you could try a non-contact thermometer. These are also good to testing cold spot on radiator or if thermostats are open.

Cooling fan: If mechanical it'll be spinning. If electrical, it should come on automatically with the A/C, or when it gets hot.

Water pump: Will be leaking from the front if it needs replaced. On a van, it's a tough job.

Good luck!

Was the thermostat put in backwards? I know this is a silly question, but I've seen it happen. The water pump will fight it if it is. Another thing are the coolant passages, are they flushed out? Also, if that fan clutch is the original one, I would go ahead and replace that as insurance against the water pump breaking.