Engines cooling systems acting odd, somewhat like a blown head gasket, yeat no water/steam seen in exhaust, and none in oil.
Im wondering the direction of water flow. By looking at the pump and the fan direction, Im thinking the water goes IN at the intake and pumps out from the waterpump, pushing the water UP through the radiator.
If this is correct, Im thinking a thermostate stuck closed may be the problem.
Its pushing water out the overflow, so its eather that or a head gasket causing the pressure.
The temp goes fine then spikes and then sometimes drops off weird.
In mine it runs from the water pump to the top of the radiator. So it pushes in the top and it flows down through the radiator. just did a flush a week or so ago, so I know this for sure.
as a side note, I think it has to go that way, so that the water heats up in the motor, then the thermostat opens, lettin the hot water flow to the radiator, to get the proper temperature.
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Grover
1990 LX 5.0 HO-- center force clutch(heavy duty);Hurst competition short throw shifter; 3.55's;Pro-M air w/ K&N; 65MM throttle body and spacer;1.72 Roller rockers; 1/2 inch plenum spacer; Big Tube Headers; pizza box kit??; Fuel pressure reg and gauge; GT-40 heads(ported and flow tested); Pulley Kit; 24LB injectors, Chip(not sure which one); 180 thermostat; Flowmaster off road X-pipe(no cats); 110 lph fuel pump; mobile1 5-30 synthetic; Corbeau VX2000 racing seats.5 lug conversion..
There are two different types of waterpumps too. Standard rotation, which is typical with V-belts (it's driven by the inside of the belt), and reverse rotation, with the serpantine setup - it's driven by the outside of the belt - hence, in reverse.
I think it flows out of the engine, through the upper hose, (once the 'stat opens) through the rad, and out to the waterpump, which circulates it through the engine. There's also the 90º bypass hose which is always circulating.
Make sure you have the proper waterpump for your belt setup.
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1992 Deep Emerald Green Mustang GT Hatch, 5 speed, 2.73s, Mac fenderwell cold air intake, '93 Cobra MAF, Mac 70mm throttle body & spacer, Explorer intakes, Mac unequal headers & offroad pipe, Magnaflow catback, Walbro 110lph fuel pump, Jet adjustable FPR, MSD Blaster coil, Accel 8mm wires, FRPP aluminum quadrant, UPR firewall adjuster, stock cable, Granatelli upper control arms.
Waiting to go on: Granatelli lower control arms, and GT40 Iron heads!
Iv been running this for years with the same pump and no problems.
By looking at the waterpump I cant see how the water cant be coming OUT the lower hose, and into the intake. If this is right, a stuck t-stat would pressurize the upper hose causing the overflow to release the pressure....
What eer happened to the OLD manuals that showed these kind of details....
You're prolly right, seein how it is your car, and you should know it better than anyone ... I'd just change the thermostat, they are only a few bucks, and about half an hour-tops- to change.
I sure miss those old manuals too. Hard to come by. I had one for my 64 fairlane, that had intimate details, man that thing was handy- heavy too though.
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Grover
1990 LX 5.0 HO-- center force clutch(heavy duty);Hurst competition short throw shifter; 3.55's;Pro-M air w/ K&N; 65MM throttle body and spacer;1.72 Roller rockers; 1/2 inch plenum spacer; Big Tube Headers; pizza box kit??; Fuel pressure reg and gauge; GT-40 heads(ported and flow tested); Pulley Kit; 24LB injectors, Chip(not sure which one); 180 thermostat; Flowmaster off road X-pipe(no cats); 110 lph fuel pump; mobile1 5-30 synthetic; Corbeau VX2000 racing seats.5 lug conversion..
Iv been running this for years with the same pump and no problems.
By looking at the waterpump I cant see how the water cant be coming OUT the lower hose, and into the intake. If this is right, a stuck t-stat would pressurize the upper hose causing the overflow to release the pressure....
What eer happened to the OLD manuals that showed these kind of details....
a sticking thermostat will pressurize the upper hose. im pretty sure the flow is as stated before, starting at the waterpump, into the upper hose, down through the radiator, in the lower hose, back into the pump, through the block and manifold, and heads, then ends at the thermostat. when it opens, it recirculates. the heater core lines are the small bypasses on the side of the pump.
are you running the V belt setup on the BII or are all the front accessories from the stang you got the 5.0 from?
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Sr. Master Ford nerd.
saludos caballeros del ovalo azul
The engine and everything bolted to it are off a '88 Mustang...well ok everything on the front of the engine is.
guess once the snows gone, I will try a idea I had....
a foot or two of thread dropped into the fill in the upper hose will tell me what way its going....with me holding the other end of course.
If you look at the waterpump, and being the hose clamps on below center and the counter clockwise direction of the fan, I dont see how the water could come in at the bottom, but maybe there is something my brain dont wanna see...DAMN BRAIN
I think it would pressurize the rest of the cooling system as well.
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1992 Deep Emerald Green Mustang GT Hatch, 5 speed, 2.73s, Mac fenderwell cold air intake, '93 Cobra MAF, Mac 70mm throttle body & spacer, Explorer intakes, Mac unequal headers & offroad pipe, Magnaflow catback, Walbro 110lph fuel pump, Jet adjustable FPR, MSD Blaster coil, Accel 8mm wires, FRPP aluminum quadrant, UPR firewall adjuster, stock cable, Granatelli upper control arms.
Waiting to go on: Granatelli lower control arms, and GT40 Iron heads!
a stuck open thermostat will pressurize the upper hose because its not supposed to be open when its cold. when you first start the car, there will be immediate pressure throughout the cooling system.
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Sr. Master Ford nerd.
saludos caballeros del ovalo azul
If its stuck anything its stuck closed, not opened...
Again the fill cap is INLINE in the upper nose.
It pushes out water cold or warm, but isnt acting anything like a head gasket, other then the pressure in the upper hose.
Last I drove it, the temp went way up as it did the time before that, but last time it suddenly dropped off to a normal temp, leading me to beleave the t-stat was stuck closed, so not to let the water curculate.
IF the water flows INTO the intake, and the t-stat was stuck CLOSED, that would cause higher then normal pressure in upper hose and would explane EVERYTHING.
5.0 water flow direction.... 
