4age cooling system 101

yikes · 06-26-2005, 09:31 PM

The water pump pumps water into the block, it circulates into the head and comes out of the head on the passenger side above the water pump. The water travels passed the cap and forward to the radiator, entering at the top. The water comes out the bottom of the radiator and travels rearward to the thermostat housing. When the thermostat is open the water travels through the pipe under the intake manifold to the water pump. This water travel is backwards of what it looks like it should be in the car. This also means that almost all of the water in the system must be heated before the thermostat opens. This helps the motor to get to operating temps quicker, but also means that if there is a delay in the thermostat opening, the block and head can overheat very quickly. It is for this reason that the system MUST be bled correctly. An air bubble by the thermostat would cause it not to open. I guess that is why there is a bleed valve there.
I have also heard that the flow of water through the head goes from #1 cyl through to #4 cyl. Therefore #4 cyl gets preheated water, gets less cooling effect and is the normal cyl to have head gasket problems and the exhaust manifold crack.

There is also an opening on the drivers side of the head. If the heater is on, water travels to the heater directly from the head and comes back to the back side of the thermostat housing. Therefore the heater does not need to wait for the thermostat to open to be able to provide heat. Also, if your car is overheating, turning on the heater can really help your motor. After all, if you are going to continue to drive, you deserve to be as uncomfortable as your motor.

When installing a thermostat, put the nipple (the triangle) into the side with the bleeder valve. The spring goes in the side with the heater hose on top (the hose that is returning from the heater).

There is also a hose on the backside of the thermostat housing that goes to the throttle body. When the water is hot enough it melts a bead of wax. This is what causes your idle to drop form 1500 to 800. Because of the age of our cars, this unit is starting to develope problems, though I have not had any.

There is a long and involved process for bleeding the cooling system. I have never done it. I park on a hill with the back end up after refilling the system and go to bed. Then I open the valve by the thermostat and close it. I do this for a couple of days or until I'm bored. But for your own piece of mind, go by the book.

Spectrum · 03-02-2007, 07:17 AM

Nice write-up.

I have an Auto model, and unless the car's up to temperature, the Overdrive is not available.

This is proving to be more of a pain as we start having colder mornings. As I quickly get onto a freeway on my way to work, it's possible to have travelled 40km (25miles) at 100km/hr (60mile/hr) in third gear before I can get the revs to drop back from a noisy, droning, fuel-thirsty 3700RPM as overdrive kicks in. If I get off the freeway for a bit, or pull over for a moment, it can help warm it up quicker.

If the car's already up to temp (and Overdrive engaged) and the temperature drops for whatever reason (cold freeway driving at speed, or I flick on the heater for the first time and flood the cooling system with unheated water from the heater's piping) then the Overdrive will actually switch off should the temp drop too far.

I'm dreading winter.

So, is it:
a) normal
b) a faulty thermostat that doesn't close properly?
c) or perhaps I could do with a few air bubbles caught in the line.
d) well, it's an auto which isn't a true MR2 anyway, so why am I even on this board?

Thanks in advance for any thoughts.

toyotaspeed90 · 03-12-2007, 03:53 AM

there are some flaws in your statements here though......

the water doesn't flow that direction..... 4ag pumps don't push water that way..... and all 4ag pumps (save the ae86 pump because it has a different pulley system for the RWD stuff) are the same.... hell, they're the same as the 4afe pumps.....

the pumps pull the water from the "upper" hose that runs off the oil cooler through the motor and out the back of it pushing the coolant through the large metal pipe on the back of the block.... the water inlet sits above the water pump on the head and is where the upper hose connects to (and this can easily be proven by opening the cap and watching the flow of the coolant).

The coolant gets up to temp in the engine quickly because the pump is forcing it through the motor, into the back of the t-stat which doesn't open until it gets to temp. The heater will heat up quickly because the thermostat housing has a bypass that leads to the heater.

also the hoses that run to the idle air control valve aren't on the thermostat housing.... they run to the 2 metal pipes that run the length of the motor on the intake side..... there is a 4" or so rubber hose that runs from the main pipe to the back of the thermostat housing

the thermostats will go in only 1 way..... instead of telling the direction it faces it would be more pertinent to explain that it's necessary to make sure none of the old o-ring has gotten stuck in the housing as to ensure a good seal of the new t-stat

as for "bleeding" the system.... anyone who does a lot of mr2 work (or a lot of cooling system work in general) should purchase a Snap-On RadkitplusA (or similar tool)..... don't fear the snap-on name... i got mine on ebay for $100 and they can be purchased brand new for $200, all you need is a compressor.... the last mr2 I used it on was an 86 with a 20V, and it took about 10 minutes to fill the system air-free w/o running the motor.

06-26-2005, 09:31 PM	#1 (permalink)
yikes Cage Fighter Join Date: Feb 2005 Location: Gnaw Bone, Indiana Posts: 230 Thanks: 2 Thanked 12 Times in 4 Posts iTrader Rating: (0/0% )	4age cooling system 101 The water pump pumps water into the block, it circulates into the head and comes out of the head on the passenger side above the water pump. The water travels passed the cap and forward to the radiator, entering at the top. The water comes out the bottom of the radiator and travels rearward to the thermostat housing. When the thermostat is open the water travels through the pipe under the intake manifold to the water pump. This water travel is backwards of what it looks like it should be in the car. This also means that almost all of the water in the system must be heated before the thermostat opens. This helps the motor to get to operating temps quicker, but also means that if there is a delay in the thermostat opening, the block and head can overheat very quickly. It is for this reason that the system MUST be bled correctly. An air bubble by the thermostat would cause it not to open. I guess that is why there is a bleed valve there. I have also heard that the flow of water through the head goes from #1 cyl through to #4 cyl. Therefore #4 cyl gets preheated water, gets less cooling effect and is the normal cyl to have head gasket problems and the exhaust manifold crack. There is also an opening on the drivers side of the head. If the heater is on, water travels to the heater directly from the head and comes back to the back side of the thermostat housing. Therefore the heater does not need to wait for the thermostat to open to be able to provide heat. Also, if your car is overheating, turning on the heater can really help your motor. After all, if you are going to continue to drive, you deserve to be as uncomfortable as your motor. When installing a thermostat, put the nipple (the triangle) into the side with the bleeder valve. The spring goes in the side with the heater hose on top (the hose that is returning from the heater). There is also a hose on the backside of the thermostat housing that goes to the throttle body. When the water is hot enough it melts a bead of wax. This is what causes your idle to drop form 1500 to 800. Because of the age of our cars, this unit is starting to develope problems, though I have not had any. There is a long and involved process for bleeding the cooling system. I have never done it. I park on a hill with the back end up after refilling the system and go to bed. Then I open the valve by the thermostat and close it. I do this for a couple of days or until I'm bored. But for your own piece of mind, go by the book.

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03-02-2007, 07:17 AM	#2 (permalink)
Spectrum No Skills Join Date: Jan 2007 Posts: 21 Thanks: 3 Thanked 1 Time in 1 Post iTrader Rating: (0/0% )	Nice write-up. I have an Auto model, and unless the car's up to temperature, the Overdrive is not available. This is proving to be more of a pain as we start having colder mornings. As I quickly get onto a freeway on my way to work, it's possible to have travelled 40km (25miles) at 100km/hr (60mile/hr) in third gear before I can get the revs to drop back from a noisy, droning, fuel-thirsty 3700RPM as overdrive kicks in. If I get off the freeway for a bit, or pull over for a moment, it can help warm it up quicker. If the car's already up to temp (and Overdrive engaged) and the temperature drops for whatever reason (cold freeway driving at speed, or I flick on the heater for the first time and flood the cooling system with unheated water from the heater's piping) then the Overdrive will actually switch off should the temp drop too far. I'm dreading winter. So, is it: a) normal b) a faulty thermostat that doesn't close properly? c) or perhaps I could do with a few air bubbles caught in the line. d) well, it's an auto which isn't a true MR2 anyway, so why am I even on this board? Thanks in advance for any thoughts.

03-12-2007, 03:53 AM	#3 (permalink)
toyotaspeed90 No Skills Join Date: Jun 2006 Location: Bellingham, WA Posts: 2,401 Thanks: 0 Thanked 148 Times in 143 Posts My Google Map iTrader Rating: (0/0% )	there are some flaws in your statements here though...... the water doesn't flow that direction..... 4ag pumps don't push water that way..... and all 4ag pumps (save the ae86 pump because it has a different pulley system for the RWD stuff) are the same.... hell, they're the same as the 4afe pumps..... the pumps pull the water from the "upper" hose that runs off the oil cooler through the motor and out the back of it pushing the coolant through the large metal pipe on the back of the block.... the water inlet sits above the water pump on the head and is where the upper hose connects to (and this can easily be proven by opening the cap and watching the flow of the coolant). The coolant gets up to temp in the engine quickly because the pump is forcing it through the motor, into the back of the t-stat which doesn't open until it gets to temp. The heater will heat up quickly because the thermostat housing has a bypass that leads to the heater. also the hoses that run to the idle air control valve aren't on the thermostat housing.... they run to the 2 metal pipes that run the length of the motor on the intake side..... there is a 4" or so rubber hose that runs from the main pipe to the back of the thermostat housing the thermostats will go in only 1 way..... instead of telling the direction it faces it would be more pertinent to explain that it's necessary to make sure none of the old o-ring has gotten stuck in the housing as to ensure a good seal of the new t-stat as for "bleeding" the system.... anyone who does a lot of mr2 work (or a lot of cooling system work in general) should purchase a Snap-On RadkitplusA (or similar tool)..... don't fear the snap-on name... i got mine on ebay for $100 and they can be purchased brand new for $200, all you need is a compressor.... the last mr2 I used it on was an 86 with a 20V, and it took about 10 minutes to fill the system air-free w/o running the motor.