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Discussion Starter #1 (Edited)
I have a Spyder and swap the stock 1ZZ-FE to 2ZZ-GE engine. At beginning, the temperature runs fine. But suddenly one day the after I start driving, the water temperature gauge passed the midline. Luckily, I cold start the car and test drove only in my neighborhood for anything might happen. It is only 500 yards away from home. I was able to stop the car shut down the engine, let it cool down, and drive home carefully.

When that happen, I found the radiator upper was warm, but the lower hose is as cold as ambient temperature. It means the coolant is not circulating. My first thought will be a stuck thermostat. After doing some searches, I found it is not that simple. I did not bleed (brup) coolant correctly. The trapped air in the system might caused overheat.

So here comes to my questions, why MR2 Spyder is sensitive to trapped air?

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After doing some research, I guess the design of the engine coolant flow might be a reason.

Unlike the other engine design, the Toyota ZZ engine has a thermostat at the cold side, not the hot side. It controls the cool coolant flow back the engine, not the hot coolant flow out of the engine.

This is a "classic" engine cooling system:

78495

The thermostat is at the "hot" side. The current of coolant caused by waterpump constant flush the thermostat. When the coolant reaches certain temperature, the thermostat will turn on, and the coolant can go out to radiator.

However, in 1ZZ, 2ZZ as well as many new engine design, the thermostat is located at the "cold" side, which mean the water pump is constantly "push" the coolant away from the thermostat.

78496


78497

From above diagram, we saw when thermostat is closed, the waterpump (3) throw coolant over the engine block and then cylinder head. Then the coolant is return from cylinder head and hit thermostats. Because there is no gate at the water outlet, the hot coolant can get into the radiator pipe and travel to radiator (4).

If the system is full and the thermostat is closed, it is difficult for coolant to get out of cylinder head and reaches the radiator. The hot coolant can only diffuse slowly. Most of hot coolant will return from cylinder head and flush the thermostat, until it reaches the designated temperature and open the thermostat.

However if the system is not full, everything is different.

The MR2 Spyder's coolant hose is long and in large diameter, and located lower than the engine, which created a lot of space for coolant. Coolant can escape from cylinder head, flow into the space in the hose and accumulate somewhere. The activate coolant in the engine block is fewer and fewer and there is not enough hot coolant to flow back, hit thermostat and open it. Meanwhile there is not enough coolant to cool the engine and you have an overheat issue.
 

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Discussion Starter #3
You are kind of on the right track but not totally. I solved this issue 5 years ago. 2ZZ swap coolant system investigation
Thanks, I just read your post. You thoughts is pretty appealing.
When my car still run 1ZZ-FE, I use the bleeding procedure from factory manual and everything works fine. But 2ZZ-GE, I do have issue even follow the same procedure. Your thoughts that difference between 2ZZ and 1ZZ is probably true.
 

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The 1 zz and almost every automotive engine built in the last 25 years uses what is known as an "inlet thermostat". It controls temp going into the pump instead of the old way which was controlled at the engine outlet. The main reason this was done is that an inlet stat gives a steadier temp since their is much less hysteresis in the control loop. A steadier temp is better for emissions and fuel economy. Also the stat tends to last longer at the inlet since its average temp is lower. The location of the thermostat has only a little bit to do with how difficult it is to get all the air out of the system. The real difficulty is that the Spyder is mid-engine car with the radiator and heater core in the front, and you fill the system in the back.

I have written this a number of times, and will do it again: Follow the factory shop manual directions for filling the system!!!. You need to have the 2 front vents open with tubes attached and you need to take your time filling because in order for the whole system to fill from back to front the coolant has to pass through the small bleed hole in the thermostat and the small tubes to the coolant fill tank. Just keep filling at the tank until the coolant starts coming out of each front bleed point. Each time you pour more coolant into the tank the level will slowly drop and air is displaced through the front bleeders. The whole filling process can take an hour or more, but it will work every time and you do not need to even start the engine.
 

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I have a Spyder and swap the stock 1ZZ-FE to 2ZZ-GE engine. At beginning, the temperature runs fine. But suddenly one day the after I start driving, the water temperature gauge passed the midline. Luckily, I cold start the car and test drove only in my neighborhood for anything might happen. It is only 500 yards away from home. I was able to stop the car shut down the engine, let it cool down, and drive home carefully.

When that happen, I found the radiator upper was warm, but the lower hose is as cold as ambient temperature. It means the coolant is not circulating. My first thought will be a stuck thermostat. After doing some searches, I found it is not that simple. I did not bleed (brup) coolant correctly. The trapped air in the system might caused overheat.

So here comes to my questions, why MR2 Spyder is sensitive to trapped air?

---------------------------------------------------------------------------------------------------------

After doing some research, I guess the design of the engine coolant flow might be a reason.

Unlike the other engine design, the Toyota ZZ engine has a thermostat at the cold side, not the hot side. It controls the cool coolant flow back the engine, not the hot coolant flow out of the engine.

This is a "classic" engine cooling system:

View attachment 78495
The thermostat is at the "hot" side. The current of coolant caused by waterpump constant flush the thermostat. When the coolant reaches certain temperature, the thermostat will turn on, and the coolant can go out to radiator.

However, in 1ZZ, 2ZZ as well as many new engine design, the thermostat is located at the "cold" side, which mean the water pump is constantly "push" the coolant away from the thermostat.

View attachment 78496

View attachment 78497
From above diagram, we saw when thermostat is closed, the waterpump (3) throw coolant over the engine block and then cylinder head. Then the coolant is return from cylinder head and hit thermostats. Because there is no gate at the water outlet, the hot coolant can get into the radiator pipe and travel to radiator (4).

If the system is full and the thermostat is closed, it is difficult for coolant to get out of cylinder head and reaches the radiator. The hot coolant can only diffuse slowly. Most of hot coolant will return from cylinder head and flush the thermostat, until it reaches the designated temperature and open the thermostat.

However if the system is not full, everything is different.

The MR2 Spyder's coolant hose is long and in large diameter, and located lower than the engine, which created a lot of space for coolant. Coolant can escape from cylinder head, flow into the space in the hose and accumulate somewhere. The activate coolant in the engine block is fewer and fewer and there is not enough hot coolant to flow back, hit thermostat and open it. Meanwhile there is not enough coolant to cool the engine and you have an overheat issue.
I think you are not understanding that in an inlet thermostat system the thermostat is a double-poppet and that the temperature-sensing element of the thermostat is in the bypass flow path, so it always knows what the bypass circuit temperature is. As long as the engine is full of coolant the bypass circuit will flow and the thermostat will have no trouble controlling temp.

Dave
 

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2zz swaps have a checkered history of having overheating problems with the stock radiator. I threw a Mishimoto radiator into mine (some ppl have mixed feelings on if it’s necessary) & still had very intermittent overheating issues. I had a feeling it was related to running the overflow tank low when the car went into lift & then sucking air into the system. I found Funkycheese’s post about the restrictor piece on the overflow inlet tube which helped validate what I felt the issue was. I haven’t had an issue in months since adding that piece and bleeding the system yet again.
 
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Time for me to step in. I have done probably more than fifty (50) engine swaps and replacements on MR2's including MK1, MK2, MK3 over the years. Yes my former shop had a lot of history with MR2's. I have never, I repeat never had an overheating problem after a service that was not related to either A. cooling fans, B. blown head gaskets or just a stuck thermostat. I always followed the factory air purge procedure and lifted the rear of the cars VERY high in the air. I have had plenty of new customers bring in cars that were overheating due to air in the coolant system. In fact we had a flat rate just for the coolant air purge because it happened so often. I have also seen (very rarely) damage to the under belly hard lines cause issues.

One of my current cars is a MK3 that has a 2ZZ swap WITH a water cooled EFR turbo and the OEM radiator. I have zero issues at idle (100*F day), in stop and go traffic with AC on full blast or heavily loading the engine under boost for half an hour or so up in the thin Colorado air at 12,000 ft. The cooling system on these cars has decent capacity. You should not have issues with a simple 2ZZ swap or even a V6 NA. I have datalogs to show actual temperatures and not just a guess based on the dashboard.

TL:DR If your car is overheating you didn't purge the air from the coolant system correctly.

Protip: After you follow the coolant purge procedure let the car cool fully up in the air with the overflow fully filled. Once its cooled TOP OFF THE COOLANT TANK ALL THE WAY. With the car lowered on the ground cold. TOP OFF THE COOLANT TANK ALL THE WAY AGAIN. Close the lid. Go drive the car and extra coolant will piss out. Now your tank will not suck air. You will find that when it cools afterwards you will be at the max line.
 

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I read horror stories about the bleeding procedure when I did my 2zz swap and prepared for the worst. I literally just followed the procedure in the manual slowly filled the overflow tank with the bleed vlaves cracked and 'massaged' the coolant lines as I filled the reservoir. Never lifted the car or the reservoir. It filled up perfectly and sits dead center on the gauge. I don't know if I just got lucky or what.
 

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The specific issue I’m referring to is related to the additional drop of the coolant level in the reservoir compared to the 1zz. There are many threads from approx 10 yrs ago (and older) which talk about this as well as how the 2zz seems to have fewer temp issues with the coolant level filled at least to, if not slightly past the ‘high’ mark in the reservoir. I am not referring to an incorrect fill procedure or air existing in the system...I am referring to air being sucked in to the system once the overflow reservoir level drops too far during operation compared to when the system is not pressurized.

Essentially it ends up being what Node says about overfilling and letting it level off, but personally I’ve always just filled a little past the ‘high’ mark; never to the top.
 

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Since we're on the subject, my kswap threw an overheat code. Went to check and the reservoir is empty and nothing came out the rad when i cracked the screw where you put the hose to. Anyway I filled the system like dave mentioned and all is good except the line going to the reservoir never got hot even after 4 or 5 times the rad fans tripped. Is my thermostat stuck closed or should I give it another try? I even reparked the car in the opposite direction since there was a slight incline.
 
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