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Discussion Starter #21
First of all if the pump is faulty, then a larger radiator will indeed cause a larger pressure difference accounting for the poorer cooling.

Also any time you play with the cooling system, radiator, hoses, whatever you can get losses somewhere that create air pockets no matter how well you think you have burped it, flushed it or added enough coolant. Sometimes it takes up to 3-4 attempts of burping and/or adding more coolant to fix it.

Consider waiting until car is cold, like first thing in the morning, get up 20min earlier, and then opening the cap to the turbo reservoir and inspecting to see the coolant level there. If it looks sparse, add some.

Also be sure to look at the reservoir next to the radiator as it should read between full and low. There are two lines, one about 1/4 from the bottom, the other 1/4 from the top: your level should be in between that. Don't necessarily add it there though, it's more efficient to avoid bubbles if you add from the turbo reservoir. Keep doing this until your level is as high as it will be. If it won't rise but the coolant still has not hit close to "full" in that reservoir next to the radiator, then add more coolant directly there.

You may have a pressure drop from the faulty pump, but it also sounds like you may have air bubbles somewhere which can interfere with temperature sensing and you get these unstable, quickly changing coolant temps. The above should be the easiest, low cost, low time first step to resolving it.
Thanks for your tips it is really informative. but why the same behavior is not happening in the stock Radiator.

1. I drained the coolant installed the KOYO. installed the Greddy Sensor in the KOYO added new coolant in the Radiator slowly. started the car and started adding slowly in the turbo reservoir. took the car for a drive and logged the Temp. it reached as per the log 105C (221F)

2. I burped the car again for 20 mins adding more coolant. took the car for a drive and logged the Temp. I took exaclty the same route reached the same 105C (221F)

3. I drained the coolant installed the stock Radiator, move the the greddy sensor back on the hose using the greddy adapter. added same coolant in the Radiator slowly. started the car and started adding slowly in the turbo reservoir. took the car for a drive and logged the Temp max i reached. 92C (197F)
 

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You may not be sufficiently burping the larger KOYO rad. You may be getting all the air from the stock but leaving some in the KOYO.
Like others are saying, its hypothetically impossible against the laws of physics that a smaller stock radiator is cooling more than the KOYO.
There are other variables we aren't considering here,
 

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If he's able to burp the stock radiator just fine, I'm sure he's doing a fine job at burping the Koyo. However, even with a spill-free funnel, it will take some time to ensure all the air is out (multiple heat cycles).

Just because the piece is bigger, does NOT mean it cools better. Given everything is the exact same design, I would agree with you, however, don't let size be your total indicator. With that said, Koyo is a well built radiator, so I'm assuming there is either an issue with it that may need warrantied, or something is being overlooked.

Are you reusing the OEM cap for this radiator?
 

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There are a few key things that make heat exchangers work.
1) Surface area
2) Hot side flow rate, and temp
3) Cold side flow rate, and temp
4) Turbulence
5) Enough fluids
6) Construction material that is conducive to transferring heat, but will survive the cold/hot media, and elements ( I would assume this isn't a issue).

With that said I would start checking flow rate I.E. is the internal fluid making its way through the rad fast enough? If it is then check if the air is flowing through the fins fast enough. If it is then assuming that the rad is made of the proper materials you can start checking for things like Air pockets (burp the system). Is your thermostat ok? Is the water pump ok? Does hot air come out of the vent? There are alot of things that can contribute to loss of cooling in a car. Do your research, make a list, and go through them starting from easyest to hardest.

Hope this helps

Scott
 

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Discussion Starter #27
I returned the Koyo Radiator to the owner. Thanks for your support guys. I will try to buy another radiator and test. Should I get another koyo as I suspect this one was faulty.
 

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Discussion Starter #28
I should have replaced the Thermostat with the Radiator with lower temp one. Just wanted to update the thread.
 

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a larger radiator will indeed cause a larger pressure difference accounting for the poorer cooling.

Is a Koyo taller, thicker or wider than stock. I thought thicker but not going to look it up on a cell phone. Wider or thicker should result in lower pressure drop. Only taller should increase pressure drop. There are other dimensions that affect pressure drop, but generally i would expect a decrease in pressure drop in our cars with common larger radiators that hold more water and fit in the same place.

The water flows top to bottom, correct?
 
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