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Discussion Starter · #1 ·
Hi guys,

I just replaced my radiator and thermostat with the "kit" from Mishimoto (the stock one went kaput). The thermostat I installed is (5-10°?) colder than the stock one. I didn't like how my engine temp gage stayed well over half way (1/2 to 3/4) most of the time and I figured it wouldn't hurt to run a little cooler. I'm currently in Florida so I figured better cooling would be needed anyway.

However, my check engine light just turned on the other day and I checked it only to find my engine thinks it's too cold now.

"Code P0128 - coolant temp below thermostat regulating temp."

I'm wondering if there's a way to get around this (or if it's bad to run the engine cooler, temp gage is at 1/4 to 1/2 now). It's kind of a pain to have to reset it every couple days.

Thanks
 

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Discussion Starter · #2 ·
Just drove my car pretty hard to work and the OBD read about 172 degrees after the drive (180 peak, took maybe two minutes to get to 160, and 8 minutes to get to 170).

The car is stock except for the radiator (and the AC condenser is broken so no AC).

I'm pretty sure everything is fine, just looking for a second opinion.
 

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I used to think a little cooler was OK. But don't any more. Engine was designed to run at a certain temp (with a 'range' of okayness.) The code is telling you it is outside what is expected from OEM design. I would be concerned if the cooling system (t-stat, radiator, pump, coolant passages, hoses, etc.) wasn't functioning properly, but I would not put in a cooler t-stat just to run cooler. You are probably "OK" with a cooler t-stat but engine is not running at optimal design state. Cooler engine will affect all sorts of things: emmissions, sealing of gaskets, power,.....)
 

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I would reinstall the OEM thermostat. There is a reason the thermostat is set to the OEM temp.

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Discussion Starter · #5 ·
From what I understand the engine is fine if I drive normally, but if I drive too much like a granny it takes longer than the car thinks it should to reach operating temperatures (and it trips the check engine light). I'm surprised this hasn't happened more often on here. I installed the thermostat recommended by Mishimoto, it's making me wonder how much I should trust them now.
 

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Did Mishimoto give a reason for the suggestion?

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Discussion Starter · #7 ·
No, that's the only thermostat they sell though, made for 1993-2010 impreza's. I just did some reading and the stock t-stat opens at 172.4F, there's opens at 154.4F ...seems like a pretty big jump to me! Worries me a little.
 

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Discussion Starter · #9 ·
Yes, it is bad to run an engine below its optimal operating temperature.
Do you happen to know what is "hot enough?" I've heard if you're above 170F that's enough to burn off any bad stuff (not sure what it needs to burn off ...maybe fuel/oil/air contaminants?). After about 20 minutes of driving my engine is around 175-180F, it used to be around 215-220F (which I thought was way too hot).

Still haven't been able to find what the recommended operating temp is of my engine. I'm guessing it's around 172F though based off the OEM t-stat.
 

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Do you happen to know what is "hot enough?" I've heard if you're above 170F that's enough to burn off any bad stuff (not sure what it needs to burn off ...maybe fuel/oil/air contaminants?). After about 20 minutes of driving my engine is around 175-180F, it used to be around 215-220F (which I thought was way too hot).

Still haven't been able to find what the recommended operating temp is of my engine. I'm guessing it's around 172F though based off the OEM t-stat.

The main concern I would have is actually the oil temperature (since the stock "oil cooler" is basically a heat exchanger between coolant/oil.. helps warm up the oil faster when the engine is cold, and cool it down when the engine is hot). You want your oil temperature in the 210-220°F range when fully warmed up so that it can burn off moisture and contaminants.

The other thing I would be concerned about is how the car is tuned via input from the coolant temp sensor. If your sensor is reading low coolant temp then the ECU is going to be running the engine in warm up mode all the time. This will effect fueling (it runs a richer mixture during warm up) as well as other things (I wonder if it could be low enough to trigger the TGVs to close since you're getting a CEL for it). None of that is going to be good for the engine.


Ideally, I would want to see warmed up coolant temps in the 180-200°F range, topping out around 210-215°F. That's the range the engine was designed to function at.
 

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Discussion Starter · #11 ·
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Ideally, I would want to see warmed up coolant temps in the 180-200°F range, topping out around 210-215°F. That's the range the engine was designed to function at.
Thanks! That's exactly the info I was looking for. I think I'll buy an OEM t-stat and stick that back in.

I don't have a tune so it makes sense to stick with what the engine thinks it has (as you said).
 
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