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Discussion Starter #1
Modded my 2012 sti

Cobb access port
Stage 2 - SF intake and map
Invidia highflow cat down pipe
Corsa exhaust
Mishimoto racing radiator
Mishimoto 10 degree cooler racing thermostat

Intent is primarily a daily driver but also something that could be fun and be taken to the track / hpd or other enthusiast driving events.

Ever since the new thermostat and new radiator I have gotten the p0128 code for engine undertemp - as well as a check engine light.
I’ve also had flashing cruise control and the “s” drive mode - turning the car off and resetting the ecu temporarily resolved it.
I have burped the system.
tested for exhaust gasses - none detected
No leaks and oil is fine so is the water pump.

I have since flashed it to the econo map stage 2+ sf and taking it super easy till I can get to the bottom of this. I’m inclined to take it to a shop but want to exhaust every DIY effort I could do before doing so as I’m allergic to bleeding $. If you have any advice for things to try I’d appreciate the help or even speculation of what it could be.
 

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Discussion Starter #4
This was happening on the stock thermostat and stock radiator - the racing radiator was an upgrade that would help this problem given that the car is stage 2+ ... Gonna put a new coolant sensor and also ordered a new stock thermostat... maybe to swap back ... I called the guys at fast wrx and will take it to them next week. Who knows maybe the sensor will fix it before then.
 

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I can't speak to the issue on your car but from other vehicles I've owned it's usually the thermostat because they are designed to fail open. I've also personally not had much luck with anything with the tag racing slapped on.

Even if you solve this a giant radiator isn't going to help. Unless you are having issues keeping the engine at temp from sustained racing, or work, the radiator is a waste.

I would start simple. Check the sensor. If that doesn't solve it use OEM parts.
 

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Thanks for your reply - the car is modified stage 2+ so I’m not just willy nilly slapping parts on for no reason - the goal is to eventually be able to also take it to the track and drive it hard and these Whips are known to overheat.

It had a stock (new) thermostat - which had been replaced prior to the new shortblock and that has since been swapped now with a new aftermarket Mishimoto one albeit 10 degree cooler I figured it would help keep the temps down - why else would they sell it for the car?
I agree getting to the bottom of the issue is the main effort not covering up the symptoms with a larger radiator... so I’ve ordered another stock thermostat and sensor and will install the sensor once it gets to me and if not it’s off to the shop for the pro’s 2cents and I’ll bring the stock thermostat just Incase it’s needed.
I’m going to also try the exhaust gas leak test again. The prior owner crashed the car so hoses may be in order too.

I can tell that the new radiator already has made the situation better but something is still up.
 

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Thanks for your reply - the car is modified stage 2+ so I’m not just willy nilly slapping parts on for no reason - the goal is to eventually be able to also take it to the track and drive it hard and these Whips are known to overheat.

It had a stock (new) thermostat - which had been replaced prior to the new shortblock and that has since been swapped now with a new aftermarket Mishimoto one albeit 10 degree cooler I figured it would help keep the temps down - why else would they sell it for the car?
I agree getting to the bottom of the issue is the main effort not covering up the symptoms with a larger radiator... so I’ve ordered another stock thermostat and sensor and will install the sensor once it gets to me and if not it’s off to the shop for the pro’s 2cents and I’ll bring the stock thermostat just Incase it’s needed.
I’m going to also try the exhaust gas leak test again. The prior owner crashed the car so hoses may be in order too.

I can tell that the new radiator already has made the situation better but something is still up.
The radiator is specifically making your problem worse. If it's designed well it's increased cooling capacity is making the situation worse.

The car is reading under temp and a cooler thermostat was the solution? That's opposite of helping as well.

Let's go back. You had the vehicle rebuilt. When they rebuilt the block did they mention anything about using cooler thermostats? Even at stage 2+ I've not heard anything about overheating issues.
 

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Discussion Starter #8
No they used the stock thermostat. I which was new as I had it replaced at the dealer prior to them doing the new block. It over heated originally due to a cracked cylinder on the old block.

no leaks, no exhaust gasses, fans running ....

So I then personally installed the cooler thermostat and radiator as the overheating was happening with stock radiator and thermostat - figured it would help - it was only after the new radiator and thermostat that I got the p0128 - but even then after 10mins it still ends up overheating 235F monitored by my Cobb AP, thermostat gauge 3/4 of the way up and AC blowing warm.
 

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You've got bigger issues.

You said you burped it did you use the highest coolant point like the ej255 had the spot near the turbo. Did you run the heat full blast?

Did you by chance replace the water pump? Something isn't adding up in my mind
 

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Discussion Starter #10
I know that’s why it’s driving me mad. I have the ej25 it’s an sti -
I used a sealable funnel over the radiator which brought the level higher than the top port no real bubbles came out - squeezes the hoses and all that.
 

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Burp from that upper reservoir if you can. You may want to pressure test the system too.
 

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Thanks I’ll try that. So today I tried to heat it up fast and drove it a bit hard it managed to get to temp and not throw the p0128 code - then temps got up to 235F by the end of my short drive maybe 30 mins some stop and go. That doesn’t seem like a normal temp. Also I could have sworn in the past the radiator fans would come on after parking - they don’t seem to do that now. Is that a feature and should they come on when parked and the car is off after driving with elevated temps? I left it with the hood open to cool off. It doesn’t seem to overheat as much with the econo Cobb 2SF+ OTS map vs the 2+ SF ... wonder if a pro tune would be worth it / help.
 

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Discussion Starter #13
Anyone know - For a stage 2+ modded car what are “normal” coolant operating temps?
 

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Anyone know - For a stage 2+ modded car what are “normal” coolant operating temps?
Similar to stock.

170-190 deg C
 
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