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Old 02-09-2019, 01:38 PM   #1
bugeyerex02
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Default 02 Wrx P0128

Hey guys,

Acquired a project 02 wax a few months ago, and now that its starting to warm up a bit here in Vermont, I have begun tinkering and seeing what's going on under the hood.

Specs:

2002 WRX, 91k on the odometer. Was hit, motor pulled, heads completely redone, cleaned up the short block, reassembled and reinstalled after body work. I was TOLD this included new water pump, timing belt, thermostat, tensioners, the usual 100k maintenance on these vehicles.

However, it has an odd issue. The car takes ~5 mins to warm up (inner gauge reading above the first line). Turn the heater on, works 100% fine, plenty of heat. However, once you begin driving, whenever I get on it a bit (make any sort of boost), the temperature gauge drops to ice cold. However, I still have GREAT heat coming from the vents.

Once this happens, it takes awhile for it to build up temp again (at least according to the gauge), even though the heat in the car is fine. It also throws P0128. I get an active one, and a pending every time.

First thought is thermostat, but it doesn't make sense that if the thermostat failed, it should fail open, and I would get NO heat in the car.

Coolant Temperature Sensor?

No coolant leaks, no white smoke, no chocolate milk oil, so I'm not worried about HG.

any ideas?

thanks guys!
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Old 02-09-2019, 02:46 PM   #2
gagg maggot
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Check fan and thermostat operation. Sounds like you have an issue there.
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Old 02-09-2019, 03:04 PM   #3
bugeyerex02
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Can you elaborate? Fan as in radiator fan operation?
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Old 02-09-2019, 03:45 PM   #4
gagg maggot
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Quote:
Originally Posted by bugeyerex02 View Post
Can you elaborate? Fan as in radiator fan operation?
Yes the radiator fan. The fan cycles and the thermostat opens/closes to regulate coolant temperature through the engine. I'm pretty sure these cars have an electric fan and not a fan clutch. They work differently but the systems do same thing on all vehicles.

You only want the Subaru OEM factory thermostat or AISIN. Any other brand is junk. From what I understand, a t-stat that runs cooler than factory can mess with the tune and kill power. Whatever the factory temperature rating is: stick with that.

If your thermostat is simply stuck open, the engine will over-cool. Likewise, if your radiator fan is always on, it will also overcool. Neither is good. You can still get heat with both being bad, just depends how much it is overcooling.

This all makes sense for a P0128 which was a quick, easy Google search. You could have a bad ECTS but I doubt it, or you'd likely get no reading at all.

Last edited by gagg maggot; 02-09-2019 at 03:59 PM.
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Old 02-09-2019, 04:12 PM   #5
bugeyerex02
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Yes, I googled, which is why I'm here. Radiator fans do not run constantly. The reason I am leaning away from the thermostat is because I continue to get heat, even when the temp drops to ice cold on the gauge. If the thermostat was truly stuck open, wouldn't I get NO heat?
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Old 02-09-2019, 04:42 PM   #6
bugeyerex02
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So I just took it for a ride with my live data scan tool attached. Engine Coolant temps were 135-160 for the whole ride. Generally around 145 for normal driving, got up to 155 or so when going slow and dropped to like 135 after a hard pull. Full heat the whole time.

Threw P0128 (pending) when I got back to the garage.

Ugh.
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Old 02-09-2019, 10:50 PM   #7
gagg maggot
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Quote:
Originally Posted by bugeyerex02 View Post
So I just took it for a ride with my live data scan tool attached. Engine Coolant temps were 135-160 for the whole ride. Generally around 145 for normal driving, got up to 155 or so when going slow and dropped to like 135 after a hard pull. Full heat the whole time.

Threw P0128 (pending) when I got back to the garage.

Ugh.
That's cold, but warm enough to blow heat. Normal operating temp for most cars is 190-220. With your scanner on, can you read the sensors without the car running? I'd pull that ECTS and hold a lighter to it to rule that out. If your fan isn't constantly running then you likely have a ****ty T-Stat.
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Old 02-17-2019, 03:10 PM   #8
bugeyerex02
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Just as a follow up, it was definitely a ****ty t-stat. I replaced it with OEM this morning, coolant temps were 190-210, with HOT heat.

Thanks for the responses.
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