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How to bypass the heater core to stop the engine overheating

#1 User is offline   boostin 

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Posted 19 April 2010 - 9:52 AM

A blocked heater core is worse than you think. 90% of the coolant flow when the engine is cold is through the heater core. It is this flow that opens and keeps open the thermostat. If the flow is insufficient, and it doesn't take much to restrict it slightly, the thermostat won't open and you get overheating issues.

Bypassing the core completely is one option, but that causes another set of problems in winter with defrosting windscreens.

What I do is get two 5/8" plastic tee's from Repco and splice them into the heater hoses and run a link between them. Its a fairly cheap repair as its around $20 in parts. This ensures good coolant flow to the thermostat, but still allows some flow through the core for heating and demisting. You need 2 of the tee's (Repco part # is 28634EA), and 6 hose clamps to suit 5/8" heater hose.

Here's a shot of the twin turbo I fitted this to today.

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I find a suitable section of heater hose, chop out about 20mm of hose, fit the tee, and then do the same for the other hose. Then with a new piece of hose, join the 2 tee's together. Quite simple and effective, and takes about 30 mins at most.
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6 stars, 6 cylinder and 6 speed. 666. It's the devil car!!
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#2 User is offline   Shale 

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Posted 20 April 2010 - 3:30 AM

If I did it right...this is now a sticky ;)
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#3 User is offline   Max Headroom 

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Posted 20 April 2010 - 8:32 AM

I di the same mod to my BF7 when it overheated a few weeks AFTER fitting a recored radiator . . . .

Seems the fresh coolant/antifreeze dislodged some of the fine crud in the cooling system which slowly blocked the heater core. The result was the temp guage soaring while cruising DOWN a long hill under no load on a cold evening. The coolant was taking so long to get through the heater core that it was cold when it eventually emerged, fooling the thermostat into thinking the engine was cold and thus closing up.

The clue was that the radiator was stone cold at the thermostat end, yet it was hot at the other end. Initially we suspected a faulty thermostat (even though it was a new Subaru item) but it tested OK. We replaced it with another genuine thermostat as a precaution but the overheating remained.

Interestingly, I called in and mentioned this episode to the radiator guy who'd done the recore, and he was stunned. He said, "wow, that explains a lot. You wouldn't believe how often I've had customers cook their Subaru's not long after a recore!"
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#4 User is offline   boostin 

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Posted 20 April 2010 - 9:58 AM

I first did this on my BC7 about 10 years ago. It ran sweet until I hit the open road, and then it would steadily keep getting hotter.

The final straw was when we were heading down country one weekend. It started overheating on Auckland's Southern Motorway. I got pissed off with it doing it, so pulled over in Rotorua (only 220km away) and ripped out the thermostat. It stayed that way for about 4 months, until the weather got cold and I got sick of not having a heater for about 30 mins of driving. A bit of research and I figured out what was going on. The engine never overheated again, and I reused the thermostat I pulled out too.
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#5 User is offline   RS-YES 

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Posted 31 August 2010 - 11:14 PM

What's the story if my needle in the water temp. slowly lowers when I put the heaters on and/or when I'm doing about 110k(it's actually 100k, speedo reads incorrectly) on the motorway. Is that a thermostat / waterpump / heatercore issue? I'm always doing 90k's to get the needle to stay in the middle....
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#6 User is offline   boostin 

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Posted 01 September 2010 - 7:30 AM

Sounds like a thermostat issue.
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#7 User is offline   Alfred 

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Posted 15 April 2011 - 3:16 AM

Do you make the bridge as close as you can to the firewall for this mod?
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#8 User is offline   boostin 

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Posted 15 April 2011 - 6:24 AM

Nah, just somewhere convenient.
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#9 User is offline   ferrugem 

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Posted 01 June 2011 - 11:43 AM

Could this be the issue with my BF? Heater electronic components seem to work fine, just no hot air blowing through... :S
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#10 User is offline   boostin 

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Posted 01 June 2011 - 12:05 PM

It could be the control motor not working or the link arm disconnected. If you have ign on and everything off, can you hear the electric motors buzz when you change settings? Listen when you change the fresh/recirc button.
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#11 User is offline   spark_38 

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Posted 01 June 2011 - 1:14 PM

 ferrugem, on 01 June 2011 - 11:43 AM, said:

Could this be the issue with my BF? Heater electronic components seem to work fine, just no hot air blowing through... :S


I suspect the blower is poked - same problem I have

Luckily I have a spare wagon for parts, it's when I can be bothered to swap the parts over :lolz:
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#12 User is offline   ferrugem 

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Posted 01 June 2011 - 9:04 PM

 boostin, on 01 June 2011 - 12:05 PM, said:

It could be the control motor not working or the link arm disconnected. If you have ign on and everything off, can you hear the electric motors buzz when you change settings? Listen when you change the fresh/recirc button.


Yeah I've noticed I can hear a slight buzz sound when I change the settings of the heater/air con (Air con doesn't go either :L). Also, the other day when I decided to take it for a spin, I noticed the temp gauge just kept on gradually getting hotter and hotter, until I stopped the car. . . .
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#13 User is offline   boostin 

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Posted 02 June 2011 - 8:29 AM

Try putting some water in it. Low coolant level will cause slow overheat and the heater to not work.
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#14 User is offline   ferrugem 

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Posted 02 June 2011 - 10:40 AM

 boostin, on 02 June 2011 - 8:29 AM, said:

Try putting some water in it. Low coolant level will cause slow overheat and the heater to not work.


Done that. No signs of slow overheating, heater still a no go. Just buzzes/whirrs when options are selected..
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#15 User is offline   boostin 

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Posted 02 June 2011 - 10:58 AM

When the engine is hot and running, set the heater to hot and high speed, and feel both the heater hoses down under the intercooler. If one is noticeably cooler, the heater is blocked. This is due to the coolant passing through the heater core too slow, and all the heat is being pulled out of it.
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#16 User is offline   ferrugem 

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Posted 02 June 2011 - 12:45 PM

 boostin, on 02 June 2011 - 10:58 AM, said:

When the engine is hot and running, set the heater to hot and high speed, and feel both the heater hoses down under the intercooler. If one is noticeably cooler, the heater is blocked. This is due to the coolant passing through the heater core too slow, and all the heat is being pulled out of it.


Alright I'll try that. Could something be disconnected also? There is no air , cold or hot, coming through. I shall give it a look-see tomorrow!
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#17 User is offline   boostin 

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Posted 03 June 2011 - 6:37 AM

Oh ok. In that case, check to see if the blower fan fuse is blown, or that the fan is actually connected!
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#18 User is offline   ferrugem 

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Posted 04 June 2011 - 1:59 AM

I checked every single fuse and everyone one of them is fine (what a PITA...) So Im guessing the fan may be disconnected? How do I go about reconnecting it? Also, the AC doesn't have a belt on it so thats why it doesn't work at the moment...

EDIT: Remover heater blower motor, checked for continuity etc and the motor itself seems to be fine. Voltage doesn't seem to be going through the wires/circuit itself, so maybe it could be the fuse or a relay to the motor, or it might even be shorting out somewhere. Its showing about 0.2v through some of the connectors but not all of them :S Where is the heater motor fuse/relay located? Im guessing the problem is between the heater motor and the control switch, since the switch is getting power but the motor itself isn't? -_____________- I hate electrical.

EDIT2: If this website is anything to go by http://www.surrealmi...com/vrg3/fuses/ , then my fuses are fine, and Im stumped on what to do next :L

I decided to stop derailing this thread and make another one :P See http://www.dirtyolbc...-motor-problem/
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#19 User is offline   matdaymon 

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Posted 15 June 2011 - 12:06 AM

Your pic is broken, here is the same mod on my EJ22E
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Works a treat. Went from not being able to cruise the motorway 10km to being able to do Albany via SH17 to Silverdale and back along SH1 to Albany with the temp at normal the whole way. Very impressed
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#20 User is offline   boostin 

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Posted 15 June 2011 - 12:14 AM

Nice work. I'll fix the broken link now.
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6 stars, 6 cylinder and 6 speed. 666. It's the devil car!!
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