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Discussion Starter · #1 ·
Hello fellow Outbackers,

I have a 2005 Outback 2.5 A/T that has recently began to act weird. The car works and drives flawlessly, no issues whatsoever apart from sudden loss of cabin heat when engine RPM goes above 4000 on the highway. Temperatures around here are currently around 0C (32F) so cabin heat is rather necessary.

Cabin heat works strong and reliably in urban driving and up to 120-130km/h when accelerating smoothly. When kick-down accelerating and RPM goes to 4000-5000, it is as if water feed to heater core cuts off and the air coming out of the vents goes down to outside temp within 30 sec. It stays cold until I slow down enough so that engine goes below 2000 RPM, then it goes back to normal.

I've bled the cooling system quite thoroughly a couple of times, the problem persists. Monitoring the coolant temperature through OBD, it stays between 90C and 95C (194F - 203F) at almost all times, so I believe the thermostat works correctly. Coolant temperature does not deviate from normal when the heater malfunctions.

I've never encountered such an issue, and my search online has so far been futile. I hope someone can point me to a possible cause as I am out of options now.

Thanks
 

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can you duplicate that while standing still?

might be a coolant line sucking itself shut, or a loose clamp on a coolant line opening.
 
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It does seem like a coolant flow problem but one other possibility is a vacuum leak. I have no idea if the door that directs air through the heater core is vacuum operated on Subarus. If so it is possible that a leak could cause the door to lose vacuum. Under heavy acceleration vacuum drops so if there is a leak in any of the vacuum lines there may not be enough vacuum to hold the mixture control door where it is set.

If vacuum isn't used on Subarus then this isn't relevant at all.
 

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2017 Outback Limited w/all the goodies. 2013 Outback Limited (sold)
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I had this happen to me in an old 4cyl car that after the head gasket was replaced the coolant had air pockets in it because the mechanic didn't bleed the system correctly. Ended up over heating the engine again. After second repair and proper coolant system bleed the heat worked again all the time.

Just my 2 cents.

Hope it's simple to fix.

Sent from my SM-G930R4 using Tapatalk
 

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Discussion Starter · #7 ·
So I just tried to replicate the issue at standstill and couldn't, which only adds to the mystery.

I checked radiator coolant level when cold before I bled the system this morning, it was at the filler neck.
 

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Discussion Starter · #9 ·
I use a home made version of this, a plastic bottle with the bottom cut out and the neck sealed at the radiator filler. I keep a couple of inches of coolant in and keep the car idling untill it gets up to temperature, then rev it several times until level in the bottle stops dropping. I also squeeze all the coolant hoses I can get to a few times through the procedure and keep the heater at max temp, min flow.
 

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Our heater hoses are formed and pretty dang rigid. I'm not saying they wont collapse, just unlikely.

If there have been no external loss of coolant, and not overheating, I'd say try burping it again. Making sure that the radiator fill is the highest point. And a little more flow (RPM's) for a longer period.
 

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Did a little quick searching and not surprisingly the actuators on the '05 Subaru appear to be all electric so my vacuum idea doesn't seem to fit here. I have seen vacuum leaks cause all sorts of problems on the older vehicles I am more familiar with though.
 

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If you are not moving and you hold the higher RPM do you have the same result? If you do, I would use a non-contact thermometer and check the coolant lines to the heater core to see if you have a drop in temperature to the heater core at the higher RPM. At the same time you can look for lines that may be collapsing. Looking at that should allow you to confirm or eliminate the controls/blend door.
 

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Discussion Starter · #14 ·
I've tried to replicate the issue at standstill but couldn't. In addition, it only seems to manifest on longer drives. I tried to repeat it yesterday after 15-20 mins driving, the car was fully warmed up but no issues with heat up to 6k RPM.

I fear it could be a gasket that trickles exhaust gas at higher RPM, and once enough accumulates it gets sucked into the heater core. Coolant level stays constant in the radiator though
 

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Discussion Starter · #15 ·
So I bled the system again with a funnel for about an hour, coolant temp reached 92C (198F) and all hoses were hot. No major air came out, but every now and then (say every 2 mins or so) an errant bubble would come out. Sometimes the bubble would appear when revving a little (blipping the throttle up to 2k RPM and letting go). These bubbles appeared all through the procedure. No bubbles when revving higher, or for longer. I don't know if this is enough of a symptom to point to head gasket, or if there is another explanation. As the system was running without pressure at around 92-92C at the time, maybe it is cavitation from the water pump.

After I got tired of bleeding, I took the car out for a drive. As soon as I reached the highway and hit 4K RPM the heat went. Coming down to 2.5K it came back again. Did this a couple of times quite predictably. I then pulled over, and attempted to recreate the issue at standstill, this time successfully. As I was alone, I could not monitor for sucked-in hoses and such.

What I learned from this is that the car has to be completely warmed up for this issue to manifest. Thats why I couldn't replicate it at stand still last time I tried, the car had been running for about 30 mins then which seems to be not enough.

At the moment, I am trying to find a way of testing the heater core valve, or so called Heater **** Solenoid Valve. I can try to bypass it completely, or disconnect the connector once I have it in the open position to see if it makes a difference.
 

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I don't believe there is a solenoid valve that restricts coolant flow through the heater core circuit. Generally, the coolant flows unrestricted through the heater core and the blend doors are used to regulate heat flow to the passenger compartment.

Have you identified what you believe to be a valve that regulates coolant flow through the heater core?
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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any difference with the proble if the the climate system is set to RECIRC or FRESH? or set to defrost?

no total loss of coolant? - radiator (always look when you have odd coolanr issues) and overflow levels are the same?

any chance of oil from a seeping cam or crank seal getting on the timing belt? you migh inspect for that. WP is driven by th e smooth backside of the belt.

Has the thermostat been changed? If OEM, they are are extremely robust, but anything can fail, if aftermarket - the typical small ones are no good in subarus.





because the wax capsule is smaller, there may not be enough hysteresis to keep the t'stat open with cool air flowing across it's 'down-under' exposed housing. not sure though (???)


as a lasyt resort, snding a coolant sample to Blackstone or polaris labs might help determine if combustion products are present.
 

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Discussion Starter · #18 ·
dougmac I thought there isn't one but it seems like there is. I tried to attach a picture in my previous post but it made the whole post dissapear. It's a black cylindrical valve, slightly smaller than an oil filter for example, attached to one of the tubes going in the firewall. The manual lists it as a Heater **** (Four letter rooster synonym that gets censored, starts with a C :D) Solenoid Valve.

1 Lucky Texan, I don't think fresh/recirc makes any difference, although I will test for that next time. No detectable loss of coolant, level in overflow goes up and down as it should, and every time I check the radiator coolant is up to the neck. All belts look dry, and for the moment I don't think it's a water pump issue - coolant temperatures stay normal when the problem presents, strong flow is visible when bleeding at high RPM and generally no problems with engine cooling are observed. Thermostat is OEM 82C (180F), I haven't tested it in a pan but both upper and lower rad hoses get hot at about 90C (194F), below that the bottom one stays cool which seems about right. Once again, I think I should have seen some coolant temperature anomaly if the thermostat was the culprit.

Thanks you all for the input, the investigation continues
 

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@ivomet

Where are you located? That valve might be unique to non-North American (or to EC) model.

(Found it mentioned in a 2004 EDM Legacy/Outback FSM; a solenoid-controlled valve in the heater line, controlled by the Body Integrated Unit. Have yet to figure out what criteria are used for opening/closing . . . )
 

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Discussion Starter · #20 ·
@plain OM I'm in Bulgaria, Europe, though the car is bought from Switzerland. It is MY05 body, though it was first registered at the end of 2004, so it could be a single-year engine or some such curiosity. The valve is described in the 2004 Legacy Service Manual at page AC-47, it is not present in the 2005 Manual.
 
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