Help with 91 Legacy Postal(RHD) wagon heater core

[BoostedSubie]

Hello all, been awhile since I've posted on here. I'm working on a 91 Legacy postal wagon for a customer and have stumbled across something I've never seen on a 1st gen and curious if anyone has more knowledge about it. On the Temp/Heater controls, there is a button that says "On Heating", I had never seen it so I dug into the dash and found that the button actuates a valve via a vacuum line up by the heater core on the left side of the dash behind the glove box(RHD). The vacuum line was not connected when I got the car and it came to me with overheating issues, I ended up having to put a new engine in it. Well, I hooked up the vacuum line and the valve works now. Now, I am having issues burping all the air out of the coolant system and I have triple checked every possible scenario except the heater core. So, I'm curious to know if the valve is just a flow valve that slows down coolant flow through the heater core so it can dissipate more heat to the interior while the car warms up. I believe there is air trapped in the heater core somehow because I don't think it's a bypass as it only has one IN and one OUT, unless it has a bypass built into the heater core housing. Also, the heater hoses were hooked up backwards from a LHD car but Im also wondering if it is supposed to be since the shape of the hoses look like they are meant to be reversed from LHD cars. There are no leaks and the system holds pressure but the car will run on the hot side to where the fans want to stay on. Has anyone had experience with this mechanism and how it works, thinking maybe something is screwed up with it, maybe that's why someone unhooked it in the first place and what could of been causing the previous overheating issues.

Thanks

[scoobiedoo22]

Do you know how old the coolant temp sensor is on the new engine as they are notorious for going out leading to both fans running continually??? When you say it runs hot, are you using just the fans to gauge that or are the temps above normal on the coolant temp gauge? Usually, if you have air trapped in the system, especially the heater core in my opinion, your temps will run slightly above normal and then will spike and then drop back down as soon as you turn off the ignition. Standard practice for me on any need to flush coolant is to top everything off, crank the heat/fan in the cabin, and let the car idle while working the throttle every now and again to push out any of that locked up air in the heater core. Feel the supply and return hoses also as the engine starts to heat up as you should notice the temps rise in both as you get up close to operating temps if they aren't air locked.

[BoostedSubie]

Both coolant temp sensors are new, and I have a new aftermarket gauge monitoring coolant temps. The temp has gotten as high as 210°, with the fans coming on around 200° and continue to run because the temps won't drop back below 200°. I've always done the same procedure for bleeding as you mentioned except I use a burp funnel, basically a big funnel that locks into the radiator cap so you don't make a mess and makes it easy to see air bubbles coming out, I'm sure you've seen them. I had some luck last night, got more air out after running it for probably 45 min while pumping all hoses including the heater core hoses, also turning that formentioned flow valve on and off. I drove it around and it finally is running satisfactory temps. I'm just curious as to why they use that valve on the postal wagon and what it's intended purpose is.

[scoobiedoo22]

Sounds like one serious case of air lock! Glad you got it worked out of the system. If you find out any more about the postal valve throw up a quick update for posterity.

[rallyak]

Just curious, it the thermostat OEM or aftermarket?

[Legacy777]

Just curious, it the thermostat OEM or aftermarket?

That was going to be my question. If you don't have a Subaru thermostat, replace it.