Thermostat Slow to Open on Subaru 2.5L Engine

[youderian]

Would appreciate any help with this cooling issue.

I recently replaced my coolant lines (new SS) and heater core lines. In the process I also removed the rear heater core lines. The rear heater core was removed years ago but the previous owner kept the coolant lines in place and just connected them to form an open loop.

Now I'm having issues with the thermostat taking a really long time to open when the engine is cold. It will take probably ~10 minutes driving with temps creeping as high as 215 to 220F (scan gauge) before the thermostat finally opens.

Once open and flowing to the radiator all is well and temps are normal in the 180 to 190 range.

I'm guessing the open rear heater loop allowed hot coolant to flow more easily to the thermostat. But when I removed that loop the hot coolant is having a harder time getting to the thermostat to open it. Backing up this case is that the engine seems to run cooler and the T-stat opens earlier when I open up the front heater and allow coolant to circulate.

So my question: what's the best way to get hot coolant over to the Thermostat when the engine is cold? Should I install a bridge from the hot and cold heater core lines to replicate my setup before? I've heard Tom Shields has some kits to this end but not sure if he's selling them anymore. Something else?

The conversion was done originally by Buslab. Appreciate any thoughts you may have, thanks!
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1990 Westy Syncro with 2.5L

[vanis13]

Yes, you removed a feed-back loop.

so many different solutions. Many on here have done this swap.

Have you asked BusLab? they would be the most familiar.

Sketching out your current layout would be the easiest for us to comment on.

Possibly for the most comprehensive ideas may be at this group as that is all this group is about - https://groups.io/g/SubaruVanagon
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83.5 Westy with Subaru 2.5, 4 spd manual, center seat, COLD A/C on 134a!, Winter camp heated with an Espar B4 gasoline furnace

www.SuperVanagon.com - some stuff I make

[vanis13]

PS, my favorite solution is the water pump that has an extra nipple in addition to the heater core return one. Plumb a line from the coolant manifold to the second nipple and it duplicates the Tom Shields solution.
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83.5 Westy with Subaru 2.5, 4 spd manual, center seat, COLD A/C on 134a!, Winter camp heated with an Espar B4 gasoline furnace

www.SuperVanagon.com - some stuff I make

[skemems]

Does seem you removed your feedback or bypass to the thermostat.

You can post pics of your current setup or a drawing to comment on.
There are also a few threads on coolant diagrams.
Here is an example

https://www.thesamba.com/vw/forum/viewtopic.php?t=747918&highlight=diagram
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87 VW Westfalia - EJ25
86 VW Westfalia - EJ22
85 VW Westfalia - WBX 2.1 project

[alaskadan]

Yep. Did you remove the rear heater lines altogether? As in now you have two straight runs all the way to the front heater? If so I'd put a "T" in both lines and a short pc of hose between them. There needs to be a restrictor inside that short hose that reduces the 5/8" ID to 3/8". I used a pc of 5/8" steel rod with a 3/8" hole drilled through it and jammed it into the hose. Do this somewhere towards the rear of the van. This assuming that your feedback loop was working fine before and all you did is remove the crossover hose.

Makes me wonder about air trapped in the front core after draining the system. Without the rear crossover hose would trapped air really inhibit feedback? Or would air be forced out of the front core easier because the coolant doesn't have anywhere else to go?

[wesitarz]

Two PEX Tees 3/4x3/4x1/2 between the two heater lines near rear of engine is a quick fix. Or 3/4x3/4x3/4 Tee off hot heater line through expansion tank to a 3/4 to 1/2 restrictor to heater return

[Wildthings]

I see no reason the T has to be towards the rear of the van, instead put it where it is easy to access. Mine can be accessed by dropping the spare. I bought 3/4 x 1/2 tees and used a piece of 1/4" ID hose that fit snugly inside of a piece of 1/2" hose as an orifice.

[youderian]

Thank you all for the replies! This help a lot and, yes, I removed the heater lines that previously went to the rear heater core so I think this is the issue. I'll make this change and report back to confirm results.

Two quick follow up questions:

Can someone help me understand what about the design of the Subaru engine block requires this to be needed? The thermostat sits in the coolant in the engine block so I always assumed it would warm up along with the rest of the coolant. But obviously it doesn't.

Is it because it's at the bottom and the hot coolant naturally rises, so the warmer temps don't transfer down to the TStat for a while? Or some other reason? Just seems like a questionable design to need to separately route hot coolant from the heaters back to the Tstat to get it to open.

And secondly, whats the reason the bridge line across the hot and cold heater coolant lines need to have a reducer? Is this related to ensuring enough coolant flows to the front heater when needed vs. just taking the easy shortcut back to the block?

Thanks!
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1990 Westy Syncro with 2.5L

[wesitarz]

The Subaru t-stat sits at the bottom and cold coolant in. The Subaru is designed with an always open feedback loop through it's short heater system. Early on with conversions it was determined that you only need a 3/8 inch opening (1/2 inch) hose fitting for the feedback loop using the heater lines for MOST applications and still get good heat to the cabin. Tom Shiel's sandwich adapter has a 3/8 inch inlet and Smallcar's feedback to the black heater pipe is 3/8 inch and is connected straight to the coolant manifold. Some have used smaller (1/4 inch) or full 5/8 inch (RMW). Do what works for your climate.
Using the rear heater full 5/8 inch as a feedback loop is the least desirable. Using the water pumps with a full 5/8 inch extra nipple also works.

[Wildthings]

The Subaru system is designed to prioritize flow through the heater for use in colder climates. Since the Subaru heater loop has no valves in it it will give full flow through the heater during warm up bathing the thermostat in luke warm water even after the water in the engine gets quite warm, this allows the engine to produce even hotter coolant making the heating system that much more effective during the coldest weather. I did my conversion a dozen plus years ago, and went back in maybe six or seven years ago and changed the bypass design my system used, it has worked well since.

[alaskadan]

So what about my thought earlier, that was whether or not totally restricting the bypass loop to purge air trapped in the front core after coolant system work?