TheSamba.com :: View topic - coolant Subaru underbelly expansion tank advice

Tue Jul 29, 2025 5:20 am

Hi,

I need some advice. Coolant expansion tanks.

For a decade I have run a Subaru conversion I installed in my T2 bus and its been good.

Time has moved on, I have learnt a lot and I am questioning my coolant setup. I don't have a catch tank like a T25 (and others) and my coolant tank I think might be borderline for ok.

I have an under belly radiator, pipes run in on one side then back out the other. The underbelly radiator and pipework holds about 16 litres of coolant so quite a bit.

The expansion tank is appx 1.6L , so 10% of the setup.

But I am doubting whether its sufficient, should I have more, overflow/catch tanks or maybe a bigger 2.0L expansion tank just to give it a bit extra room. How would you install this if needed.

Or should I just leave it as it is, becuase it has worked and I'm over thinking it.

Picture attached. I have just got running again after a gearbox sheared spline problem and I want to get my head straight on this, don't want any more issues that I could avoid.

Very grateful for any advice. My reading so far seems to suggest more capacity in the expansion tank would be ok but its right as it is. and of course the fact its worked for so long..

PG

little more fluid to reach min level.

Tue Jul 29, 2025 7:02 am

IMO, Your over thinking the non-issue.

I have the same expansion tank in my 03 2.8 Eurovan and, never over flow the tank.

Don't fix what ain't broke.

Tue Jul 29, 2025 10:15 am

It's that with a Subaru underbelly radiator ?

Tue Jul 29, 2025 10:59 am

pgtips wrote: Is that with a Subaru underbelly radiator ?
No, just a stock VW 24 VR6 2.8ltr. with an automatic trans.

Tue Jul 29, 2025 6:15 pm

You're overthinking it IMO.

I'm running that exact same VAG coolant ball in my EJ25 swapped late bay, and I run a LARGE aftermarket dual pass aluminum rad under the belly. Zero issues with the coolant ball size for the past ~7 years on the swap.

I don't pay much attention to the min/max marks on the ball either. Usually just make sure I try to keep the min level near the min mark, but don't get hung up on a little over/under. I've never observed the ball being overfull due to expansion based on my coolant capacity.

Wed Jul 30, 2025 1:47 am

Vee Dub Nut wrote:
I'm running that exact same VAG coolant ball ...
I don't pay much attention to the min/max marks on the ball either. Usually just make sure I try to keep the min level near the min mark, but don't get hung up on a little over/under........

Thank you. That's given me peace of mind.

Nice setup by the way, the install looks clean. I still haven't decided where I want my air filter to go (L or RHS).
Is that a bleed valve you have on the bottom RHS coolant pipe. ?

thanks again though, that's what I needed to see, same setup etc.
I've just changed to 5w30 fully as well so it'll be interesting to see if it runs any cooler.
Pg

Wed Jul 30, 2025 5:17 pm

pgtips wrote:
Thank you. That's given me peace of mind.

Nice setup by the way, the install looks clean. I still haven't decided where I want my air filter to go (L or RHS).
Thank you! I've been very pleased with the end result. We've driven the wheels off of it since I finished it. Its been a great family cruiser

Quote:
Is that a bleed valve you have on the bottom RHS coolant pipe. ?
So the hose coming off the LHS coolant pipe goes to the small barb on the top of the coolant ball. This is the highest point in the system, and the main de-gas route for the engine.

The hose coming off the RHS coolant pipe is the large (~5/8" by memory) hose that connects to the large bottom barb on the coolant ball. This allows coolant to flow into the recirculation loop from the ball/reservoir, as required.

Quote:
thanks again though, that's what I needed to see, same setup etc.
I've just changed to 5w30 fully as well so it'll be interesting to see if it runs any cooler.
Pg
Happy to help!

Thu Jul 31, 2025 1:32 am

Hi,

My feed into the ball and out are on opposite sides to yours but its worked so maybe its not fussy.

I'll hopefully be out for a test run later if it stops raining.

Thanks again Dub Nuts, its helped me get my head right on it :-)

Thu Jul 31, 2025 5:01 am

pgtips wrote:
My feed into the ball and out are on opposite sides to yours but its worked so maybe its not fussy.

Are you sure? Looking at your picture, I think you're routed the same? Maybe we are miss-understanding each other?

Here is more detail on mine:

The engine coolant outlet side, LHS coolant pipe, has the small degas line that routes back to the top of the coolant ball.

The engine coolant inlet side, RHS coolant pipe, has the large hose that connects to the bottom of the coolant ball.

The idea is that as coolant flows out of the engine, any air will degas through the small hose into the top of the ball. That point on the LHS coolant pipe is the highest point in the cooling system. In operation, a small volume of coolant is always flowing through that line, continually providing degas to any trapped air in the coolant recirculation loop.

The coolant suction side of the engine is connected to the bottom of the ball, so that as any air degasses out of the system, air free coolant can flow back into the recirculation loop. This is also the path that coolant flows into the loop when filling from empty.

Hope that helps.

Thu Jul 31, 2025 11:28 am

Vee Dub Nut wrote:
Are you sure? Looking at your picture, I think you're routed the same

OMG, how embarrassing, in my head it was different to what it actually is and I'm the one who made it lol !.

yes, it is the same as yours I think I've had my head in the engine bay for too long :lol: .

Fri Aug 01, 2025 9:23 am

Ha! No worries, it happens.

The only thing I'd suggest looking at yours, is that your small bleed line coming off the LHS of the motor is lower than the actual outlet on the engine coolant manifold, which means you can still trap air.

Ideally you want that bleed line at the very highest point to properly degas.

In actuality you may be fine, but that is where you theoretically want it.

Fri Aug 01, 2025 10:15 am

Well spotted, I'll attend to that and move the pipe up.

If I can do it without introducing too much air and coolant loss I'll be happy. Might have to wait until September now.

Will keep in touch. Thanks again Steve !