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My cooling system works fine, but it strikes me as overly complicated. I don't know why they built it the way they did, and I wonder if I could simply it.
The one problem I do have is that the coolant level in the billet AST / thermostat housing seems to randomly flucutate by an inch or two, which trips the coolant level warning.
Any ideas of what might be causing this? Is the sensor simply placed an inch or so too high? I don't seem to be burning or leaking coolant, and the level in the overflow is usually at "full."
Maybe I just need to bleed it? In which case, how should I bleed this system?
If I was to refresh the system, do I really need to use a 1.3 bar cap?
Could (or should) I get rid of the swirl pot?
On an unrelated note, my car is finally street legal in Japan! (It only took me a decade to get around to it.)
So the purpose of the "swirl pot" is to act as an AST, right? And you have an AST / filler neck / thermostat combo thing where the stock filler neck / thermostat is.... And I see you have the stock overflow tank but you don't mention whether it is being used...
The top nipple on your AST near the intercooler is supposed to be for the coolant overflow, it looks like its going down to the radiator. That whole unit looks like a prime location for trapped air since your swirl pot near the firewall is acting as the source with the pressure cap and not the AST as it was designed for. This front AST is meant to have the pressure cap on it so any air would be pushed out of that top hose and into the overflow reservoir, then as the system cools, it would pull coolant back into the system essentially self bleeding over time. I would definitely tackle this as a "start over" project and look at what options are out there at the moment. You can probably ditch the swirl pot by the firewall and just run one in front. The one you have looks to be missing a nipple or two in the lower portion which we would typically see to go to the radiator and factory neck (which has been replaced by this unit.) There are other aftermarket solutions like what you have, but I'm guessing that front AST not being able to bleed is your main issue, there is nowhere for the air to go from the AST.
So the purpose of the "swirl pot" is to act as an AST, right? And you have an AST / filler neck / thermostat combo thing where the stock filler neck / thermostat is.... And I see you have the stock overflow tank but you don't mention whether it is being used...
Yeah, I'm confused by your setup...
The overflow tank is being used. How well it is functioning is not clear.
The top nipple on your AST near the intercooler is supposed to be for the coolant overflow, it looks like its going down to the radiator. That whole unit looks like a prime location for trapped air since your swirl pot near the firewall is acting as the source with the pressure cap and not the AST as it was designed for. This front AST is meant to have the pressure cap on it so any air would be pushed out of that top hose and into the overflow reservoir, then as the system cools, it would pull coolant back into the system essentially self bleeding over time. I would definitely tackle this as a "start over" project and look at what options are out there at the moment. You can probably ditch the swirl pot by the firewall and just run one in front. The one you have looks to be missing a nipple or two in the lower portion which we would typically see to go to the radiator and factory neck (which has been replaced by this unit.) There are other aftermarket solutions like what you have, but I'm guessing that front AST not being able to bleed is your main issue, there is nowhere for the air to go from the AST.
I almost wonder if the reason they added a separate swirl pot was because this thing https://www.auto-staff.net/tuning2/t-SRS-FD_2L.jpg (left) didn' take a radiator cap.
Athough there is a radiator cap version available.
It looks like they drilled a hole in the top of the AST cap at one point, but then plugged it back up.
I almost wonder if the reason they added a separate swirl pot was because this thing https://www.auto-staff.net/tuning2/t-SRS-FD_2L.jpg (left) didn' take a radiator cap.
Athough there is a radiator cap version available.
It looks like they drilled a hole in the top of the AST cap at one point, but then plugged it back up.
You would probably be fine running the radiator cap version of that and eliminating the swirlpot. Would have to cap off the radiator hose and loop the hose on the back of the thermostat housing but you need a good way to bleed the system.
I'm guessing OE parts aren't hard to find in JP so I'd just return it to stock-ish, AST and all, and loop or block off the turbo feed and return to be done with it. The AST eliminator isn't doing any good if the old AST has to be replaced with that because there's no pressure cap.
You would probably be fine running the radiator cap version of that and eliminating the swirlpot. Would have to cap off the radiator hose and loop the hose on the back of the thermostat housing but you need a good way to bleed the system.
That is what I was thinking.
I suppose the current setup has the benefit of bleeding air from the block via the rear AST (SARD swirl pot), regardless of whether the thermostat is open, while the front AST (Autostaff) was intended to bleed air from the radiator itself, but is probably failing at that.
Unless I am burning or leaking coolant without knowing it. Or maybe the system isn't sucking coolant back into the system.
The SARD swirl pot and Autostaff AST were actually added before the V-mount was installed. It seems he had a hard time with water temps before the v-mount.
Apparently, the air bleed nipple on the top tank was added before the stock-mount Koyo radiator was converted to a v-mount. Not sure how it was plumbed then.
Some old pictures suggest the Autostaff AST was originally plumbed differently.
I'm guessing OE parts aren't hard to find in JP so I'd just return it to stock-ish, AST and all, and loop or block off the turbo feed and return to be done with it. The AST eliminator isn't doing any good if the old AST has to be replaced with that because there's no pressure cap.
From a pure performance standard, the SARD swirl pot should be much more effective than the stock AST, since it shoots the coolant in an oblique angle. That's the best way to remove air, as far as I know.
The system works fine except the coolant level warning turning on. It might just need more bleeding.
May or may not be the case as you could have a joint pulling air on cool down but that sounds very much like the start of failure which I had. Took a long time to progress but got to the point where when hot I was getting an idle miss caused by coolant entering the chamber and starting on one rotor.
I'm thinking this was the problem, lol
I wonder if I should get a 1.1 bar, or stick with 1.3...
That could be part of the problem if it was sucking air back in there, but you still have the issue of that AST being an air trap. There is a reason why it keeps getting trapped in that specific area.
That could be part of the problem if it was sucking air back in there, but you still have the issue of that AST being an air trap. There is a reason why it keeps getting trapped in that specific area.
That's the point of an AST, so you don't get air pockets in other places, if the rad cap is drawing air on cool down, every time you top coolant up, it will expand and bleed to the overflow tank then draw air back from the cap.
That's the point of an AST, so you don't get air pockets in other places, if the rad cap is drawing air on cool down, every time you top coolant up, it will expand and bleed to the overflow tank then draw air back from the cap.
That's the original purpose of the AST yes, in this case it is not routed that way and does not have the pressure cap, so it does collect air, but has no good way of bleeding naturally the way you described. The port that would usually expand into the overflow is connected to the radiator, so all the air collects at the top of that AST now and eventually triggers the low coolant light if the air pocket gets large enough. So the cap is possibly drawing air into the swirlpot, possibly making it down to the AST and has nowhere to go because of the design.
Looks like you would probably want something like this RaceOnly version with the provision of the hose for the radiator and the overflow hose with the pressure cap. There may be other options out there but this was easy to find. Others may be able to chime in. Note the placement of the sensor vs the other where the sensor is at the top where all the air is supposed to collect.
Looks like you would probably want something like this RaceOnly version with the provision of the hose for the radiator and the overflow hose with the pressure cap. There may be other options out there but this was easy to find. Others may be able to chime in. Note the placement of the sensor vs the other where the sensor is at the top where all the air is supposed to collect.
If I need to, I can get a radiator cap boss welded onto my AST.
I’m wondering whether I should ditch the 75 C thermostat and get a stock unit. I don’t see what benefit it would have, but I know it takes a while to warm up.
If I need to, I can get a radiator cap boss welded onto my AST.
I’m wondering whether I should ditch the 75 C thermostat and get a stock unit. I don’t see what benefit it would have, but I know it takes a while to warm up.
Changing the thermostat won't make it warm up any faster or slower. The function of the thermostat is to keep coolant from cycling through the radiator until it reaches that temp. So the temp of the engine should increase the same with either thermostat prior to opening. If you want to keep your fully warmed up temps lower, then you have the lower thermostat. If you drill holes in it like some people do, then that will allow coolant to cycle through the radiator before its supposed to and cause the engine to take longer warming up.
FWIW, My experience with thermostats is that using a lower temp thermostat results in a lower peak operating temp. It took the same amount of time to get to the thermostat opening temp. Proved this out in my Spec Miata. I suspect the behavior would be similar in other cars.
I use a 180* tstat and get operating temps on track in race conditions of about 195*.
When using a stock tstat with a 190* ish opening temp, operating temps on track were always over 200*
When using a 160* tstat, operating temps in race conditions were around 180-190. Sometimes lower if ambient temps were low. Which is rarely the case here in hot California tracks.
YMMV but can't hurt to test it out on your own car.
From a pure performance standard, the SARD swirl pot should be much more effective than the stock AST, since it shoots the coolant in an oblique angle. That's the best way to remove air, as far as I know.
The system works fine except the coolant level warning turning on. It might just need more bleeding.
Maybe the SARD swirl pot is a superior part in a vacuum, but the way this system was set up by the PO is a mess, with long runs that aren't necessary hooked up in a questionable way. It is dead simple to bleed the stock configuration if you just disconnect the top throttle body hose when you fill the cooling system up, that's how I do it.
If the cracked seal on the cap isn't the problem, it's possible there's a leak in the recovery tank hose as well causing it suck air when the engine cools. The way that's supposed to work is that when the system is fully purged of air and the coolant expands enough to overwhelm the pressure cap it bleeds some coolant off into the tank, then when it contracts again while cooling down the little check valve in the middle of the cap lets it suck some coolant back in via the hose, which needs to be submerged at its end and leak free or it will just suck air. That's the whole reason for the AST in the stock system, they didn't really have room for an expansion tank configuration since it needs to be the highest point in the cooling system, so they put the AST in there and used an recovery tank. Flow through the AST is bypassing the radiator core, there is a pressure drop across the core so some coolant goes straight from the the filler neck to the bottom of the radiator near the water pump inlet, there is always going to be net flow through the AST when the thermostat is open. If the system is low, air will be pushed through the hose from the filler neck to the AST but it won't go downward to the radiator tank and will bleed out when the system pressurizes enough, the pressure differential across the AST keeps the air from flowing back to the filler neck. The recovery hose is deliberately notthe highest spot in the system because if it is, there is a chance it will never fill with coolant and when the system cools no coolant will return to it and fill the void. In the stock configuration there will always be a little bit of air in the filler neck above the AST level so that the AST should be able to purge all air from itself and the recovery tank hose and pull coolant back in when the system is cold. Plus the height differential between the coolant reservoir and the AST is a little less than the filler neck. People often eliminate it to no ill effect but it must be understood that the stock system was engineered with the intent of requiring no attention outside of the maintenance schedule in a variety of driving and environmental conditions, the way it is set up coolant level is able to be monitored entirely with the dipstick. It doesn't always hold up for decades, but that was the design intent.
Maybe the SARD swirl pot is a superior part in a vacuum, but the way this system was set up by the PO is a mess, with long runs that aren't necessary hooked up in a questionable way. It is dead simple to bleed the stock configuration if you just disconnect the top throttle body hose when you fill the cooling system up, that's how I do it.
If the cracked seal on the cap isn't the problem, it's possible there's a leak in the recovery tank hose as well causing it suck air when the engine cools.
The way that's supposed to work is that when the system is fully purged of air and the coolant expands enough to overwhelm the pressure cap it bleeds some coolant off into the tank, then when it contracts again while cooling down the little check valve in the middle of the cap lets it suck some coolant back in via the hose, which needs to be submerged at its end and leak free or it will just suck air.
That's the whole reason for the AST in the stock system, they didn't really have room for an expansion tank configuration since it needs to be the highest point in the cooling system, so they put the AST in there and used an recovery tank.
Flow through the AST is bypassing the radiator core, there is a pressure drop across the core so some coolant goes straight from the the filler neck to the bottom of the radiator near the water pump inlet, there is always going to be net flow through the AST when the thermostat is open.
If the system is low, air will be pushed through the hose from the filler neck to the AST but it won't go downward to the radiator tank and will bleed out when the system pressurizes enough, the pressure differential across the AST keeps the air from flowing back to the filler neck. The recovery hose is deliberately notthe highest spot in the system because if it is, there is a chance it will never fill with coolant and when the system cools no coolant will return to it and fill the void.
In the stock configuration there will always be a little bit of air in the filler neck above the AST level so that the AST should be able to purge all air from itself and the recovery tank hose and pull coolant back in when the system is cold.
Plus the height differential between the coolant reservoir and the AST is a little less than the filler neck. People often eliminate it to no ill effect but it must be understood that the stock system was engineered with the intent of requiring no attention outside of the maintenance schedule in a variety of driving and environmental conditions, the way it is set up coolant level is able to be monitored entirely with the dipstick. It doesn't always hold up for decades, but that was the design intent.
I don't quite understand why the put the SARD tank there, rather than, say, somwhere right next to the water pump.
It's a good thing I decided to replace the radiator hoses, since I found one of them was almost burned through behind the heat wrapping. Guess I let it get too close to the turbo.
I'm now just realizing that my description of the system is inaccurate. The turbo coolant feed is capped off, and the two hoses going to the SARD tank are both connected to the throttle body to water pump nipples. This seems to make even less sense, since the water should be flowing the wrong way.
Last edited by Valkyrie; May 27, 2026 at 11:50 PM.
I’ve spent the last few days trying to clean out the cooling system. I would have done the final fill with coolant, but I accidentally threw the block drain bolt out of my garage window along with the drained flush water. Woops… It’s a good thing the drain bolt isn’t NLA, or very expensive, and I needed a new crush washer anyway.
If the low coolant buzzer issue doesn’t resolve itself, I will probably cap off the radiator bleed plug and the TB return, and route the fill nec bleed nip to the SARD AST. That seems to be closest to the stock arrangement.
Note to self… it’s best to do radiator flushing outside, instead of in the garage…
It turns out I did not actually yeet my drain plug out of the garage; it was sitting my subframe.
I did lose the o-ring to my filler neck, so I'm going to have to source a new one before I can button everything up. It probably needed replacement, anyway.
