This is probably going to sound like a weird question, but hear me out.

My FC is an early '86 model, and originally came with the coolant expansion tank mounted on the drivers side wheel-well and the nipple for it on the thermostat housing. The hose went to the cap, and then down a straw type thing to the body of the tank. There was a little hole drilled in the cap to allow air to escape. Over about 6 months I would get some coolant evaporation and need to top it up.

A few years ago I had to relocate the tank, so instead I went with a generic aluminum tank mounted over in the corner by the passenger shock tower (where the ECU harness comes through the firewall). This tank did not have a vented cap so I put a rubber hose cap on one of the spare nipples and drilled a tiny hole in it so that pressure wouldn't build in the tank. I also used the nipple at the radiator, and swapped to the aluminum FD part with no nipple for the cap at the thermostat housing.

This worked fine, but resulted in a noticeable loss of coolant as it would evaporate through the hole I poked in the rubber cap. Noticeable enough I could smell it when idling on occasion.

Today I did an experiment where I capped the nipple on the expansion tank entirely, so that no air could escape. I drove around and the car seemed to cool a bit better than normal (although I'm probably just imagining it).

When I got back the coolant tank was really pressurized. I didn't pop the cap but it was difficult to turn from the outward pressure on it.

On the one hand it may have cooled a bit better with the tank pressurized (and I stress the "maybe", because I doubt it), but on the other hand I don't think the tank is supposed to pressurize that way.

It also makes me wonder if the rad cap was actually working as intended (my understanding is that the rad cap bypasses at 13psi and extra coolant goes into the overflow). If there's no place for the air to escape wouldn't it just max out whatever pressure the pump can provide?

Then again maybe my understanding is flawed. I recently did a coolant flush on my mom's Miata and the tank works by flowing coolant through it and then the cap just bypasses to a vent tube.

Any insights are appreciated.

 

Minor update, all research I've done tells me the tank should be vented or it won't allow coolant to be pulled back into the rad. Meanwhile I went out last night after the car had cooled and the rad was full of coolant, with the level in the overflow tank level exactly where I'd left it.

I'll probably put the vent back on, but I might try something to get it to not lose coolant anymore. Maybe a longer, upward sloping hose attached to the vent nipple so the steam condenses back into a liquid and drips back down into the tank?

 

ive seen people update the cooling system to the more modern style where there is a pressurized coolant air separator tank thing, and then the overflow can just be an overflow.
Mazda did it with the AST on the FD, 626/Mx6 and Rx8.
VW used that ball looking thing for years and years, Higgi used on in his FC for a long time too

HPI, has a kit actually, https://www.rhdjapan.com/hpi-evolve-...ment-38mm.html
you add that guy, and then keep the overflow standard

 

Tuning in... interested in this. I have an early S4 as well that I think originally had a driver side mounted overflow tank, but it has had a front mounted tank for as long as I've owned it. I've been chasing air getting into my cooling system that I have narrowed down to leak on vacuum/suction due to some bad rad caps, coolant seal leak (passes block test), or localized boiling. I've tested with OEM/tested working caps and still eventually find some air getting into my cooling system. My current setup is flat cap at thermostat housing, 13lb cap at radiator w/ nipple that flows to overflow tank at the front. Air will pool up at the thermostat housing cap/upper rad hose, probably because it's a high point. I might try switching back to a pressure cap & nipple here at the thermostat housing, in hopes that air will get pushed to the overflow tank and bleed off to atmosphere.

Not sure if you're aware of the TSB for early S4 regarding coolant loss, but it might add some insight. Solution was to move the driverside tank to the front (typical spot), change to a coolant level sensor with a nipple, then nipple with line that went to a little rad cap assembly mounted near the now front mount overflow. I'm curious why they decided to use an line at the radiator/coolant level sensor, rather than just use the original nipple by the thermostat housing. The added line would have to be pressure rated too, since it is on the pressure side of cooling sys. Maybe so that they can bleed the air right at the coolant sensor which will prevent the coolant light from coming on. Also original radiators didn't have a radiator cap on the radiator.

TSB attached, taken from alldata. Please excuse if I've misread any of the TSB lol

 

that is a good point, on an S4 there were like 3 different setups, and if someone mixed and matched you can have something weird

 

On my previous engine (with the stock setup) I also had air pooling at the thermostat neck. This is despite having a known-good rad cap in that location and coolant in the reservoir. I never did figure it out, but that engine was a disaster inside. Amongst other things it had broken coolant seals on two irons but never consumed coolant or overheated. So unfortunately it's hard to draw a conclusion as to whether the air collecting there was due to the coolant seals or the reservoir setup.

This is interesting. I knew they'd updated the coolant tank setup but it's interesting they did it specifically due to coolant loss.

If I had a bit of coolant evaporation over 6 months like I did with the stock setup then I wouldn't really mind. The only thing I don't like about my current setup is that the coolant dribbles down the inner fender and leaves a green trail, and I can occasionally smell it. I'd really rather not smell coolant so that when I do smell coolant I know there's a problem.

I wonder if it's due to the tank location. Maybe in the stock setup the coolant in the overflow hose is much colder by the time it reaches the tank and less likely to evaporate (since the hose is long and the tank is far away from sources of heat). In my case the aluminum reservoir is at the highest rearmost point of the engine bay on the passenger side, so it might be that exhaust heat is contributing to this.

What I might try is just attaching a hose to the overflow nipple and directing it upwards someplace. The theory being that the coolant will condense back into a liquid in the hose and drip back down, while still allowing air to pass.

 

Also tuning into this. My FC still has the driver side tank and I've always been interested in a) why the recall was never done and why it never got switched and b) if there are any real cons to keeping it in the driverside location (besides the overflow line dumping on to the coilpacks lol)