This one has had me stumped for about a week.

I did some major work on the coolant system (Dropped the radiator, flushed, RX8 fan upgrade, thermoswitch, new coolant sensor). On the plus side temps are very stable and low now, with the engine never breaking 95C.

However, I've noticed every time I drive it there's a small amount of coolant missing at the cap.

At first I chalked this up as air in the system and kept topping it off, but after two weeks I've declared this fairly abnormal and began to investigate further. What I quickly noticed is that the overflow tank is... well... overflowing. It's miles past the 'F' mark.

It also appears the system isn't really reaching what I typically expect as "normal" pressure. This may just be bad data on my part, but after shutdown at 85+C the hoses can still be easily squeezed and don't feel much firmer than normal. I'm not brave enough to remove the cap though...

The amount of coolant missing varies a bit, but anecdotally it appears to be tied more to how hot it got more than how far it was driven.

If I remove the cap. as it warms up coolant is just slowly pushed out of the cap. I let it go all the way to operating temp and didn't see any bubbles.

Engine also doesn't smoke outside of a few seconds immediately after a cold start. 52k on the clock.

I'm kinda stumped on this one. I've found a bunch of assorted information in searches but nothing that quite lines up with this particular situation.

I do know it was fine before I did the coolant system work, as coolant was right at the cap when I went to drain it; so I assume it's something I did wrong (Or just really bad timing). However without a clear leak point it's hard to really point my finger at something, so hopefully someone has good test ideas, or knows exactly what this is.

pleasedon'tbecoolantseals

edit: Realized this is the wrong forum, but can't seem to delete or move. Perhaps a helpful mod could bump this in the right direction~

 

*IMO the slightly lower level at the filler neck is normal. It should only be a couple ounces...but normal.
*If you've continued to fill up the level of the coolant to get rid of the "missing coolant", it's being pushed into the overflow tank. And that may be why it's level is raised.
*Coolant being pushed out an open filler neck as the engine warms is also completely normal. As it warms, it expands. As it expands, the level is raised. With the cap installed, it would be forced into the overflow bottle. On shut-down that coolant would be drawn back out of the overflow by the naturally occurring vacuum that forms as the fluid cools and contracts so long as the system has no leaks.

I'd get the overflow level to 'full', install a new cap just to eliminate any other possibility and monitor temps closely. Chances are, things are just fine.

On a side note, why the RX8 fans?

 

What AST do you have? How old is your AST cap? If your AST doesnt seal, or the cap is bad, the coolant wont flow back into the AST, and your level will be a little low. My filler cap is always full to the top.

 

I have an aftermarket aluminum AST which was installed by the previous owner. It looks nearly identical to the Pettit racing one (Except it's a speckle grey instead of black), with a Motorad lever vent cap.

A quick inspect of the cap seals doesn't show signs of obvious failure, but they're of unknown age so I'll go ahead and replace them anyway.

Edit: Went withe the RX8 S2 fans because one of my stock fans had died, the other sounded a bit unhealthy. The RX8 units are both cheaper and nicer so it seemed like a no-brainer. They bolt right in other than needing to swap the tails.

 

Sounds like you have a Pettit AST. My old one was colored as you described. The Pettit AST is made of flimsy aluminum. The top warps over time, and wont seal properly even with a new cap. I replaced my crappy Pettit AST with one from Tri Point Engineering six years ago. It still seals fine.

Tri-Point Engineering

 

Not trying to scare you but I'll give you my experiences that had similar symptoms.

My previous R1 had the exact same thing happening. Car seemed to run fine and never really got hot but it was consuming coolant. I too found out it was going into the overflow tank and not being reclaimed. The guys above telling you to check your AST cap are spot on. Definitely replace that first as it's a cheap cheap part and takes 2 seconds to replace.

Unfortunately, in my case, the car had a tiny hole in the coolant jacket on the combustion side of the engine. This hole was small enough that the 16PSI from a pressure test would not cause coolant to blow through; so the pressure test results looked fine. However, when that coolant seal was exposed to the thousands of PSI from combustion....the combustion gasses would squeak past it into the cooling system. This lead to lots of air in the system that would push out the coolant into the tank. Then when the car cooled, it wouldn't reclaim the coolant.

It was tricky for me to VERIFY this was indeed happening. The way I did it was I took one of those funnels you can mount to the filler neck and I filled it with coolant and let the car run for about 15 minutes. After a few minutes of idling most of the bubbles should have been released. Then the thermostat opens and you'll see some more. Once the car is fully up to operating temperature the bubbles should STOP. If they don't....RIP.

 

We've all been there before....

 

That's a very sad picture

 

So far the cap/AST look to be a likely suspect. The cap does not sit cleanly on the AST neck and can be rocked back and forth a bit.

Rotating the cap 90 degrees improves fitment, which further implies warping of either the neck or cap.

I have a new cap on order and will update this thread when I have more data either way.

 

Get a straight edge like a flat piece of steel and put it over the top of the neck of the AST to see if it's true or it's warped.

 

She got rebuilt. Currently couldn't be better. Even in the Texas heat. New owner is an older fella who simply ADORES the car. He was the perfect buyer.

 

I had this problem recently.

Traced it to my own screw up. I'd installed a jubilee on the small coolant pipe connected to the filler neck.

After doing some work on the car, I'd tightened up the clip such that when I fitted the coolant filler cap back on, the jubilee worm drive interfered with the coolant cap as I installed it back onto the filler neck, apparently damaging it and causing a poor seal.

Spun the jubilee around and replaced the filler cap with a new one, problem solved.

 

I had the same experience as the OP and while my coolant system pressure test checked out it was failed coolant seals and my car was doing the same thing as you describe.

 

New caps didn't help.

AST neck is only very slightly warped, so I was hoping a fresh cap might sort it out but no dice.

Who makes a good AST then? Looks like the re amemiya ones are solid but hard to find. I don't see any other alternatives beyond stock.

Amusingly the car has started leaking coolant.... from the overflow tank

 

Look at post #5. Or you can delete it properly like many have with an S5 filler neck.

 

I did, but it looks like they don't sell them anymore (the only thing in their store are ECUs.

Might have some luck used though. I'll poke around. I'm not too keen on removing the AST if I can just replace it with a good quality part.

 

I picked mine up on the forum in the for sale section. Found it at the end of 2015. It was either new or slightly used cant remember. The build quality is Outstanding If you can find one. I had a ast from mazdatrix I was going to have installed, it looked pretty flimsy compared to the tri point.
Grey

 

Good news?

Did another inspection of the coolant hoses and noticed the hose that goes into the bottom of the AST is rotting. This wasn't obvious before as the damage was covered by a rubber sleeve (some kind of anti-kink maybe?). It doesn't appear to be leaking at that point, but it's suspect regardless. It's far enough up that it might simply be allowing the car to suck air in, but coolant being pushed out still makes it into the tank. (it could also be leaking and just getting insta-dried but the radiator/fan)

Going to tear it back down tonight and replace all of the small diameter lines that go to/from the AST and overflow tank, then wait for the new AST to arrive. Luckily the vacuum line kit I ordered came with replacement hose for these lines as well, so I've got some nice silicon on hand for the purpose.

I swear if it ends up being a coolant seal after all this....

 

Late update, been busy but here we go anyway.

I replaced the radiator and AST caps, but saw no improvement (Unsurprising). So I ordered a RE AST and installed it.

For about a week much less coolant was being lost than usual, after which it resumed it's usual consumption.

Then I borrowed a pressure testing kit. Pressurized it to 17, and it falls to 13 in about 30ish minutes (This isn't overly surprising, as the AST has a 0.9 bar cap on it

After this is holds semi-stable and will drop to about 9PSI after 12 hours. I've not tested further but I would assume it continues to fall.

I re-pressurized it a few times over the course of 30ish hours to get a good baseline, then started the vehicle. It fired in two and ran smooth, as usual. No white smoke, but there was a strong sweet smell to the exhaust.

My guess at this point is that I have a fairly minor seal failure. I'd love to be proven wrong, but it seems the only way I'm going to get any better data is either pulling the plugs and going in with an inspection camera, or just running it until it gets worse.

Unless anyone has any bright ideas on what else it might be.

 

My condolences dude.

 

My coolant seals had failed and it had passed a static cooling system pressure test. It only pushed the coolant when it was running. I had no exhaust smoke or smell really. Sounds like you are going to need to rebuild very soon.

 

Try not to wait too long. Coolant/water will sit in the seal grooves and start to corrode and pit. That can render your irons unusable and make the rebuild just that much more expensive.

 

I have a lot of prep work and unrelated crap to deal with before I'm going to have a chance to tackle this, so the car will likely sit for a bit. Is there a good way to prep the engine?

My hunch is that running the engine for a few seconds after letting it totally cool should push/burn out any coolant that seeped in during cooldown?