The OEM pressure cap won't work on the RX-7.com aftermarket AST. That uses those weird Stant caps with the lever, and they're some American radiator cap. You should be able to find the same Stant cap at a parts store.

The radiator itself doesn't have a cap on it. The two caps are the only caps on the system.

If you have a Mityvac, hook it up to the hose going to the overflow tank and see if you can pull a vacuum and suck coolant from the overflow tank. A pinhole leak in that hose can cause a loss of vacuum, making the coolant recovery system not work.

Dale

 

Thanks for the suggestions, I'll try replacing those items and see what happens.

 

Missed one. If the filler cap is defective it will also cause this to happen. Be sure to get the $24 Mazda cap. It's different. Even if the rubber insided does not look worn, replace the cap anyway. I've been through this with my FD. Also the line from the overflow tank can get clogged with sludge from dirty coolant. This line may need to be replaced. When the rebuild was done, did they change these lines because when an engine has a coolant seal failure, exhaust gases can cause sludge buildup in these lines. If they are only slightly clogged this can interfere with the overflow return as the engine cools. Also whenever you change the coolant which should be often (twice per year), the overflow tank needs to be flushed out well. I would also use a 70% coolant mix or better yet go to Evans NPG coolant and follow the directions carefully when changing over. Oh and also replace the thermostat. It's cheap and could also be the problem.

This is all my best stuff learned the hard way except I have not tried the Evans yet but it comes highly recommended by the best racing shops.

 

Defective filler cap? Isn't the filler cap just flat and not pressurized? I thought only the AST cap is pressurized.

The more I look at my AST cap, the more I think I need to replace it actually. It looks kind of worn

I'm going to replace the AST cap and the line from the overflow to the AST, as well as remove my overflow bottle for inspection and I'm going to empty it and fill to the proper level to ensure I don't have too much fluid in it to start.

 

On mine the intermittance was caused by how hot it was outside. For example it never did it at night when I would drive it. If the temp outside was below 85 then it would not do it. It's related to how hot the coolant is getting and how much expansion into the overflow happens. So my conclusion is that you may have a combination of things going on. Try all the things I mentiioned and see what happens. A shop can change those coolant lines to the overflow and your thermostat and it shouldn't cost very much. Check and make sure you are not topping off your coolant too high in the overflow tank. Siphon a little off if above the full line on the dip stick.

 

I had this EXACT problem and it was my overflow tank. They crack over time. A quick replacement solved my problem.

 

The filler cap is meant to *hold* pressure and not release it at 16 PSI as the AST cap is designed. That is why there is a rubber seal inside. It is very important it does not leak, otherwise vacuum pressure isn't enough to draw the coolant from the overflow tank at cool down. See what I'm saying? Note: If you switch to Evans NPG coolant you can actually run the systme on 0 PSI and can remove the rubber from the filler cap. This is the ideal solution but switching the coolant over to this type is a major pain. Do some searches and you'll see what I'm saying.

 

Yeah those dang things cost about $120 too. And they come with new lines so the lines could have been the problem as well. I had some cracks in mine but only in the filler neck section. I got a new one because they look nicer than cracked ones and they come with new lines.

 

Holy **** $105.78 for an overflow bottle.

I sure hope mine isn't cracked lol

 

Yeah seems to be nothing cheap on these cars even when you are talking about caps and overflow bottles. Just be glad you don't need new turbos or engine!

 

Haha....I already replaced the turbos and the motor

 

Well from the look of your profile documentary, there isn't much you haven't replaced on that FD. You are to be commended for following through with such a huge project.

I've been scanning the forums regularly and have noticed that this coolant dump at shut off is not all that uncommon. Seems to be a very common problem with FDs unfortunately. One guy who had a lot of experience told me that he just doesn't drive his "toy" in anything over 85 deg weather. It seems that the first thing a lot of people say when this happens is "coolant seal failure." I've found thru my readings and own experience that the more likely culprit is cooling systems clogs and leaks and perhaps thermostat. Your experience with having just replaced your coolants seal and having the problem tends to bolster my belief further about this. Of course I could be wrong and there could be some small degree of coolant seal failure or the beginnings of coolant seal failure involved as exhaust gases under higher pressure push there way into the cooling system.

 

Thanks man, this car has been a major PITA but it's getting there. Besides the coolant leaks the engine is running strong, even at 10PSI on the non sequential turbos the car feels like a monster. I Can't wait to figure this coolant problem out, get it on the dyno for tuning, and take it to the track / turn up the boost

 

maybe your radiator is cracked?

 

Why would the radiator leak after shut down and not when running?

 

My $.02 I may be totally wrong:
After shutdown there little/no airflow to lower the temps of the coolant, but the engine may be hotter than the coolant and continue to transfer heat. The coolant gets hotter and increases in pressure. Perhaps the leak only appears after the coolant pressure reaches a level not seen during normal driving.

 

Well yes, that is how the coolant system works. When you shut the engine off, the coolant temps shoot through the roof because the coolant stops moving and the heat isn't transfered out through the radiator. That's why we have the overflow bottle, so the coolant can be pushed into it, and as the car cools back down the coolant gets drawn back into the ast from the overflow bottle.

It seems mine isn't working this way, instead it is pushing my coolant out of the bottle.

 

Well I pulled the overflow bottle and line from the AST last night.


The Overflow was 100% full, which I think is why it overflowed the other day. The line from the AST going to the overflow I pressure tested, and it was perfectly fine as well. So basically I think it was the overflow being too full. I probably filled it too much last time I added coolant.

 

If running an aftermarket AST with the larger US spec cap, do NOT run any brand with the red pressure release lever. Purchase the Stant brand or Stant-style from NAPA without the red lever and your should be fine.

I pressure tested more than 6 lever release style caps a year ago that were rated at 13 psi and all of them released pressure around 11 psi. Only the non-lever style caps would consistently hold the pressure level as rated on the cap. Also, I run the 16 psi cap, but that's just me.

Get yourself a good cap and keep the coolant level near the "F" mark on your overflow tank and your "overflow" issue should go away.

Good luck and keep us posted.

 

Few week ago I replaced the plastic AST to an alum unit, it was bought used which also came with a 13 psi CST cap (red lever pressure)...end result after a 15 miles drive was coolant dump out of the overflow tank. Bought a new cap (CST with red lever pressure release), drove home in 101* weather...no leaks. So I thought things are ok with the new cap.

Last night at dusk took the car out for a good 30 minutes open hwy run...it was a hard drive speed averaging 100 mph; reading from the PFC coolant temp was 95* C with AC on.

This morning there was a puddle of coolant right below front passenger bumper...coolant level in AST was half full; Overflow tank was beyond full mark.

Like others said...the CST caps are junk.

 

Empty your overflow and fill it to the proper level, that's all it took to solve my issue.