ndinunz

Hello all,

I hope this isn’t a stupid question. I’ve done a lot of research on the cooling system with these cars, and I’m having what seems like a very simple issue that i can’t figure out.

Every time I fill my cooling system after a drain/flush, I always end up overfilling the overflow tank. I’m taking all the proper steps, I disconnect the TB hose, fill it through the filler neck with a lisle funnel, AST cap still on. Even if the overflow tank is completely empty, I can never seem to get the coolant level to the top of the filler neck before it starts leaking out of the overflow tank. I have removed the overflow entirely to inspect for cracks. Maybe i’m just an idiot, but I have no clue what i’m doing wrong, and this problem is making it difficult for me to bleed the remaining air out of the system.

After I drive the car, and get it up to operating temp, then let the car sit and cool, I’ll open the filler neck cap, notice a light vacuum (which tells me the system is sealed), then I can see the coolant level drop dramatically, and the car will start pissing from the overflow. I will then try to fill it back up from the filler cap, and it will overflow the reservoir before it ever reaches the top of the filler neck. It’s just a viscous cycle of recycling the same water that the car keeps expelling, and I’m going through way too much distilled water lol.

What am I doing wrong with this?

EDIT: I also have never gotten the low coolant buzzer while driving, but I do notice the temperature start to spike only when the car is sitting uphill, which tells me there’s air in the system that I can’t bleed out. By spike, I mean I’m a hardcore gauge watcher, and I notice the needle barely move up from the center position, then drop back down to the normal position whenever the car is on level ground

cr-rex

iTrader: (114)

Pause.......

Are you pouring fluid into the overflow bottle? The one behind the headlight? The only place you pour coolant in is through the filler neck next to the alternator. Delete the ast and this process will be much simpler than it already is. Put the funnel on, fill it with all the hoses connected.

You only want to remove the tb line if the system is completely empty. Run the car until the coolant starts to steam and bubble, shut it off and let it boil. Once it cools down, do it again. Holding the revs up will help some and squeezing the radiator hoses will help some as well.

If I'm reading right then you're simply doing it wrong. It sounds like you're just pouring coolant in wherever coolant is present and hoping for the best

scotty305

iTrader: (2)

Does the car have a completely stock cooling system, with caps at the AST and at the filler neck? If so, which cap has a completely flat bottom and which has the pressure relief spring? There's a photo in this thread showing which cap is the filler neck or the AST.
https://www.rx7club.com/3rd-generati...thread-571088/
It's an old thread but most of the info is still relevant. If I could make edits to the original thread I would mention how important it is to flush the coolant often, and also I've changed my opinion about how much coolant-to-water to use. After 15+ years of wrenching on my RX-7 I think it's more important for most cars to run a 50/50 mix to prevent rust than to run a 70/30 or 100% water mix for max heat transfer.


If you're using a Lisle funnel while the engine is running, the Lisle funnel should be on the AST, not at the filler neck. On my setup which has the AST deleted (FC filler neck) and a few small holes drilled in the thermostat, I can get a lot of air out of the system by filling the Lisle funnel to the top and waiting for an hour with the engine off. Also if you're seeing the OEM gauge move the engine is getting much hotter than I would be comfortable with.

ndinunz

Thanks for the response! No, i’m not filling it through the overflow tank. I’ve drained the coolant completely and i’m filling it through the filler neck. For some reason, the overflow is filling up and pissing out before the coolant level remains consistent at the top of the filler neck

ndinunz

The cooling system has an an aluminum radiator and aluminum AST, other than that, it has a catless downpipe. The relief spring is on the AST. I had no idea you needed to put the lisle funnel on the AST if the car is on, thanks for that piece of info!

Also, I’m pretty sure the car isn’t overheating, based on the circumstances, but I appreciate the concern. I’ve never had issues idling in stop and go traffic on 90+ degree days, but just after i drained and refilled the system, and have been unable to bleed the air due to this issue, I noticed this.

scotty305

iTrader: (2)

The cap with the relief spring goes on the AST, that's correct. Be sure the hoses to the AST are correct, the small AST hose up high near the cap goes to the overflow tank and should only have coolant venting out of the system when the relief spring is being pushed open by coolant pressure. It shouldn't vent when the engine is cold, only when it starts getting warm. You can double-check this by buying a spare length of cheap hose from an auto parts store and temporarily running the new hose that so the overflow vents into a clear container like a plastic water bottle. The AST should be the highest point in the system, so any air in the system will be vented out the overflow before any coolant gets vented out.

The small AST hose near the middle goes to the filler neck, and it receives coolant from the port at the front of the filler neck. The bottom hose from the AST goes to the radiator, that is the suction side so coolant goes down that hose and then gets sucked in by the water pump.

ndinunz

Yep, that is all correct, but went and confirmed on the car. The line going from the AST to the filler neck was just replaced because it had a pinhole leak, which is why I drained and flushed the coolant in the first place.

But this problem is happening when the car is off and completely cool. I haven’t driven the car in a few days, I guarantee if I were to go pull the cap off the filler neck, the system would be under slight vacuum, and I would see the coolant level drop significantly after vacuum escapes. Then if I begin to fill the system with coolant, it would start coming out of the overflow before it reaches the top of the filler neck

scotty305

iTrader: (2)

If the overflow tank has never been removed and cleaned, there might be crud in the tank or the lines preventing coolant from going back up into the AST. Getting at the overflow tank requires removing the fender liners, it might be easier to try the extra line and plastic water bottle thing first. It really bugs me that the OEM overflow tank is so hard to see.


I've also heard that some pressure caps are one-way and will only vent pressure but never suck coolant back in. I don't think I've seen one of these in person, but it might be worth spending the $15-25 on a new cap in case yours is the wrong one. My car is running the Stant cap intended to fit a 2000 Lexus LX470, it happens to be the same fitment but a bit higher pressure than the RX-7 cap (1.1 bar / 16psi vs 0.9 bar / 13psi). There are probably plenty of others that would fit, that just happens to be a vehicle I had access to and noticed the stock Toyota/Lexus cap had a 1.1-bar pressure rating.

ndinunz

Thanks so much for the information, i just went and checked the coolant (and drove the car), the coolant level was at the top of the AST and the top of the filler neck, so i’m not sure. I took the AST cap off before I took the filler neck off and didn’t notice the level drop like it usually does. I’m not sure if that has anything to do with it, but that’s the only thing i did differently. The car is currently only filled with distilled water and has the belly pan off and stuff because I plan to drain it again and refill with 50/50. Im really not sure why i’m having issues getting the coolant level to stay consistent.

I’ll update again once I drain and refill it. I’m also going to get an actual water temp gauge so I can properly monitor the temps

jsesq

Isn't the level of the AST lower than the filler neck? Wouldn't you want the Lisle funnel at the highest point of the system to get all the air out?

TIA

Jesturr

This is what I would think, too. I have an aluminum AST, I keep the cap on but fill through the filler neck with a Lisle funnel.


OP, if you're filling through the filler neck, with the spring cap maintained on the AST, my thought is that no coolant should be flowing to the overflow tank at all. The spring on the AST cap should keep the cooling system closed and self contained until it is pressurized. Coolant should only bypass this cap and go to the overflow when it sees pressure, something that wouldn't happen if your filler neck cap is off and connected to atmosphere (or connected to a Lisle funnel, which is connected to atmosphere.)

Are you running the car while filling fluid? Is the coolant somehow hot when you're pouring it in?

My thought would be to replace the spring cap on the AST. Coolant is getting past it when it shouldn't be and ending up in your overflow bottle.

ndinunz

Thanks for the information. I thought the filler neck is at a higher point in the system than the AST as well, but perhaps it has something to do with how coolant\air flows through the system making it more effective to have the funnel on the AST? Not sure about that exactly, but the person who made that comment seems to understand the cooling system on these cars very well.

As far as filling the system with the car on, the answer is no. Here has been my exact process:

I drain the fluid completely, including all of the fluid inside the overflow
Then I fill it through the filler neck with the AST cap still on and TB line disconnected. The cap for the AST and filler neck are also both brand new, and I have heard issues about the surface of some of the aluminum ASTs warping and causing leaks, so I cut the old flange off and welded a new one on.
Reconnect all the lines, secure lisle funnel, and start filling with the car off and cold. As I'm filling the system and massaging the lines and occasionally jacking the front or rear of the vehicle up to burp the air out, it eventually starts leaking from the overflow. I'll pour water into the filler neck, watch the fluid level drain, then see it dripping out from the passenger side. This only happens when it starts to get full, indicating to me that it's overflowing the reservoir before it gets to the top of the filler neck for some reason. I'm going to drain the distilled water out and fill it with 50/50 today after work, I can take a video or something and attach a link for you to see.

DaveW

I fill through the filler neck. Initially I have the TB line off along with the AST cap. When the coolant rises to the level of the AST neck, I cap it. I keep filling until coolant comes out the TB fitting and then reconnect that, followed by adding more coolant until it is at the level of the filler neck. While this does not get ALL the air out of the system, It's good enough that I can run the engine a few times to purge most of the remaining air. Then after a few drives, adding coolant through the filler neck or to the OF tank after cooling down as needed, it's done.

Jesturr

​​​​​Might be worth trying to fill it with the old AST cap you replaced, instead of the new one you replaced it with, to see if you got a bad pressure cap.

When you are filling a cold car with coolant, the only nipple->hose that goes to the overflow tank should be blocked off by the spring-loaded seal on your AST cap. If coolant is getting through, I would think either your new cap isn't sealing properly at atmospheric pressure, or maybe you've got damage/warping to the lower sealing surface on your AST, where the spring-loaded seal sits.

ndinunz

Thanks for pointing this out, Dave. Pretty similar to how I’m doing it, but i’ll try keeping the AST cap until it gets to the neck.

ndinunz

This makes sense. Maybe the interface is dirty. The cap has been replaced multiple times in the last 6 months or so because i was having suspicions that the cap wasn’t sealing properly, but im confident this one is. I also sanded the mating surface smooth before i welded the new flange on, but I will definitely take another look at that.

ndinunz

Hey guys, sorry for the delay. I’m pretty sure the sealing surface of the AST was dirty/pitted. I removed it, and cleaned the surface with a scotch bright pad and I haven’t had that issue since. Thanks again for all the help

gracer7-rx7

iTrader: (16)

Look up how to do a coolant system pressure test to test for any other coolant leaks.

ndinunz

Thanks for the advice, I have a coolant pressure tester that i’ve used to diagnose a slew of other cooling system related issues lol