ccc_nos

iTrader: (3)

OK so heres the story. A couple weeks ago my AST broke. I bought another stock replacement and installed it. Then when I reach about 70 to 80 mph for about 20 minutes, it would start to run hot. The temp would rise about 3/4 the way up. I pull over and as soon as I do the car temp returns to normal. It will run cool as can be through town but when I hit the highway that happens. All coolant hoses are new and I can't find a leak. Today stopped at a store and when I came out there was coolant under the car. The car is still running cool all the way home. When I get there I look and there is no leak or any fluid dripping on the ground. I let the car cool for a few and then go back outside. There is coolant under it again. The coolant is coming from the Resivoir. The fans are running fine. It seems like the coolant is boiling back into the reservoir after the car is turned off. What the heck is going on? More importantly how can I fix it?

Upgrayedd

iTrader: (1)

Did the car overheat when your AST broke? You're having very similar symptoms to my bad coolant seal. It's not an every day leak, but when it does leak the coolant makes its way out the overflow when the car is parked and shut off. Worst case scenario you may have damaged a coolant seal if the car overheated...

dhays

iTrader: (1)

I may be way off base. but is it possible that there is a leak in the line that runs from the AST to the overflow tank? Also, is it possible that you still have some air in the system from incomplete burping?

Just a thought. I'm generally ignorant so I'm probably wrong.

dhays

iTrader: (1)

Ouch! I hope your wrong but that is possible.

For my own education... what would be the best way to check for this? Champagne test? Pressure test? Test coolant for HCs?

crcleofdst

I suggest that you start with the simple things. Make sure that you top off the coolant, and clear out all of the air from the system. It may take a few heat cycles before all of the air is gone. I recommend using the lisle funnel. For less than 30 bucks, you have a really useful tool for removing air from the coolant system. A google search should find several websites selling this item.

If you do not already have an aftermarket coolant temp gauge, I highly suggest that you purchase one and install it. The stock gauge is not going to move into the red until it is already too late.

Above all, you would do yourself a favor by parking the car until you sort this issue out. Overheating the car may kill a coolant seal, requiring you to R&R the engine.

crcleofdst

Pressure test or hydrocarbon test. Both can be useful, but I would tend to think that the hydrocarbon test would be the most telling. A pressure test can only tell you if you have a leak, but it's not necessarily going to tell you where.

T2 Tsunami

iTrader: (3)

also, make sure you have the hoses going to the AST in their correct spots....and top off.

crcleofdst

One more thing: If you have not replaced your ast and filler neck caps, now would be a good time to do so.

ccc_nos

iTrader: (3)

Thanks for the input guys. Both caps have been replaced. The AST has been replaced. Could it have anything to do with the thermostat sticking? Just a thought.

MyRedFD

iTrader: (1)

Also, another good thing to do would be to replace the AST with a metal one, not a stock plastic one, if that's what you bought. It'll save your wallet in the long run.

Rx72Heaven

iTrader: (2)

^^ i was just going to say that, lol. When i read "replaced AST with a stock one" i was like :o NOOO!

gracer7-rx7

iTrader: (16)

Go here: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/faq-3rd-gen-other-useful-links-68640/

Scroll down to the cooling section. There are two bookmarked links there that should answer all your questions and teach you a few things.

Tim McCreary

You are probably sucking air when the engine cools down.

I have seen bubbling and the throwing up of antifreeze before on my car after stopping and shutting down. Engine would spike high temp and come back down when I stopped. It would throw up antifreeze from the overflow when I shut it down and left it there for 5-10 minutes.

What I found was a very small leak on one of the radiator hoses that actually evaporated before it dripped. It was because of the original spring clamp that I reused when changing the hoses. When the engine cooled down, it would suck air through the leak point instead of pulling antifreeze back in from the overflow tank (fluid expands out to the tank when it heats up, then gets sucked back in to the system when the engine cools). Next heat cycle, more antifreeze was pushed out to the overflow, more air sucked into the system upon cooling down. Eventually throwing up antifreeze from the overflow every time. Actually seeing a fast spike in temp when a large enough steam/air bubble reached the sensor, and it cooled down immediately when the antifreeze would hit it again after stopping and pulling over.

I had the stock spring clamps on the hoses. I just added two screw type clamps to each hose end, tightened down, and the problem stopped. No boiling noises, no throwing up of antifreeze.

May not be your problem, but an easy check and fix. It may also be sucking air from the AST hose back to the system or even the hose going from the system to the overflow tank. Check them all.

Good Luck.
Tim

adam c

iTrader: (2)

How many miles on your engine? Is it the original engine?

First, check thoroughly for leaks. Do a pressure test. That will be a big help to find a small leak.

Your symptoms sound like a bad coolant seal. If that is the case, a rebuild will be in your near future.

dhays

iTrader: (1)

Tim, you have touched on something that has bothered me for a bit and I've never found an explanation.

I understand that the overflow tank, serves as an expansion tank just as you have described. What I don't understand is how the connection is made between the unpressurized expansion tank, and the pressured coolant system. Somehow the system must vent the coolant into the expansion tank from the AST as the pressure rises. As I look at the AST, it is obvious that the line running from the AST to the expansion tank is a zero pressure line. Is there some type of baffle there at the top of the AST that will vent coolant one way under pressure and then suck it back in under vacuum? Does the pressure cap on the AST work as some type of one-way valve, only allowing coolant to go one direction over 13psi but allow it easily to go the other under vacuum?

Really basic question I know, but at 50 years, I'm still pretty new to a lot of simple automotive concepts.

adam c

iTrader: (2)

The pressure cap on the ast is a two way cap. When the pressure reaches a certain point, coolant pushes into the ast, and then into the overflow tank. As the engine cools, the vacuum on the underside of the ast cap opens the lower valve, and allows coolant to flow back into the system.

Remove your ast cap, and look at it. Pull the small metal valve on the bottom down a little. You will see that it takes very little force to open it.

dhays

iTrader: (1)

Thanks Adam. That helps.