Why does car pump coolant out of overflow tank? on 2003 Mazda Protege

My car has 110,000 mi.and I have replaced the radiator cap and had the system flushed and filled, also replaced thermostat, and overflow resovoir tank--and it still pumps coolant out the overflow and then starts to run hot. It will run fine sitting still but when you drive it, it will loose coolant untill it eventually starts to heat up. What's up with that? I recently bought the car and I have no warranty I did not notice the problem untill 30 days had passed because we did not drive the car that often. When the garage flushed the coolant system they said that it looked like it had stop leak in it. Hope you can help, love your site. --Thanks Ed

Asked by for the 2003 Mazda Protege
This does not sound good. The radiator cap keeps coolant in the engine and radiator once the coolant heats up it need to expand and the radiator cap allows coolant to push a small amount of coolant into he coolant expansion tank, as the coolant in the radiator "cools down" that coolant is drawn back into the radiator through a second valve in the radiator cap. You just bought the car and it looks like someone had put stop leak in the radiator before. I suspect someone had a coolant leak and has previously over heated the car in the past. It is likely your Mazda has a blown head gasket. To test for a blown head gasket a garage will look for the presence of carbon monoxide in the cooling system. Carbon monoxide is a by produce of combustion when gasoline is burned in the engine and can only get into the cooling system if the cylinder head gasket doesn't seal well or if the cylinder head is cracked. There is a very good two stage carbon monoxide tester many repair shops have. Is there any steam present in the exhaust, or does the engine run rough when it is started or running once warmed up?
My protege is doing this same thing. I think it might be the gasket on the valve cover. If there is one.
You should check your fuses, maybe your heater fan fuse went out... your fans arent turning on causing that to happen... the same thing happend to my honda accord.. it was steaming and throwing coolant from the resavour. It would only heat up when i came to a stop.... it took me awhile to realize it cause i had just changed that fuse... but once i replaced it and both fans turned on it stop doing it... but on the other hand you could have blown your head... you need to check all possibilities before spending money on a new head... if fan doesnt turn on check fuses or could be fan motor... it could be many things last thing you want it to be is motor head....
4 answers 6 comments
This does not sound good. The radiator cap keeps coolant in the engine and radiator once the coolant heats up it need to expand and the radiator cap allows coolant to push a small amount of coolant into he coolant expansion tank, as the coolant in the radiator "cools down" that coolant is drawn back into the radiator through a second valve in the radiator cap. You just bought the car and it looks like someone had put stop leak in the radiator before. I suspect someone had a coolant leak and has previously over heated the car in the past. It is likely your Mazda has a blown head gasket. To test for a blown head gasket a garage will look for the presence of carbon monoxide in the cooling system. Carbon monoxide is a by produce of combustion when gasoline is burned in the engine and can only get into the cooling system if the cylinder head gasket doesn't seal well or if the cylinder head is cracked. There is a very good two stage carbon monoxide tester many repair shops have. Is there any steam present in the exhaust, or does the engine run rough when it is started or running once warmed up?
there is no steam coming from exhaust and the engine runs great both cold and fully warmed up. Is there anyway I could keep the coolant from escaping the overflow canister until the radiator cooled down and sucked it back in (A larger overflow canister maybe)?
i have a 2002 mazda protege es and i have a foam like mix coming out of my overflow tank. I talked to some mechanics and they said it is either a head gasket or that i have a atf in my radiator.
I had the same problem. My radiator cap was not keeping any pressure. When I would stop the engine hot, it would spill one or two cup of coolant throught the overflow tank and I would hear the coolant boiling in the engine.
I noticed that the seat of the radiator neck was cracked (the ring where the radiator cap seal is making contact). The plastic of the radiator was turning to dust when scraped with finger nail. I used a small grinding weel (Dremel) to make the seat straight. It fixed the problem, the cap is keeping the pressure and the coolant is not spilling anymore.
It sounds like my 2001 Mazda protege (150K miles) has he same problem. After two hour at highway speed (65-70mph), the temp needle starts to climb to the max. At this point the expansion reservoir is completely full and bubbling over through the overflow valve. After letting it cool down 20 minutes the coolant starts to flow back slowly into the radiator until the reservoir is completely empty!
As far as I can see, there are no noticeable leaks but coolant is lost through the reservoir when it boils over.
I already replaced the thermostat which was confirmed bad (probably from previous overheating and age) and also preventively changed the hoses. I do not suspect airpockets being the issue (heater works normal). The radiator cap "looks" ok, although "could" be faulty from what I read in previous post?? Would loosing pressure here really cause coolant to be forced to the reservoir?

the radiator was replaced recently and pressure test is OK.

the engine check light does not come on, nor does the engine run rough. I haven't noticed anything suspicious yet that would suggest a head gasket leak.

Would anyone have any further troubleshooting ideas on what could be the root cause of the radiator sending all the coolant to the reservoir? I had 3 independent mechanics look at it and root cause still not fixed.
Sorry to revive an old thread but I figured I could answer the likely answer. From what it sounds like your cars are doing, the problem lies with either the head gasket being blown, head being warped or cracked. Another problem, rare but does happen is the water pump has failed. The head integrity should be checked first by performing a diagnostic test called a Block Test. This is done by sucking gases in either the radiator or overflow tank and mixing with a chemical. Be careful though, if your water pump has failed, coolant may back up in the radiator or overflow tank if no radiator cap is present. If the test shows no exhaust gases (hydrocarbons), then most likely if the thermostat(s) have been check, its the water pump. Now most water pumps fail by the internal gasket failing, leading to coolant leak out either the weep wholes or the seam where its bolted to the engine, however a rarer failure which does occur more often than realized is the fan blades on the inside either break or more likely erode away. This leads to reduce pressure created by the water pump to "pump" "hehe" coolant.This will lead to the pump overheating because it will spin with less resistant causing either temporary or permanent failure. When this begins to happen, coolant begins to back up, causing it to overfill in radiator by backing up the whole system, blowing the coolant out and overheating. The other way is if no radiator cap is present and a bypass exists within the overflow tank, where usually multiple hoses will connect to it. If this is the case, the coolant will backup into the overflow tank, raise the pressure and begin to overflow letting air be exchange in its place. At the same time this will cause some of the coolant to bypass to the radiator, acting like a pressure pump for the coolant, leaving the car a longer running period of the temperature fluctuating but running, until the coolant runs out. Your Welcome :)
Sometimes someone mixes up the Overflow cap with the radiator cap a radiator cap has to be flat with nothing on it just the washer overflow container has the wrong with a little plunger it's a common mistake