coolant overflow

I found a post that states the problem that I am having with my 99 3.4 alero. However, the answer is not posted. Begin paste:

Coolant leaks from the coolant tank thru the overflow tube after the car has warmed up. I need to add coolant every day. The car heats well, and does not overheat as long as i can keep coolant in it. I know i am losing the coolant from the overflow tube because i put the overflow tube in a bottle and ran the car under normal conditions. After driving some 20 miles the bottle was full of coolant. This is where its going, I just don't know what is causing it. Help.... Thanks....

end paste...


I am not a car guy and will probably not be able to fix it but I also fear taking it to a mechanic to let him guess what the problem is (no one seems to know what it is). I'd like to know what the problem is when I take it in - any ideas?

Dex-Cool is supposedly good for 150k or 5 years (i think), so if you have not had your system flushed, maybe its time. There could be a kink, or clog in your hoses somewhere.

My 99` alero, never had that problem, but at 80k, and 5 years old, the dex-cool was pretty gummed up and needed to be changed. I am no expert by the way, this is just an opinion.

The hoses look and feel good. The coolant was recently flushed so I don't think it is that. It's almost as if something is siphoning out the coolant as the overflow hose is at the top of the reservoir and even when the coolant is low in the reservoir (not close to the overflow hose), it still comes out of it...

Any other ideas?

The hoses look and feel good. The coolant was recently flushed so I don't think it is that. It's almost as if something is siphoning out the coolant as the overflow hose is at the top of the reservoir and even when the coolant is low in the reservoir (not close to the overflow hose), it still comes out of it...

Any other ideas?


Is the plastic cap on tight? Or maybe its defective. It should be rated at 15 psi.

The cap is on tight. No coolant is leaking out of the reservoir from the cap - I'm 100% sure on this. However, could it be that it is on tight enough to keep coolant from leaking out but somehow not tight enough to stop coolant from leaking out the overflow hose (below 15 psi)? How do I check/know if it is on tight enough

The cap is on tight. No coolant is leaking out of the reservoir from the cap - I'm 100% sure on this. However, could it be that it is on tight enough to keep coolant from leaking out but somehow not tight enough to stop coolant from leaking out the overflow hose (below 15 psi)? How do I check/know if it is on tight enough

I think you're on the right path. The cap should be tight to where it bottoms out and stops turning. The cap works the same like a regular radiator cap except on the Alero its on the plastic coolant reservoir assembly and not physically on the aluminum radiator.

If you look at the coolant overflow outlet where the hose connects it coincides with the center or overpressure part of the cap. Your system is either over 15 psi in order to discharge coolant or its defective (the cap). The only way to really tell is to have a pressure test of the cooling system run. The cap is probably the cheapest remedy and still hasn't been ruled out in the process of illimination during troubleshooting.

This sounds like a likely cause for the problem - defintely worth a try. I will look into it and post what I find out on this thread once I know the results. Thanks for the advice.

I had this problem too, I had changed the cap, waterpump, pulled the radiator, checked hoses, changed the thermostat (pain in the butt, since its under the throttle-body).
Not the problem.
Talked to a couple wrenches and they were stumped. I finally pulled the heads off to check the head gasket. This was the problem. What was happening was the leak was just enough NOT to make white smoke coming out of the tailpipe but enough to lower the coolant level to where it would start to boil and overheat, increase the pressure in the system, to the point when I would shut it off....it would boil over.
It was a big job of redoing the head gasket. I just hate to pay someone else for something I can do.
I would do the cheap stuff first, cap...$6-7 bucks then thermostat...$10 bucks, make sure that the air is out of the system. There is a bleeder valve on the pipe that comes out of the back of the water-pump(the pipe then heads toward the thermostat)
Your miliage may vary :)

The head gaskets or even intake manifold gasket is a possibility, although I would consider the intake gasket first because it is a known issue on the GM 3.4L V6.

You can check the condition of your motor oil. Look for coolant contamination. The oil may look milky. Vice versa, check the condition of the coolant.

Whatever the coolant system problem is it's causing the overpressure condition and its exceeding the 15 psi rating of the cap, therefore the discharge out the overflow hose.

LOL I started reading this because my car did the samething when I got home from work.. I could smell antifreeze and seen hot water falling to the concrete and remembered I left the cap off when someone mentioned it... LOL..


I've done it atleast 100 times and I leave it off the 101 time...





The head gaskets or even intake manifold gasket is a possibility, although I would consider the intake gasket first because it is a known issue on the GM 3.4L V6.

You can check the condition of your motor oil. Look for coolant contamination. The oil may look milky. Vice versa, check the condition of the coolant.

Whatever the coolant system problem is it's causing the overpressure condition and its exceeding the 15 psi rating of the cap, therefore the discharge out the overflow hose.



Question.. Is there a way to tell if it's the head or intake gasket?? The pressure for the coolant is greater than the oil pressure right? So if it is it should leak water into the oil..


I used to get oil sludge into the coolant but the reservior is always empty before I add water.. Or do I have to take it to the mechanic and they will know?


Anyone have a ballpark figure on the price for either one?

Question.. Is there a way to tell if it's the head or intake gasket?? The pressure for the coolant is greater than the oil pressure right? So if it is it should leak water into the oil..

I used to get oil sludge into the coolant but the reservior is always empty before I add water.. Or do I have to take it to the mechanic and they will know?

Anyone have a ballpark figure on the price for either one?

Are you having a similar problem? The coolant has to be going someplace so an intake manifold gasket or head gasket is a possibility.

Good mechanics can run a vaccum check of the intake system and run a compression test of each cylinder. You can also take a reading of the spark plugs.

The ballpark figure to have an intake manifold gasket replaced is around $300 - $500.

Well, I went out and purchased a new cap for the system and "installed" it. I also had to bleed it for air but that was it. Problem solved. No more coolant leaks.

This may or may not be true in all cases but it was for mine: if your system is not maintaining the pressure it should and if it is because of the cap then it will not make the "hissing" noise when you unscrew the coolant reservoir cap.

I also spoke to a couple of people about the gasket problem that is apparently common with the 3.4. I was told that you can not only check the coolant for signs of contaimination but you can also use the dipstick to look for signs of contamination in the oil. I was also told that I would know if I had a gasket problem as the car would be running *very* poorly.

Are you having a similar problem? The coolant has to be going someplace so an intake manifold gasket or head gasket is a possibility.

Good mechanics can run a vaccum check of the intake system and run a compression test of each cylinder. You can also take a reading of the spark plugs.

The ballpark figure to have an intake manifold gasket replaced is around $300 - $500.




I've ran the car in park for awhile and theres no visible leaks anywhere. I even coated the block with water paste I use from work and no still nothing....

I've ran the car in park for awhile and theres no visible leaks anywhere. I even coated the block with water paste I use from work and no still nothing....

How much coolant are you losing and over what period of time. Did you ever get the intake manifold gaskets changed out since you've owned the Alero?

How much coolant are you losing and over what period of time. Did you ever get the intake manifold gaskets changed out since you've owned the Alero?



No because I didn't get on this site or ever know about this problem until after 100k miles.. Well I could probably fill up the reservoir veryday but I usually just fill it up every 3 or 4 days... It gets old after awhile and really don't care if it fries the engine.. I have every dash light on known to that car and I really can't afford to get everything fixed... I have about 1 yr left on my loan and I hope it lasts atleast another 6 months... Then after that I'll have fun pouring 10 gals of gas on it and watching it burn to the ground...

No because I didn't get on this site or ever know about this problem until after 100k miles.. Well I could probably fill up the reservoir veryday but I usually just fill it up every 3 or 4 days... It gets old after awhile and really don't care if it fries the engine.. I have every dash light on known to that car and I really can't afford to get everything fixed... I have about 1 yr left on my loan and I hope it lasts atleast another 6 months... Then after that I'll have fun pouring 10 gals of gas on it and watching it burn to the ground...

Hopefully the next year goes by fast. With all the problems you've had I would have gotten rid of it sooner. Let it be someone elses problem.

Man I would never ever do anyone like that, but I would do it to dealer..

Man I would never ever do anyone like that, but I would do it to dealer..


Or you could stick with your burn it to the ground option or just leave the keys in the ignition and hopefully someone steals it..lol. :lol2:

But seriously you should consider getting the poor thing fixed.