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Discussion Starter #1 (Edited)
Here's what I've got going on in my sometimes overheating 94 Intrepid 3.5L: (Warning in advance, I'm definitely a newbie to any kind of automobile repair, although I'd like to learn. Also, I've been a broke and busy college student for so long that I often chose less than optimal solutions such as Bar's stop leak, rather than getting a new radiator.)

Problem: radiator fluid is entering the overflow/de-aeration tank when the engine gets hot. However, the fluid never returns to the engine/radiator. At the same time, I am NOT losing fluid through the overflow tube (the one with a free end which drains to the ground) on this tank. Also, I do not see any current anitfreeze leaks. If the engine/radiator is full of fluid and the overflow is overfilled (beyond the cold line), it doesn't overheat. However, the overflow may be completely full, but it overheats due to a lack of fluid in the radiator/engine. Also, if the overflow tank is empty/very near empty and the engine gets hot, it will dump fluid into the overflow and then begin to overheat.

History:
I did have a distinct radiator leak which I temporarly used Bar's Stop Leak to fix. Then, when the leak returned, I was waiting for an opportunity to get $$$ for a new radiator and I tried to deal with the small leak with another treatment of Bar's and keeping an eye on my fluid levels. Well, I was a little surprised when my car overheated (I was able to pull over before it hit the red, but well above 3/4) because my overflow tank was above the cold line (however I never filled it above the cold line), but I could see that the car had lost at least a quart of fluid. Recognizing that the overflow was not providing fluid to the radiator, I removed the top tube between the engine and radiator, I filled it with fluid by pouring into this tube, until I noticed some fluid coming out of the engine opening. I used almost 3 quarts doing this. Then, upon driving it, the car remained quite cool (~1/3) even when idling.

I used some external putty which solidifies hard ("like brass") over the radiator leak. (I forget the brand, but it's green and specifically sold for radiator leaks.) This appears to have worked perfectly in sealing up the leak. Also I bought a new radiator cap (16 psi). Before I went driving again, I decided to drain out the excess fluid in the overflow tank. I took it to about an inch below the cold fill line. (At this point, I was assuming that maybe the radiator leak was to blame for fluid not leaving the overflow.) After driving for ~10 minutes, it got hotter than I would have liked (~3/4), so I stopped, let it cool and noticed that the overflow was again completely full (above cool line), although no fluid left through the overflow tube on the overflow tank (it was quite dry inside). After returning fluid directly to the radiator again via that top tube between the engine and radiator, it runs cool, no leaking, and no fluid leaving the overflow tank. Still, the overflow must be completely full, or else the radiator loses fluid into the overflow and tends to overheat the car.

Sorry for the long post. I wanted to give a complete history.

My current plans are to flush the system (remove unneed Bar's stop leak), perhaps replace the overflow tank (it's looking dirty and old), and attempt to remove air in the system through the bleeding valve at the bottom of car (sorry for the vague terms).

Would dirty fluid and/or air bubbles in the cooling system cause this problem? Is there something else I should be looking at as well (e.g. bad water pump?, etc.)

Thanks tons in advance!
 

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TRY A NEW RADIATOR CAP, IT MAY NOT BE HOLDING PRESSURE.... Also pressure test the cooling system....
 

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Umm, I think you put to much Bar's Leak in there and pluged up it up. I bet the Rad. will have to be roded out now. If you could find some money to get a new Rad. that would be the best thing. Also the bleed valve is on the thermostat housing. Not at the bottom of the car. You could also need a new Thermosat too. By the way what engine to you have in your car? And when your doing any work on the cooling system, you got to make sure you bleed all the air out of it, or you could run into overheating problems again.
 

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Start with a pressure test of the cooling system, If no leaks are found then flush, fill and bleed the air out. If leaks are found don't use any more stop leak (2 cans is about the most any radiator can stand). If it still overheats I would suspect that the 2 treatments of Bars stop leak has clogged the radiator or water pump failure. Replace the radiator.

And yes dirty fluid or air in system will cause this problem.
 

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Hmm, fluid ENTERS resevoir, NEVER goes into the system...it could be as simple as the siphon hose inside your overflow tank is broken near the top. There are two hoses that go into the tank, the one at the front is just the radiator cap release hose, this is about a foot long and just dumps coolant onto the ground below your tank. The one behind it is the one leading to the engines cooling system, I think it enters in at the plenum, the highest point in your system. Take the top of your overflow tank off, have a look. If that siphon hose is fine, next thing to do is to take off that hose that leads to the back of your engine (the one closest to windshield) clean the end off of coolant, blow into it, strong human lungs can temporarily create up to 5psi of pressure and 1-2psi continuous, more than enough to tell if there are any big leaks. There are no MAJOR leaks if you CANNOT keep blowing air into it. Oh yeah, don't DRINK ANY COOLANT, COOLANT IS BAD! Only do this test if you promise not to drink any coolant. If your siphon hose in the tank is fine and you can't blow air into the cooling system, your going to have to spend money.

I should also note: AIR TRAPS shouldn't happen at all in this system, the hose coming from the very top ALWAYS angles up until it gets to the tank..this means as coolant heats up in the engine and expands, all air escapes first and into the overflow tank. Trapped air was a problem on older cars with rads lower than the engine because the overflow line didn't come off of the highest point in the system, trapping air in the engine.
 

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Discussion Starter #8
Dan said:
Hmm, fluid ENTERS resevoir, NEVER goes into the system...it could be as simple as the siphon hose inside your overflow tank is broken near the top.
Thanks. I'll look into this. Previously, I did attempt to remove this hose from the overflow tank and noticed that it almost seemed attached to the tank. I didn't want to pull it too hard. Was it just a snug fit or should I be looking at the end of the siphon hose that is closer to the engine?

(FYI, this attachment/snug fit was not seen in the lower hose on the tank. That one can be removed normally.)
 

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Discussion Starter #9
Strongt said:
Umm, I think you put to much Bar's Leak in there and pluged up it up. I bet the Rad. will have to be roded out now. If you could find some money to get a new Rad.
It's on my list one of these weekends.

I bled the system, yesterday. So far, it's staying quite cool (~1/4) even when idling at a light after driving 55mph.

When I get some more time, I may drain some fluid from the overflow (e.g. just below the cold fill line) and see if the same problem arises. (Of course, first I'm going to check the siphon line, as someone else suggested.)

Strongt said:
You could also need a new Thermosat too.
Quick question, how does the thermostat effect the fluid flow into/out off the overflow?

It looks like an exceptionally easy part to replace and I'm guessing it's fairly cheap, too?

Strongt said:
By the way what engine to you have in your car?
3.5L 26 valve
 

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you mean a 24 valve
anyways, a thermostat is extremely easy to replace, the upper radiator ose leads to the thermostat housing, just unbolt it, replace t-stat, replace gasket, re-bolt, run car and bleed air
 
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