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Discussion Starter #1
Vehicle is a 2003 Neon, 2.0 liter 4 cylinder with 4 speed automatic transmission. Odometer reading is 223,800 miles. I have noticed in the last 3 days that the coolant overflow bottle is filling with coolant. Today fluid level is at the filler neck but I am NOT adding coolant. So the overflow is coming from the radiator.

Engine is not overheating as witnessed by the temperature gauge and I have not noticed in bubbling in the overflow tank. However location of the overflow bottle and opaque plastic makes viewing the coolant level and checking for bubbles difficult. I need to lower the coolant level in the bottle and then check for bubbling.

Either I have a radiator cap that has failed and is not holding pressure or this is a start of a minor head gasket leak which will progress to a major leak. I will replace the radiator cap as a cheap solution. Any other ideas as to what would cause the overflow bottle to progress to overflowing??? Would a partial collapse of the lower radiator hose cause a problem like this???
 

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You've got a lot of miles on that cap if it's the original, Allan. I'd try a replacement first and see what happens. If that doesn't work, then you need to do a pressure test on the cooling system. Does the lower hose have a spring in it like back in the good ole days? I guess you could take a quick peek at it while it's hot and see what the hose looks like.
 

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Crack the radiator cap and see if the fluid returns to the radiator. If this happens, head gasket is starting to give way, it would be a slow leak that allows pressure out of the cylinder but not into the cylinder, meaning change it before damage occurs.
 

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On Wednesday May 8 I let the engine cool for several hours and then removed the radiator cap. It was obvious that the cooling system was low as there was very little coolant in the top radiator hose. I siphoned about 1.5 quarts from the overflow bottle and poured that into the filler neck. I inspected the radiator cap and seals and noticed no abrasions or tears and the seals were pliable. I pressed the relief valve mechanism and it seemed to move smoothly. I reinstalled the cap and decided to monitor through several days of driving.

Since this over filling incident last week I have driven the vehicle about 350 miles and it has experienced several heating and cooling cycles. The coolant level is remaining constant in the overflow bottle and is not overfilling. Engine temperature after warm up is constant and remains steady on the engine temperature gauge.

So maybe I dodged a bullet for now with this overfilling. Maybe the radiator cap caught hung up one time and the relief valve stuck open and allowed the engine to slowly push coolant into the overflow tank???

I have driven vehicles for 40+ years and have never had a radiator cap problem. Unfortunately I have had my share of leaky and blown head gaskets over the years so that was my first thought when I encountered this problem. I will watch for this condition again and replace the radiator cap if it happens again. I do appreciate the sharing of ideas.
 

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Allen, have you made sure the hose from the overflow to the radiator itself has tight fittings? It is possible that the water pushed out and as the engine cooled over several cycles was able to suck a little bit of air, and since air allows expansion simply pushed a little more fluid into the overflow and now that you have refilled it, all should be good for a while. Also, as long as there was at least a little bit of water in the upper radiator hose your chances of dodging the bullet so to speak is better than 60/40 you are OK in my book.
 

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Discussion Starter #6
dana44 said:
Allen, have you made sure the hose from the overflow to the radiator itself has tight fittings? It is possible that the water pushed out and as the engine cooled over several cycles was able to suck a little bit of air, and since air allows expansion simply pushed a little more fluid into the overflow and now that you have refilled it, all should be good for a while. Also, as long as there was at least a little bit of water in the upper radiator hose your chances of dodging the bullet so to speak is better than 60/40 you are OK in my book.
The hose that connects the radiator neck to the overflow bottle is tight and pliable and there was no leakage of fluid on those connections. So I don't think I had a vacuum or air leak which allowed air into the system. Good suggestion though.
 

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Always a possibility, so keep us informed of the results every now and then, see if it was just a fluke or something else.
 

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Discussion Starter #8
Interesting happenings with the car over the last 5 weeks. In mid-May I transferred excess coolant from the recovery bottle and added to cooling system. For about 1 week the coolant in the recovery bottle was staying consistent. I let a week go by while driving the vehicle and then on May 27 I noticed that the coolant recovery has about 1 quart of excess coolant again.

I decided to rent a cooling system pressure tester and checked the radiator cap. It was the original, factory issue. I found that it would hold 15 lbs pressure for about 1 second and then release pressure in several increments and ratchet down to zero. I replaced the radiator cap and felt confident that I had found the problem.

After driving 1 week with a replacement cap installed, the level in the recovery bottle was steady and consistent I felt confident that the problem was solved. Then on June 6 while driving with AC running the engine overheated. I found that the top radiator hose had separated from the top tank nipple. I had the top radiator hose blow off the top nipple in Dec, 2012. Then I reattached and used the original spring clamp and added 1 worm drive radiator hose clamp. Obviously with a new radiator cap the system was developing more pressure and the hose clamps could not keep the hose in place.

So I reattached the top radiator hose with 2 new worm drive hose clamps. After 2 weeks of driving the top hose is staying in place. Coolant level is consistent and I am not loosing coolant. Very strange happenings with the cooling system.

I still think the original problem with the coolant overfilling the recovery system was a failing radiator pressure cap. The top radiator hose problem was a valid distraction and could lead one to incorrect assessment of a failing head gasket.
 

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With the way systems are sealed these days, there have been times that a loose hose clamp can allow air to be sucked into the engine through a connection and not from the overflow. Given the hose did come off, just like when I said check the overflow hose for tightness, there is the possibility that when the engine heated up normally and pushed fluid to the overflow, the suction happening when cooling was simply pulling air from the semi-loose radiator hose itself, allowing air into the system and not reclaiming the fluid from the overflow. I have seen reports of this several times, usually a heater hose though, but still something on the engine side of the radiator cap. Hope this was the problem.
 
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