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I got home to find a trail of coolant down the driveway and puddles forming under the car (1991 Toyota MR2 - 5SFE). Concerned that I blew a hose, I checked them all. Nothing. Finally decided to see if I had anything left in the overflow to see how much I'd lost on the drive home. It was overflowing!

Here's the weird things: Temperature gauge was perfectly normal the entire drive home, not high, not low. The coolant in the overflow was cold (room temperature), I expected it to be hot. No smell of coolant burning/cooking off either.

Once the engine cools down I plan on checking the oil cap for residue and the dipstick for level/residue, but if it was a headgasket the coolant should be hot. Ditto for thermostat & fans. Water pump failure and the gauge should have either gone high or low, not stayed normal.

I'm baffled. No prior indication of any problems. Seemed to happen out of the blue. Ideas?

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up vote 3 down vote accepted

No one's mentioned the simplest thing yet. If the system is overfilled, it will overflow. As it heats up, the coolant expands and that is why you don't fill up the overflow bottle all of the way. (If you are pouring fluid directly into the radiator, you do fill it up completely.)

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It's not overfilled. – Brian Knoblauch Apr 21 '14 at 12:09
1  
I was wrong. I did manage to overfill it... This car has a slow leak during cold weather, so I periodically have to top it off. The coolant bottle is buried under the intake manifold and a fuse box, so it's very hard to see the level. The dark/light break which I thought was the fluid level is actually just a discoloration of the bottle. Apparently the way to determine level in this car is to use a ruler against the overflow feed hose to a known level. DO NOT try to see the level in the bottle (basically ignore the owner's manual). – Brian Knoblauch Apr 22 '14 at 21:05
    
Overfilling can also cause airlocks so the system may still overflow even if the excess fluid is removed. After correcting the level, "burp" the system by running the engine (cold) with the coolant cap removed and firmly squeeze the radiator bottom hose watching the coolant until no more bubbles appear. It's best to perform this with the cars heater set to maximum heat setting. – Steve Matthews Apr 21 at 10:45

Your radiator cap may be bad. If it's not holding pressure, you're going to have some overflow.

If that isn't it, the next thing to look at is for a head gasket leak (or possibly a cracked head). Remember, you can have a head gasket leak without oil in the water or visa-versa. You can get a kit from AutoZone or the like to test for exhaust gasses in the coolant.

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A head gasket leak on the coolant side can actually overpressurize the overflow tank. – Mark Johnson Apr 18 '14 at 19:35
    
Does not appear to be either so far. I can flog it down the road and no overflow. No coolant smell to exhaust either. Only way to duplicate the overflow is by running it in the driveway and not moving the car. Temperature still reads normal, but eventually it starts pumping out coolant. I've yet to hear the fans kick on too, so it may be an electrical issue (there are 2 independent fan circuits and neither fan is running). – Brian Knoblauch Apr 18 '14 at 23:11

If you have blown a head gasket(depending on severity)water spitting out of your exhaust is an obvious sign).A very affordable pressure test is advisable and may serve to save you money and time

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I had the same problem with my car and finally it was the Thermostat who was the culprit, it was not working properly and due to that even when the engine was heating up it did't start the Cooling fan and then ended up with the lots of heat and pressure and due to that coolalnt fluid overflows. Now I got the Thermostat replaced and its first day and i did't experience the same problem again and hoping it should run successfully now.

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Two things come to mind: 1) Your cap isn't sealing properly or the spring has worn out, meaning you don't have enough pressure in your coolant system, allowing the coolant to expand and overflow. OR 2) You have an air bubble trapped somewhere and when the engine heats up, the bubble expands and causes the coolant to overflow. To get rid of a trapped bubble, you need to "burp" your car, i.e. wait til it cools off, remove the radiator cap or coolant overflow bottle cap, turn the car on and let it heat up, when the fan comes on, put your aircon on HOT and put the blower on full. You should see a few bubbles coming up into the radiator/coolant bottle and the level should go down somewhat. After that's done, top it up if necessary and go for a drive. If the problem's gone, it was #2, else you need to replace your cap. It's about $10 (or whatever currency you have where you live).

If it still persists, you probably have a bad gasket and will need to have a pressure test done to confirm. Also look out for "chocoloate milk" in your oil or clowdy coolant, which are signs of a blown gasket.

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If the pressure test is fine and ur head gasket is not leaking try and bypass the heater radiator by joining the pipes then bleed the system and get out all the air bubbles and add antifreeze.

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I'm not sure how this answers the OPs question? – Pᴀᴜʟsᴛᴇʀ2 Jan 8 at 20:37
    
Honestly I think @Paulster2 is right, this really does not answer the OP's question. – cdunn Jan 8 at 23:15

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