Radiator overflow. Why?

[Erik S. Klein]

I took the day off yesterday and decided to spend a little of it with my car. I stole away for a quick ride to get some breakfast and when I returned to my parked car I noticed about a cup of anti-freeze under the front.

Being the paranoid type I immediately called my favorite shop to make sure they could take the car and then called AAA to get a tow.

After about 45 minutes of waiting for the truck I took a closer look (the engine had cooled sufficiently) and realized that what I had thought was the lower radiator hose was actually spillage from the overflow vent. The radiator was full and the hoses were fine. I drove to the shop and got a pressure test done which turned out okay. I put on a new cap and a new temp sender because they were needed, but didn't deal with anything else.

From what I understand this car isn't supposed to have an overflow tank or vent and I removed the one that a previous owner had bubba'd onto the car years ago (it was really a catch basin and not a true overflow tank since the contents couldn't get back in to the cooling system.) That tank had been dry for every day of the 15 years+ that I've owned this car.

So, finally getting to my questions, why would the radiator suddenly overflow?

It wasn't a particularly hot day. I've driven in much hotter weather.

I wasn't driving the car hard and only spent about 3-5 minutes on the freeway.

I just checked the oil and made sure there was sufficient oil in the pan.

The tires are fine, well inflated and so on.

The radiator is clear and clean. There are very few bent fins and no debris.

The engine temp never got above normal levels.

So why did the girl **** on the road?

 

[vettepilot]

I took the day off yesterday and decided to spend a little of it with my car. I stole away for a quick ride to get some breakfast and when I returned to my parked car I noticed about a cup of anti-freeze under the front.

Being the paranoid type I immediately called my favorite shop to make sure they could take the car and then called AAA to get a tow.

After about 45 minutes of waiting for the truck I took a closer look (the engine had cooled sufficiently) and realized that what I had thought was the lower radiator hose was actually spillage from the overflow vent. The radiator was full and the hoses were fine. I drove to the shop and got a pressure test done which turned out okay. I put on a new cap and a new temp sender because they were needed, but didn't deal with anything else.

From what I understand this car isn't supposed to have an overflow tank or vent and I removed the one that a previous owner had bubba'd onto the car years ago (it was really a catch basin and not a true overflow tank since the contents couldn't get back in to the cooling system.) That tank had been dry for every day of the 15 years+ that I've owned this car.

So, finally getting to my questions, why would the radiator suddenly overflow?

It wasn't a particularly hot day. I've driven in much hotter weather.

I wasn't driving the car hard and only spent about 3-5 minutes on the freeway.

I just checked the oil and made sure there was sufficient oil in the pan.

The tires are fine, well inflated and so on.

The radiator is clear and clean. There are very few bent fins and no debris.

The engine temp never got above normal levels.

So why did the girl **** on the road?

That's a very good question.... as a side note, I used to have a Golden Retriever that did the same thing when I came home from work.
Perhaps it (the cooling system) had an air bubble that finally burped out of the block and it pushed the coolant out the overflow. Maybe the radiator cap relief valve was not seated squarely for some reason, and there was just enough pressure to cause it to dribble out. Or the valve inside the pressure relief valve may have had a small foreign object in it that caused it to allow leakage. That's where I would begin looking, around the cap area, check for anything that could prevent proper sealing of the cap and / or relief valve.

vettepilot

 

[Vettehead Mikey]

99% of the time the cause of this is an over filled system. Leave it alone.

 

[81 Corvette]

Did you top off the radiator recently? I'd find an overflow tank and set up the system correctly.

 

[Erik S. Klein]

To answer the questions and provide some more info. . .

The old radiator cap may have been the culprit and it was replaced, but it was fairly new (mileage wise) since the entire system was replaced less then 5000 miles (about 6-7 years) ago including a re-cored radiator, new hoses, water pump and so on.

There may have been something stuck in the block, air or otherwise, but I didn't see any evidence of it.

I haven't touched anything on the cooling system since the major work except to remove the useless overflow tank. That didn't touch anything, really.

I did decide to basically leave it alone. The coolant level was fine and the car runs great. I was just overly nervous about the green puddle. . .

 

[photovette]

Could be too rich a coolant mixture--counterintuitive, but I've had it happen.

 

[Bob Chadwick]

Venus lined up with Mars then lined up with Jupiter. The car just had the right conditions and burped a little extra coolant. Watch it but based on what you have explained I doubt it will ever happen again.

With an overflow tank the process would go over and over again blowing coolant out and drawing it back in as the car cools. An overflow tank keeps the radiator completely full while one without is usually down an inch or so from the top. When you turn the car off, the coolant heats up with no radiator/fan flow, the pressure exceeds the rating of the cap and the coolant blows until the pressure drops under the radiators rating.

 

[SR'71]

To answer the questions and provide some more info. . .

The old radiator cap may have been the culprit and it was replaced, but it was fairly new (mileage wise) since the entire system was replaced less then 5000 miles (about 6-7 years) ago including a re-cored radiator, new hoses, water pump and so on.

There may have been something stuck in the block, air or otherwise, but I didn't see any evidence of it.

I haven't touched anything on the cooling system since the major work except to remove the useless overflow tank. That didn't touch anything, really.

I did decide to basically leave it alone. The coolant level was fine and the car runs great. I was just overly nervous about the green puddle. . .

Quote "I haven't touched anything on the cooling system since the major work except to remove the useless overflow tank". HINT The OVERFLOW TANK has a purpose, it collects gas pressurized liquids within the limits of your radiator cap. If you excede the cap PSI with trapped gas press you get COOLANT ON THE GROUND. See my post history and read the outcome Good lk

 

[Vettehead Mikey]

Quote "I haven't touched anything on the cooling system since the major work except to remove the useless overflow tank". HINT The OVERFLOW TANK has a purpose, it collects gas pressurized liquids within the limits of your radiator cap. If you excede the cap PSI with trapped gas press you get COOLANT ON THE GROUND. See my post history and read the outcome Good lk

Pre '73 Corvettes do not have overflow tanks, they have external expansion tanks. Not the same beast at all.

 

[John Doe]

Pre '73 Corvettes do not have overflow tanks, they have external expansion tanks. Not the same beast at all.

however.... I became much more relaxed driving my 68 bigblock when I installed an aftermarket plastic overflow tank. Instead of having the overflow hose venting to the street (or the parking lot, or the driveway, or the garage) it goes into the plastic can. The can now almost never gets any coolant, but I feel better knowing it is there. (Also required at the dragstrip)

 

[Erik S. Klein]

Just as a follow-up - I took the car out yesterday and, in spite of the fact that the weather was warmer, there was no overflow at all.

The only changes were a new radiator cap and a new thermostat. I'm guessing the old cap had given up the ghost. . .

Thanks all!

E