Radiator overflow, but didn't go out the overflow tube
 

I overfilled the radiator on the GTO, didn't leave hardly any air space at the top. Was thinking of filling a modern car with an overflow bottle I guess.

Anyway, after a drive it started to puke out a bit of coolant, this was to be expected. However, it was puking it out around the radiator cap and none was going out the overflow tube. I checked the tube and it is clear. Why or how would the coolant be boiling out around the cap and NOT come out the overflow tube? This doesn't make any sense.

Oh, and for future reference, how much air space should I be leaving in the top tank? I usually fill it to just barely cover the tops of the tubes in the core as you can see them through the cap opening and it still always pukes out just a little after a hard drive or a drive of any kind on a hot day. And I REALLY need to put a temperature gauge on this car.

Wrong/bad cap? Some tanks are marked with a FULL line. If not about an inch over the tubes.

overflow
 

you have the wrong radiator cap !

I cover the tubes with the coolant level, and that’s it. I have one interior gauge in my car.....and it’s a temp.

Could be the cap and I plan to try another I have but it looks just fine and the spring is still properly stiff. The full line is typically the case on radiators with vertical tanks. These radiators have horizontal tanks. And no markings. The top tank isn't really that large so an inch over the tubes would be almost completely full with no air space.

No, I don't.

I have just one gauge, oil pressure. But I think I'll give it a little brother called TEMP. What is the normal temperature on a stock 389? There is a whole lot of iron there. After it had sat for 10 minutes or so the heads were still pretty hot according to my infrared touchless thermometer.

Mine runs 190-200 pretty routinely, but lots of factors affect this. After I shut the engine down, gauge climbs to around 240ish

Yes, fairly typical I guess but good to know. After shut down I was seeing external temps in the 220-230 range. That is if you can fully trust one of those HF touchless thermometers.

I assume that to install a temp gauge you would just put the sender in the same spot as the temp light switch. But are there any other spots where a temp sending unit can be installed? Would be nice to have both the light and gauge.

Does anyone know at what temp the light comes on? The light has never come on but it would still be something good to know.

My Thoughts:

1) you had air in the cooling system

2) You added more water to the upper radiator tank, now
effectively leaving no room for the air to escape.

3) You had the wrong cap on the radiator so the radiator could not
release pressure in the tank at the proper pressure, or a failed cap.

4) I have been told that the light comes on at the boiling point but because of
variance in the caps and the sensors, you could be as high as 225 degrees before the light came on. Not a good thing.

5) High Water Temp drives up even higher oil temp in the engine.

Cop Cars run oil coolers when the oil temps get to 240-245 degrees F.
The water temp is nowhere near that hot. Say the Thermostat is a 195 degree
stat and actually controls at 200 degrees. You still have 12 degrees before boiling
even with the cap off. The pressurized cap allows higher temps than 212 degrees.
Example pulling a trailer under a high load (Davis Dam pull out west).

So you have a couple of things to look at.

a) Purging the air from the cooling system initially.
b) Using a proper thermostat and radiator cap.
c) Testing the sensor to see where the sensor activates the warning circuit.

Tom V

Thanks for all that, but the issue isn't really WHY it puked a little coolant. The reasons for that are pretty clear (I overfilled the radiator) and not overly concerning. The issue is, why didn't the overflow go out the overflow tube instead of out of the cap? It seems to me, if the pressure was enough to push the cap off its seat, the natural route to go out is out the overflow tube.

The cap is an AC cap rated at 15 pounds, appropriate for this car. I have another AC 15lb cap which I'm going to try next, but really, if the cap releases pressure due to too much pressure or a weak cap spring, it should still shoot the overflow out the overflow tube not out from under the cap.

As an additional FYI, the coolant level settled to just barely covering the core tubes once the puking was done and once it had fully cooled down. Will have to see if it has any more to puke after the next drive and now with the alternate radiator cap in place.

On my 65 Chevelle coolant just covering the top of the tubes to be the right air volume to run well without puking. It has a similarly shaped and structured radiator,

You are a smart guy, I am sure you will figure it out.
Have a nice evening. Tom V.

As already stated, it could be a wrong cap, poor fit, or possibly too thin of a gasket in the cap that seals to the radiator. I'd do a little measuring just to make sure all dimensions match up. After market is not always good stuff.

Note the position/location of the over flow hole in the radiator neck. It is possible that it was not located as per the original style cap, so when the pressure builds up, it cannot go out the over flow tube because it is being covered/blocked by the radiator cap - ie not lifting high enough and/or maybe stopping in its travel or binding before it can dump out the over flow, so it blows out the cap instead.

If the pressure did build up and push the over filled coolant out the cap, and you know the cap is good, then I would insert a piece of flexible wire out the overflow hole at the radiator and snake it down through the overflow hose just to make sure there is no obstruction in the overflow hose. Here in the south, we have mud dabbers that love to get up into small openings to build a mud cocoon to lay eggs in. They will go anywhere and the opening of a radiator over flow tube is a perfect candidate.

Use the laser temp gun on the radiator at differing points, this will give you a better number on coolant temps. Shoot the top and bottom hoses. You should see a marked difference in the temp of the water coming out of the engine and the water temp going back in. As noted, the minute you turn off the engine, temps will climb as water is no longer circulating and all the engine heat is now going into the coolant raising the temps - this is normal and you should not be concerned. The running temps are what you want to know.

Both caps are GM built caps. They've always worked just fine, held pressure fine and if they puked they puked out the overflow tube. I removed the tube and I can blow through it easily. I can pass a small screwdriver through the metal nipple on the radiator neck. There is no obstruction in the overflow tube.

The only thing I can think of at this point is that the very top rim of the radiator neck isn't as flat as it should be and is not sealing to the metal seal on the top inside of the cap. There are two seals involved, a rubber one on the end of the spring that seats against the bottom of the neck opening and is thus what holds in the pressure. There is a secondary "seal" at the top of the cap made of metal and has a slight spring to it which should more or less seal the top of the cap to the top of the neck thus if the spring loaded seal is pushed off the seat the resulting release of pressure should be forced to go out the overflow tube which is built into the side of the neck. A simple visual inspection of the cap and neck don't show any nicks, cracks or anything out of line but I guess I'll have to look over the neck closer with perhaps a straightedge and see if maybe it isn't as flat and concentric as it appears at a glance.

I didn't mention earlier but I'll mention it now: the car has been together and running for about 6 years now without any cooling or coolant issues. No parts have been changed, nothing is new. The radiator is the original and was completely recored during the restoration and the heater core was the original and repaired to new at a radiator shop. The heater works fine. All the hoses and clamps were new at the time and there are no leaks anywhere. In the past on a really hot day it may have puked a tiny bit of coolant, but it always did it out the overflow tube. This is nothing abnormal, all my old cars do this on occassion on really hot days. The thermostat is stock, Gates as I recall, the cap is GM >> AC and at the proper 15lb rating. The coolant is nothing special, just the low tox green stuff. It has never overheated, never set off the temp light anyway (and yes the temp light works, I've tested it). When the cap wasn't puking, there was plenty of pressure in the system, the hoses were firm as you'd expect on a hot engine. External temps taken at the time were not excessive. The issue seems entirely related to the cap and neck, but it still doesn't make a lot of sense. It didn't puke one tiny bit until the car was sitting off and further heat soaking. So, in my mind the issue is not THAT it puked coolant (again, I know it was over full), but HOW it puked coolant.

Is your cap of the dual gasket variety like the later coolant recovery type cap? One in the body of cap itself as well as the one at the bottom. If it doesn't have the two gaskets, when it purges it will come out from the cap itself. If you are using a cap by application then it will only have the lower gasket. From the top both caps look the same.

As I explained above, the caps are correct original caps. The bottom seal is rubber mounted on a pressure spring. The top "seal" if you want to call it that is merely a flat piece of metal that loosely seals the top of the cap to the top of the neck with its only purpose being to force overflow pressure to go out the overflow tube. The metal "seal" is typically brass.

Like this one sold by Ames (which is a reproduction though).

https://secure.amesperf.com/qilan/De...A&web_access=Y

Exactly my point...The cap is correct for your year and model. The metal seal is of the generation of when the overflow was not meant to be recovered, meaning as the car car cooled the coolant would be siphoned back to the radiator. The next generation cap is for coolant recovery and that typically brass seal as you put it at the top had a rubber gasket so it wouldn't suck air on the way back. Back in the day when coolant was vented to the atmosphere.

https://i.ebayimg.com/images/g/QFQAA...98/s-l1600.jpg

Correct on above post by Frankie.

No cap I have ever owned for the radiators on my 64s or 65s have ever had the larger rubber gasket as pictured above. None of them have coolant recovery systems. I guess I am missing what the whole point is about whether I have cap A or cap B, as long as they seal at the base of the neck either one should force overflow out the overflow tube if the pressure spring is pushed of its seat by pressure in the radiator.

Yes, If you change the cap to the overflow tank to one that has a nipple on the inside where you can put a piece of hose on it to go down to almost the bottom of the tank and put on the style of cap I provided your system will recover and still look factory, instead of belching coolant out to atmosphere. Your coolant level should maintain.

I don't have an overflow tank. Never did. Don't want one. Talking about a 64 here, they never had an overflow tank nor a recovery system.