I have a 1969 Corvette with a big block. I have a new big block radiator, thermostat, and various hoses. The car does not overheat, however, everytime I drive it for about 10 minutes, coolant goes into the expansion tank and overflows into the overflow tube. This shouldn't happen, however, for the heck of it, I installed two 1-quart overflow tanks in the system, and it even overflows out those as well. Recently I tried a higher pound cap (19-21) and that didn't do anything. Many times, it starts overflowing after the car is turned off.
Does anyone have any recommendations? Keep in mind, the car is NOT overheating and the gauge is reading correctly. Thank You!

 

I think yours actually came with an expansion tank from the factory which is basicaly like a second rad with the pressure cap on it and not overflow tank. If you bought a direct replacement rad it might have a provision for a cap but still function like the old rad meaning the overflow tube is an open line to the overflow tank you are using so coolant flows freely between the 2. Just a guess but worth looking at.

 

Yes, you're correct. I have a radiator and an expansion tank with a cap. On the expansion tank, there's a overflow tube, under the cap area. Coolant comes out of that tube and just dumps on the ground. So I installed two overflow tanks to hold the coolant. I ran the overflow tube to those tanks, and now it still runs out of them, and onto the ground.

 

I went through this when I first put mine together ( I was not the one who disassembled the car!).

You need to be sure you have the correct caps AND you must have the correct cap on each device. If you mix them up, then it will do as you said. Since I got the configursation correct, it has worked flawlessly.

 

Sorry, must have glossed over the exp tank reference in your first post. You might try taking it a rad shop and have them shoot the exp tank with a pyrometer (sp?) to see what temp it reads. Only other think I can think of is that the exp tank cap is not getting a good seal if it is still boiling over with the new cap.

 

There should only be one pressure cap, on the expansion tank right? The radiator should not have a cap. In my case, I have a radiator with a cap. I found it was very easy to overfill the radiator which will cause overflow. The odd thing is if you overfill it does not just blow off the excess until the correct level is reached. A siphoning action is started and you lose a lot of coolant. Not sure if the same thing happens with an expansion tank equipped system.
For radiators with caps, the manual says fill to within 3" of the top. (3" is a LOT of room!!). :)

So you might be starting with too much coolant in the system, ie overfilled. MJ


[Modified by MNJack, 12:50 PM 6/24/2003]

 

OK, first of all, thanks alot for the reply guys. I've been loosing hair on this for a while.

I have one place for a cap. I think the cap is suppossed to be a 15 # cap. Is that correct? I have tried a 13 # cap, and somebody told me to get a larger # cap like 20#. They said for each #, it increases the boiling point, hence less expansion.

 

Stock cap is an RC-26 cap, rated at 15lbs. If you have A/C and a brass expansion tank, the stock cap is RC-15, but still rated 15 lbs. MJ

 

Thanks. I would think I could just buy any 15# cap. Or do you think that buying the original makes a difference?

 

Very interesting!!!!
I have a 1969 with a brand new 509 cubic big block in it.
Brand new Griffin HP series aluminum radiator (no cap), brand new aluminum water pump, all new hoses, new overflow tank, new stant 195 degree thermostat, dual spal 13 inch electric fans, and brand new cap on the overflow tank rated at 16 lbs. My car does over heat at all or at least it has not yet. I have the exact same problem with the overflow tank belching out a little antifreeze out of the overflow tube that sits directly under the cap everytime I start the car when its cold. Once the car is hot it does not do it. I have yet to figure out the problem. I only fill the overflow tank half way up when its cold and it still does it. Once the car is running its fine. Once the car is hot and I start it again its fine. I have checked the routing of the hoses against the AIM and as far as I can tell they are routed correctly. My next plan was to try a 20 or a 21 lb cap but it sounds like that did not fix your problem so I doubt it will fix mine. Someone has to have the answer to this puzzle somewhere.

 

OK... listen up.... The most likely cause of this venting is.. The routing and or configuration of the upper hose.... You MUST have the "U" shaped trap and the U must go in front of the alternator. Most aftermarket hoses are not correct. You must use a factory configured hose. Any other method of routing puts the highest point of the system above the pressue cap. Air Always goes to the highest point (remember you H.S. science class). That will force coolant out of the pressure cap when it reaches the psi on the cap. What should escape is air, but since it is not at the cap it will force coolant out. Now the system sucks in more air when it cools and, with more air in the system there will be more expansion to force out even more water. A viscious circle has now started .... The pressure cap MUST be the highest point of the system in order to work correctly. After you get that accomplished and fill the system ... Let it vent the first time to esablish the correct level. That will be about half full when cold. This is the single most cause this problem.



[Modified by bluevetteman, 5:53 PM 6/24/2003]

 

Bluevetteman I have the factory upper and lower hoses on the car.
I think have the upper hose is installed correcty as you described but maybe I need to shift it down a little more.
I will send you a e-mail with a pic and let me know what you think.
Thanks.

 

OK. OK. Now we're getting somewhere. I admit my knowledge of cooling systems are limited but what BlueVetteMan said makes sense, especially for me. I bought that hose off the wall of a parts store. It's close to the original, but must not be close enough. I will try and find an original hose for this system and post back the results.

 

You could try jacking the car up on the filler kneck side. I seem to always have some are trapped in the upper hose, can feel it when you squeeze it, but after a few times driving it is seems to push that air out, feels very firm after.