Moon86

When I fill the coolant system from the remote fill I installed in the throttle body heater line, coolant comes out of the overflow tube on the radiator neck. The rad cap is on and fully tightened. The cap is a brand new lever release Stant #10331. The car is not running and everything is cooled down when I try to fill it. AFAIK it's the original radiator. Being that the rad would be 26 yrs old could the the mating surface for the inner gasket on the cap in the plastic tank be worn or shrunk from age? It would seem like with the cap on and tight no water would should be coming out of the overflow when filling. I'm at a loss on this one.

SJW

No liquid should be moving past the radiator cap unless there's enough pressure in the system to exceed the cap's rated pressure-relief release point. The cap has a spring that applies pressure to seat its sealing surface against the mating interface on the radiator fill port.

One of three things is going on here:

1. There is a flaw of some sort with the mating surface on the radiator fill port. A crack, a chip, a nick, it's warped, or some such. A visual inspection might reveal this.

2. There is a flaw with the sealing gasket on the cap, or its pressure spring is broken or has otherwise failed. Or it's not the correct cap for that radiator.

3. The return inlet valve in the radiator cap is not sealing properly. This is the smaller disc in the center of the sealing gasket, and it is the return path by which coolant is drawn back into the radiator from the external reservoir after the engine has cooled and the liquid has shrunk in volume.

First, check the inlet port of the radiator carefully for any signs of flaws. If none is found, replace the cap, and verify that the new part is in fact the correct part for the application. The fact that the cap you're using now is brand new does not indicate that it is not faulty.

Live well,

SJW

Joe C

don't know if i'm following your thread correctly - you installed a remote fill port on the throttle body coolant line - correct??? if i am, i have to ask why? why not just fill the radiator directly at the radiator? i do agree with SJW though, nothing should get past the radiator cap unless cooling system pressure exceeds the pressure rating on the cap. the plastic overflow tank, or the plastic cap should have nothing to do with your issues. seems to me that, if you modified the cooling system, you may have opened up a can of worms, but most likely, it's the pressure cap. my suggestion, get the correct ac delco radiator cap from your local dealer, and return the cooling system back to its factory configuration.

Moon86

I put a remote fill in so I could get that last bit of water in. It's just a flush fitting I put in the line. I went on the Stant site and the # for a Corvette cap is different than a regular Chevy cap. Don't know what the difference is as the press range is the same. If the inspection of the filler neck is ok I'll pick up the right Stant cap and see if that helps. I wonder if this is a more common problem than just me but people miss it.

Moon86

Bought the right Stant cap, tried filling and it still leaked out the overflow tube. Cut a piece of plastic from a milk jug and slipped it between the center gasket and the metal backing. Reinstalled the cap and it didn't leak out the overflow tube while filling anymore. Have to wait and see if the water level will hold. Strange problem. I wouldn't have found it if I hadn't heard gurgling by the rad cap.

SJW

Did a careful inspection of the radiator's fill port reveal anything that's not right?

This has me wondering if this car has had a replacement radiator installed that differs from the OEM unit in some way at the fill port.

You did fully seat the cap onto the radiator (to the second stop), I presume?

Live well,

SJW

TWISTERUP

Have you checked the height of your new cap beside the old cap--are they the same ? Further , does the same thing occur with the old cap?
Lastly the "lever" is down/closed when this occurs--correct?

Moon86

Couldn't find anything wrong with the rad neck. I tried 3 other used rad caps I had including the original AC cap, all with the same result. I think the plastic tank/filler neck must have aged over time and shrunk or something making just enough clearance for the water to seep by. I wouldn't be surprised if a lot of the older c4's have this problem and the owners don't know it. I may make a rubber or plastic gasket to fit into the filler neck and try the other caps.

TPI BOY

SJW

At this point, I think I'd be shopping for a new radiator if I were in your position. Last time I checked, Corvette Recycling had spank-new OEM radiators for a very attractive price.

I wouldn't encourage you to add another gasket to the mix. Depending upon what's really going on, it could cause the pressure spring in the cap to compress more than its design intent, leading to a higher-than-designed pressure release point. This could lead to a rupture of the radiator, heater core, or hose. I wouldn't take the chance...

Live well,

SJW

Moon86

I had thought of that myself. The shim I put in the rad cap is the
thickness of milk jug plastic so I didn't think it would change the pressure rating of the cap. If I put the same shim in the neck it would be just to try out the other caps. I wouldn't use the shim in the cap with the shim in the neck. Now when I open the remote fill I get a little bit of pressure noise and I can only add about a tablespoon water. This is after letting the car sit over night. I think this is how the system is supposed to work

SJW

I'd recommend against adding a plastic shim between the fill neck and the gasket on the cap. It's not likely to seal well, as the plastic isn't anywhere near as compressible as the gasket.

I'd spring for a new radiator (check out Corvette Recycling) and be done with it. You'll probably never have confidence in the cooling system until you get that radiator out of there.

Live well,

SJW

Moon86

UPDATE-- Picked up a new hd radiator at Advance Auto for 168. It's double the core thickness of my old one. Problem solved. Coolant does not come out the overflow in the neck with the radiator cap tight anymore. As I said before, this could be a possible reason for unexplained gradual coolant loss.