In line rad hose filler cap question

So I'm in the middle of converting my 90 to an open system. Installed the Morroso inline filler but haven't hooked up the overflow line yet. After flushing the system and then refilling it, the overflow line is letting water out. No, the hose is not hot or under pressure I'm just filling up the system.
Does anyone have a part number for the correct rad cap for this conversion? I've tried two different 16# that are for a 3/4" neck which appears to be the height of my fill neck so I don't think the caps are both bad. I'm wondering am I missing something obvious ?

Thanks for all the help

I just used the stock cap from a 91-later when I did that conversion on my old '88. How are you filling the system with the cap on? Why don't you fire it up and see if it builds pressure?

I haven't pulled the plastic football yet so I'm filling ol' school style. Once I finish the flushing I was going to pull the football and install an overflow tank. The rad cap I'm using is supposed to fit later year XJ and it seems to fit. If I fire up the engine the coolant comes out.

Does the overflow line need to be connected until the system equalizes before the rad cap seals off the overflow? I didn't think that is how it's supposed to work. The overflow line should only open up with over pressure (expansion of hot coolant)

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winterbeater said:

According to the shop manual, the vent valve in the cap center allows a small coolant flow through cap when coolant is below 212. temperature. It closes completely when the boiling point is reached.

Put an overflow bottle on.​

Click to expand...

Sounds to me like the caps he is trying don't have enough "reach" or "depth" to seal in the filler neck. If Moroso tech can't help you then pull the filler neck adapter and take it into the parts store with you--you should have to press the cap down against spring pressure to turn it.

I sent an email to Moroso , I'll see what they respond with. maybe I'll try to contact Stant since they make the rad caps. Right now I'm leaning towards Winterbeater's post based on the following. I also suspected that the cap wasn't fitting to seal so I smeared some lithium grease on the mating surface to see what transfered to the cap gasket, looked like it was fitting fine.

I pulled the hose filler out and wrapped one end in saran wrap sealing it enough to fill the tube with water while holding it vertical with the rad cap on. While it was filling some water started leaking out the overflow tube then it stopped rather quickly as the water level began to fully fill the filler tube. It looks like the little metal plate on the bottom of the rad cap which enables the suction of fluid from the reservoir doesn't stay closed until there is enough pressure in the system or the fluid level is high enough. I was also able to simulate a vacuum by pressing my palm against the open end of the filler tube. I could get the vacuum mechanism to open and close since water would come out of the overflow tube.

So I've ended up just capping the tube outlet so I can refill the system and run it to get a good flush of the block and system before putting it all back together without the 'football' and adding an overflow reservoir.

I know the rad cap is sealing fine since after doing this and driving it around to get up to temp the football is hissing and leaking and the filler cap is tight and dry.

Now I just have to wait for a break in the weather here in 'sunny' Or-a-gun to finish the job.

Whereabouts art thou, homie?