This was posted into a long thread, but I think it's useful information about an interesting problem.
-Jim Blake

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I have a similar problem.

New hoses all around. I just replace the cabin heater inlet hoses and valve (valve was leaking), and then filled and burped the system. The car runs fine, but when I park and let the system cool, the lower radiator hose collapses a bit from vacuum in the cooling system. If I open the cap, the air escapes and the lower hose returns to normal.

The level in the overflow reservoir does not appear to be raising or lowering at all.

I pulled the hoses from the reservoir to check for blockage. Free air flow all the way up to the neck of the radiator, but the nipple on the neck of the radiator seems like it is eiter blocked or has very restrictive airflow.

I've included some pics, and from this photo it looks like the overflow bleed should come from the outer ring inside the neck, and there should be a hole where I have drawn the red circle.

https://www.hondaaccordforum.com/for...1&d=1418064743

1) Is the airflow through the overflow nipple of the radiator neck supposed to be highly restrictive? Or should it be relatively free flowing?

2) Can you tell from the pics if I have the wrong type of radiator cap?

Thanks. John

Looks like correct cap to me. It should have a pressure release of 14-18 psi. If you get coolant overflow at normal operating temps, then cap may be out of spec.

Have you installed a new thermostat recently? Thermostat should have a bleeder to allow pressure equalization across the thermostat. It's a small pin that is positioned at TDC in thermostat housing. Absence of this bleeder pin might cause hose collapse on cooldown.

good luck

Another thing to look at is the very center disk on the underside of the radiator cap. It's a vacuum-break with a relatively weak spring. You should be able to easily move that disk to expose the center opening.

If that center disk is somehow stuck, or screwed up in a way to prevent it from opening, that would explain having a vacuum in the system on cooldown.

yes, new thermostat and hoses ... the system warms, circulates, burps as expected. Lots of heat inside the car. Temperature gauge always shows comfortably below the halfway point. On the radiator cap, the outer spring is resistant but gives when I push on it, and the center valve is easy to open and close.

There is just no airflow when I blow on the overflow nipple coming out of the radiator.

I edited my first post to point to the location where I think there should be an opening.

Is it possible that this is a manufacturing defect, that the hole was never drilled? !!

Can you pull the radiator cap off your car sometime and verify if you can see the hole there or if it's hidden?

Thanks. John

Yes there should be a hole where you put the red circle. It's not a very big hole, are you sure it's not there? (You can see it firsthand, better than the photo)

And the way you're holding the cap, the edge of that innermost disk is visible. That should open easily by pulling downward. But if there isn't any hole for the overflow hose, that would explain everything.

Open the overflow reservoir, that hose goes down (like a straw) to the bottom of the reservoir. You should be able to blow into that hose & it blows out through a little hole where you put the red circle. Don't go crazy with air pressure. I'd be careful about blowing with your mouth because you really don't want any antifreeze in your mouth.

If needed, pull the hose off the plastic nipple at the radiator neck. Poke in there with a wire or something?

I already pulled the overflow tubes and checked them all ... free and clear, ~3/16" ID on these. Plenty of air and fluid flow.

I then connected a short piece of plastic tube directly to the overflow nipple at the radiator neck, and blew. Nothing, and I think the only air I was able to push through the tube escaped at the nipple connection.

I also poked a rod inside there to see if it was clogged with caked on material ... it felt solid to me. This is why I suspected a dropped breather channel inside the plastic ... maybe that's overthinking it ;)

I will take another good look when I get home. If you think there should be a nice 1/8" to 3/16" hole staring at me, then it looks like this radiator has been working for the past 2 years that we've owned the car -- and possibly it's entire life -- without a coolant overflow system! (One of the upper fan housing bolts is missing though, so there may also be an undocumented radiator replacement sometime in the past.)

If/when you get to your Honda, if you wouldn't mind popping the radiator cap and confirm the hole position and size, maybe take a pic, I would appreciate it. I can't find any pics of the Honda radiator online that show this. (All other auto radiators have a nice big hole there.)

Thanks. John

I just looked at the 97 SE outside, there is a small hole there. No where near the size of the tube...more like the size of the "straw" you get with a can of carb cleaner.

You ight have to take a paper towel and wipe inside the lip to remove the fluid/coolant in that lip to see it.

Yeah, you should have been able to blow air through it without too much effort.
I don't recall it being very difficult to see that hole inside the radiator neck.

Well, guess what? No hole.

I took the cap off this morning, wiped the outer ring of the neck dry and looked in and felt around in there ... only a slight dimple, but no hole.

So I started poking a straight pick in through the nipple, and I eventually broke through to the outer ring of the neck, and was able to route it out to maybe 1/16" of an inch.

This was not debris or crud -- I had to break through a thin film of hard black plastic to get through.

So, in the injection molding process of making this radiator top, someone forgot to route the passageway. And the car has been running with a fully pressurized cooling system with no relief valve. That helps to explain the cause of the leaking water pump and exploded top radiator hose which started the whole process of me revamping the cooling system. (I'm glad I did though -- the car really needed a timing belt change and the belt pulleys were actually wobbling).

When I get home again tonight I will clean out the hole a bit more and then go for a test drive. I will probably pick up a new cap and overflow hoses just for good measure as these are looking old.

Thanks for providing the reference info on your radiators ... another future crisis averted!

And I guess this is something that should be noted on the forum ... if your coolant reservoir doesn't seem to be doing anything, make sure the vent tube actually has an opening into the radiator neck.

Thanks. John