Leaking Coolant Overflow Cap

[emmvette]

I've had small amounts of coolant spilling out of my white plastic overflow tank from under the cap for a while and its time to correct this. I called my local dealer and the part is ZR-1 specific and no longer available.

Has anybody else had this issue and how did you correct it?
I'm wondering about a home made gasket? Has anybody tried this?

This tank is not pressurized, right? Mine must not be since fluid is leaking out....

[Paul Workman]

Ah, well...the cap has to breathe. And, there is a (paper) gasket under Ami's 91 cap, but none under my 90: tho it could be it is just missing on the 90, I suppose. And, there's some sort of splash barrier diaphragm(?) or dirt filter, integrated with the cap between the 3 vent holes on the inside of the cap and the single air vent hole on the top of the cap. I suppose you could make your own or adapt one from most any secondary coolant reservoir bottle cap.

BUT! The bigger question is: Why is that holding tank reservoir over-flowing in the first place??

As coolant expands and contracts as a function of heat, that plastic tank functions as "breathing space" for coolant - allowing for it to do so without being lost.

Things that usually cause that holding/reserve tank to over-flow:

• Too much coolant in the system, especially if over filled when the motor was cold. The "COLD" line on the dipstick means pretty much just that: The coolant level should be approximately at the line when cold.
• A leaking pressure reservoir cap (Thanks, Marc Haibeck). With heat, the coolant expands and escapes past the pressurized coolant pressure cap to fill the holding tank (white plastic) reservoir. And, as the motor cools, the coolant contracts, resulting in vacuum in the system that "draws" coolant back into the system past the pressure cap. If, however, the pressure cap is leaking so that air gets into the system when the coolant in the motor contracts, then the coolant in the holding tank reservoir is NOT drawn back into the system. The coolant level in the tank remains high/full. So, with every heat cycle, more and more coolant gets pushed into the holding tank, but cannot be drawn back into the system - which may well result in elevated coolant temperatures which could accelerate the issue. Replacing the pressure cap is cheap enough to do.
• Air is getting into the system due to a leak in a hose or (worst case) a head gasket leak failure. (Ouch.) But, before jumping to conclusions, one can do a cooling system pressure test to see if if there isn't a leak in the system elsewhere e.g., a hose connection etc. where air is getting into the system. But, if a head gasket is the issue, there will be other symptoms outside the scope of this question, and certain procedures used to verify absolutely if in fact there IS a true head gasket failure - not a small fix on the LT5 (as the motor has to come out and the head(s) removed and cams re-timed. But, then again, it might be an opportune time for a cam upgrade too).

Let's cross our fingers and hope for a leaking pressure cap or a cracked hose or nipple leading to the holding tank!

P.

[emmvette]

Originally Posted by Paul Workman

BUT! The bigger question is: Why is that holding tank reservoir over-flowing in the first place??

Its not over-flowing; it only leaked / splashed out during an auto cross / road race event from spirited driving. It is coming out from the overflow cap, which suggests I just need to fabricate a simple gasket for the lid, or replace the lid.

[We Gone]

I've had mine do the same after spirited driving, I'm sure the liquid gets splashed around some. I just lowered the level down to just a small amount left in tank when cold.

[Paul Workman]

Originally Posted by emmvette

Its not over-flowing; it only leaked / splashed out during an auto cross / road race event from spirited driving. It is coming out from the overflow cap, which suggests I just need to fabricate a simple gasket for the lid, or replace the lid.

Ah! Glad to hear that. Mine was over-flowing, due to the pressure cap (on the black surge tank) leaking. A new cap fixed that problem of coolant not being drawn back into the system as the block cooled.

My 90 doesn't have a gasket (on the over-flow cap), but I fill the over flow tank to the "COLD" mark (on the attached dipstick) when the motor is cold (with the pressure reservoir filled completely). The level when hot never sees the coolant go over the HOT level in the tank, w/o something being amiss.

Ami's 91 Z apparently has a "selfee" paper gasket, cut from gasket paper, but is otherwise identical.

[bdw18_123]

The '90 ZR-1 I used to have had this issue. But it wasn't coming out from under the actual cap.

There is a brass part that the cap locks onto, which is held on by the plastic neck of the tank. On mine, the brass part could be twisted back and forth, only being stopped by the overflow tube, indicating it wasn't fully sealed anymore. So fluid was leaking out from between where the brass part meets the plastic part.

So I just covered the whole seam all the way around (including around the overflow tube, but allowing for the hose to fit on still) with some JB Weld and that fixed the issue.


You must have a very early '90 Z if your overflow tank is white plastic. I've only seen the white plastic tanks on the prototype cars.

[emmvette]

Originally Posted by bdw18_123

The '90 ZR-1 I used to have had this issue. But it wasn't coming out from under the actual cap.

There is a brass part that the cap locks onto, which is held on by the plastic neck of the tank. On mine, the brass part could be twisted back and forth, only being stopped by the overflow tube, indicating it wasn't fully sealed anymore. So fluid was leaking out from between where the brass part meets the plastic part.

So I just covered the whole seam all the way around (including around the overflow tube, but allowing for the hose to fit on still) with some JB Weld and that fixed the issue.


You must have a very early '90 Z if your overflow tank is white plastic. I've only seen the white plastic tanks on the prototype cars.

Hmmmm, My car isn't an early '90; its middle of the pack. However I can't vouch for how much is original on it. I have no brass piece like you mention above, just a white plastic tank with a thin walled (cheap) black plastic cap screwed onto it. Here are pics of my cap and the overflow tank.

You can see a little fluid on top of the tank, this is after a gentle drive. Looking at the cap, there is a good size hold about 7/16" dia in it that goes directly up and out to the outside. This is definitely where coolant is splashing out. I wonder if there is supposed to be some type of button or smaller insert so the hole isn't so big? Does this look right to you guys - is this what is on your overflow tank?

[WVZR-1]

Originally Posted by emmvette

I've had small amounts of coolant spilling out of my white plastic overflow tank from under the cap for a while and its time to correct this. I called my local dealer and the part is ZR-1 specific and no longer available.

Has anybody else had this issue and how did you correct it?
I'm wondering about a home made gasket? Has anybody tried this?

This tank is not pressurized, right? Mine must not be since fluid is leaking out....

The cap isn't and never was ZR-1 specific BUT it is discontinued from GM. Corvette Central has new that appears to be correct for $7 - I'm sure there are others that have also. The tank is not pressurized.

Corvette Central cap:

http://www.parts123.com/parts123/yb....sort~1~cadefib

I would investigate the hose connection to the surge tank, I believe it should be clamped.

[PhillipsLT5]

My new & old cap does leak a bit, not really leaking, but evidence of coolant on seam on top of White plastic tank, my friends firebird did the same, I do not think cap is ZR-1 specific, try to run white tank at cold mark when cold, if white tank is filling up and not going back to cold level when cold then cap or head gasket

[gbrtng]

Eric, it appears there is way too much coolant in the tank. My 91 shows "full" in the dipstick with about 1" of coolant. Somebody get him the correct cap ...