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Paul Workman · 07-14-2014

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.

07-14-2014	#2
Paul Workman Join Date: Nov 2007 Location: Squires (near Ava MO in the Mark Twain N'tl Forest) - Missouri Posts: 6,466	Re: Leaking Coolant Overflow Cap 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.