Coolant level going down very slowly

A4 2.8 1996 - 46,000Km
Coolant is slowly decreasing. About 1 month from max to min. No leaks, smokes or smells. Temperature - oil & coolant - Ok.
Curiously, when I topped the coolant to Max I noticed a few hours later some coolant in the floor. (engine cold - not turned on in any moment)
Can it be just a minimum fracture in the higher part of the reservoir or should I be looking at somethng else? I imagine that the coolant works under pressure - cannot understand leaking with the engine cold and no pressure at all. BTW: after the mentioned leak - no more than 12 hours later - no leaks at all. (and reservoir at MAX level) Thanks in advance for your help. Jorge
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

could it be your coolant bottle cap?
you notice coolant on the ground or garage floor? or what do you mean "in the floor"? later, dave Reminder........ Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you are a mile away from them, and you have their shoes. Frieda Norris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave, I should have said "on the floor" - the garage floor. (english is not my native language...) No chance of the bottle cap being the culprit. Topping was a clean operation. Thanks, Jorge
snipped-for-privacy@aol.comANTISPAM (dave) wrote in message (Jorge Fonyat)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I was thinking that the cap should hold a minimum pressure and sometimes they fail to do that. When that happens the cap can allow some pressure and fluid out of the bottle. Ah it was just a though! :-) later, dave Reminder........ Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you are a mile away from them, and you have their shoes. Frieda Norris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I think you could use a UV lamp to detect small traces of coolant. On alt.lasers, they had a discussion some time ago about using UV LEDs for that purpose. Apperently, it works quite well and the UV LED shouldn't be expensive. You might have to clean your engine first, since they said those thing detect extremely small quantities.
I also saw somewhere mentioned that you could fit a tyre valve (e.g. from a bycicle tube) to the cap of the reservoir (best get a spare cap for this) and then pump it up to some pressure to detect leaks.
Good luck,
Christoph
--
Important: Emails sent to me which contain my full name
in the "to:" or "cc:" field will bypass my spam filter.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Christoph, Thanks for your points. I think it would be difficult to get an UV light here. (have to check) As for the cap + valve I am not sure I can produce this small tools. (BTW - Audi uses exactly the same technique) Best Regards, Jorge

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Jorge,

Shouldn't be too difficult. They suggested LEDs because you should be able to get them much easier than normal UV lamps.
I don't know where you life, but RS Electronics is in a lot of countries. Their no 454-4403 is a UV LED which is most likely suitable (Cannot guarantee, but it looks like what I would try. I just did a search for "uv led" on their web page, http://www.rs-components.com/index.html )

That shouldn't be difficult either. A normal valve has a diameter of 5 to 6 mm (just guessing), so all you need is a drill of that size plus maybe an o-ring.
Christoph
--
Important: Emails sent to me which contain my full name
in the "to:" or "cc:" field will bypass my spam filter.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Had this on a 90 a few years ago , it was a microscopically small crack in the rad matrix , so I suggest a bottle of "bars leaks" in the rad as this will gum up minor holes for a while , should prove the theory , and costs less than 2.00
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
First of all thanks to all for the support. Now an update: - I tried to replicate the topping/bleeding mistery. Mistery ended: no bleeding at all. - On the other hand I was finally able to spot tiny drops coming from the rad matrix (probably). Saw them in two places. The origin may be the same.
I am leaving for a trip and will follow this after my return. In the meantime: what are the typical brands of the "bar leaks" products? Are they effective in an oily environment such as the coolant? And can't they create problems with the overall efficiency of the system ? Thanks again, Jorge

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.