calaban

1995 Z28 - So it's done this forever it seems. I can't find a leak anywhere but I can top off the radiator and drive the car with no issues at all. Then after it's cooled I take the cap off the radiator and the coolant is low enough in the radiator that I can't see it without a flashlight. I can fill it back up with the amount that would be between the top of a two liter soda bottle label and the cap. The car doesn't run hot or anything. I assume that when the pressure builds some is pushed into the reservoir but I wanted to ask if anyone else has seen this issue cause I think as it cools any coolant pushed out by pressure should be drawn back into the radiator, right?

Is the cap spring weak (it's the original from 95), is there a crack in the line to the reservoir that would allow air to get pulled in instead of the coolant (I think it would leak coolant out if it pulls air in though)?

Injuneer

Yes, if the hose from the radiator overflow is cracked (or the metal part of the line, under the battery) the vacuum produced in the radiator as the coolant cools down pulls air in through the crack, rather than coolant from the reservoir. Will not necessarily leak coolant out, since there is no real pressure after the coolant passes the cap, and it flows by gravity.

Failure to pull coolant back from the reservoir would eventually overflow the reservoir, as the radiator repeatedly pushes your replacement coolant into the reservoir, and doesn't pull it back out. Have you detected in increasing level in the reservoir?

Always a good idea to run a pressure test on the system. The pressure tester also has the ability to test the cap relief pressure.

moparman

I had a similar problem a few years ago. Coolant was disappearing with no signs of a leak. Bought a new cap, pressure tested the radiator, used dye with the funky glasses, and also used the combustion dye, to test the head gaskets, in the system, and a leak never showed up. Then one day I went to a shopping center about ten miles away. Temp was in the nineties. Came back to the car and I saw the coolant on the parking lot. I got lucky twice, though. A buddy of mine owned a repair shop a couple of miles away. I was able to drive there and get the car on the rack. The leak was then revealed. The radiator was cracked on the passenger side. My second lucky break was he happened to have the correct radiator in his shop. He said he had ordered it for someone else and they never came back for it. He'd sell it to me, and install it, for $125. I took him up on the deal. I suspect the crack opened up when the coolant got hot, and closed when it cooled off. I also suspect that on that hot day, the crack finally expanded enough to really leak and reveal itself.

calaban

I thought about the radiator but it's been replaced a while back and I've parked on concrete and let it run for a while and have seen no drips and cannot visually see anything. This is why I'm wondering about the radiator overflow tube.

In all honesty, since it's not running hot and I can't find a leak I haven't been monitoring it as regularly as I probably should be so I haven't topped it off enough to see it progressively get deeper in the reservoir as suggested above. Thanks for the input guys, I'll replace that tube and see if still disappears.

Injuneer

The overflow tube is rubber. It connects to a metal tube under the battery. The metal tube also needs to be inspected.

And a final check - you indicate the radiator has been replaced. Is it possible the lines for the overflow and the steam pipe from the back of the heads (via the throttle body 95-97) have been switched on the connections at the radiator? We've seen this problem pop up, although when that happens, the reservoir fills up rapidly, and starts to overflow.

See lines 5 and 27 in this diagram:

http://shbox.com/1/95-97_hoses.jpg

Fatdog2

Also check the radiator drain plug.You may think it's tight,but give it an extra nudge with a pair of pliers or something.After driving for a while,put a piece of cardboard under the car and wait to see if this is leaking.Hope this helps.

artic94

my 94 does this every so often but after I shut it off sometimes there is some coolant under car around the area of the overflow tank think mine could be the hose as well

Injuneer

Where is the level in the reservoir? If it continues to build up to the max level, it's going to drip out the overflow. This could happen if the hose is not allowing the radiator vacuum to pull the coolant back out of the reservoir as the engine cools down, but only if you are regularly adding coolant directly to the radiator because it isn't able to pull coolant back out of the reservoir.

You also need to check the pressure cap. If it is not holding the required pressure (18psi) it may be allowing the coolant to boil when you shut the engine off, and hot spots develop in the heads. The boiling coolant will blow out of the cap and into the reservoir. Or, worst case scenario - blown head gasket.

Finally, check the reservoir for cracks.

artic94

its not really a constant thing to add it it only happens every now and then I have checked the level in the tank after driving about 15 miles and its not really all that high and pressure cap holds pressure pretty well id say

Fatdog2

I had the same problem.I would drive 200 mi or so and I would check the radiator and it was low.Every day I would add 50/50 and check it every day and add more coolant.What the hey?(Back up for a minute,had the car flushed out at goodyear and they put a stant cap on it)So I ordered an original radiator GM cap from parts train just for the heck of it,and that solved the coolant leak problem.It took about a week or so checking every morning when the engine was cold and added coolant to top it off,and after that it was fine.(I also pumped out the old crap from the reservoir and replaced it with new 50/50.)It;s still right up to the top going on 3 weeks now.Hope this helps.

Injuneer

The cap needs to be tested.... very simple.

Fatdog2

Calaban.Did you ever resolve this issue?

Injuneer

"calaban" has not returned to the site since his post (#4 12/30/14) above.

BMX Dad

Injuneer - Thanks for the advice on this thread.

I had an issue with the return tube to the overflow. Fixed the leak and fixed the problem.

Injuneer

Thanks for the feedback, and thanks for using the "search" feature!!