1987 Corvette.

Been a mechanic for 40 years, but I am having trouble with this one.

My Corvette will hold a 17 lb. radiator pressure test for hours without dropping even 1/2 lb. and not a drop of water leaks out anywhere.

However, the car may run fine and hold water for weeks, and make long desert trips without overheating nor losing water. But every once and a while the low coolant light will come on, and I find the car anywhere from a quart to a gallon low on water.

It is on it's second new radiator cap, the overflow tank never gets too full, cooling system has been flushed and tested, it has a new 180 thermostat, etc. and as I said, with 17 lbs. of pressure on the cooling system it sat on concrete for several hours without a single drop of water leaking out.

Where and how the hell is it losing water sometimes??

Thanks,
vettebob

 

Coolant takes different paths sometimes when the car is hot and running. A head gasket leak or crack is not always going to show up with a 17 psi radiator pressure test. Might want to check all hoses and fittings and throw in a can of stop leak. Sometimes that will stop small, mysterious coolant losses. I use Irontite. Good luck.

 

I had a similar problem on my chevy truck and it was the upper radiator hose to water neck connection. When the engine was cold it would sometimes leak water between the hose and the water neck, as soon as the engine warmed up the leak would go away. Check the radiator hose connection on each end and look at the water pump weep hole for any leaks when the engine is cold. (actually, look at all hose connections)

 

I don't think the heads have ever been off of this one. I replaced all the water hoses, and I've been all over the front end of the engine removing the smog equipment, replacing the a/c compressor, etc. I replaced the water hoses down for the oil cooler down near the exhaust/oil filter with high quality braided hoses. I can't see any staining or dripping anywhere. Usually you can smell a heater core leak inside the car and/or see steam on the windows sometimes, and I don't experience any of that.

Right before I bought the car it was run low on water and got up to about 300 degrees. I was there at the time and made sure no water was added to the engine until it had thoroughly cooled down, and even then made them add water slowly and with the engine running. I'm worried that maybe it has a cracked head that only leaks when thoroughly warm and running. But I have made 7 hour trips across the Mojave desert with the car with no water loss and no overheating so that doesn't make much sense either....

I forgot to mention... Plugs look normal, a little cleaner on one side than the other because it is leaking a bit of oil down the valve guides, but they all look the same. (Need to install the valve guide seal fix.) Also, no signs of oil in the water or vice versa. Car has 96,000 miles. Sometimes it will hold water for weeks, sometimes I have to add water twice in one week!


vettebob

 

I'd inspect the water pump seal weep passage for signs of fluid.
Just to eliminate the water pump of course........

 

Maybe attach a paper towel near pump and various places and see if you can detect any leakage.

Might also want to get one of those kits to detect if there is exhaust in the coolant.

Have you looked at the heater core or area?

 

On my 86 I had a leak from the small line that goes from the back of the intake to the heater hose. It would leave coolant on the intake/head so I changed the intake gasket for no reason because I didn't initially see the crack in the hose.......oops......anyway, have been getting the intermittent "low coolant light" ever since. I've tried the rev up to highway cruise rpm with the radiator cap off and top off the coolant and that seems to work for a while but then the light will return but not consistently...........and like you I have no leaks. I know the coolant sensor is correct because every time I check the level with the light on it is below the sensor..............if I ever figure it out I'll be sure to get back to you.
After having said all that. My suggestion is: to see if you have an air bubble in the system.

 

Ok, thanks for all the input guys. I'll keep looking...

vettebob

 

Eather the heater core or the engine has a leak in the head gasket or intake.Check the spark plugs to see if any of them are white instead of it being brown in color.

 

Heater core good suggestion.

Take the plugs out then do your pressure test again.

89 same issue as you

Eventually onnnly on the fwy after cruising for about 15 min eventually started creeping up in temp.

Head gasket seemed temp sensitive, it was very slight?! Fine around town but on the fwy would warm up. Eventually got worse. PRessure tested rad checked out good.

Do you have a random miss or stumble at idle?


hopefully this isnt your problem.

 

What part of the plug do you look at to see if it is white?