Cooling SystemDear Ole Mechanic;

About a week ago the ‘Check Gauges’ light came on in my 2002 S-10 Chevy pickup.  When I checked my gauges, the temperature gage was almost all the way over to  hot.  I pulled over and shut the truck off immediately.  When I opened the hood, I could tell that the 2.2 Liter four-cylinder engine was hot just from the heat radiating from under the hood.  I called my husband--thank God for cellphones--and told him what had happened.  By the time he got to where I was, the engine had cooled down to the point that you could touch the engine, and nothing came out when he loosened the radiator cap.  He had me start the engine and he filled the radiator with water.  I drove home and after the truck had cooled down, I took off the radiator cap and it was still almost completely full.  It seemed to be running fine for the next several days, but then it started to overheat again.  I stopped before the ‘Check Gauges’ light came on.  I let it sit and cool down awhile until I could get the radiator cap off with out water spewing out.  It was low, so I filled it with water--which I now carry in the truck.  Later that day, both my husband and I checked it for leaks; there were none that we could find.  Now the truck takes about a quart of water every time I drive it.  But if it sits for a couple of days, the water doesn’t go down. 

My questions are: Why did it lose water and overheat the first time?  Since there are no leaks that we can find, where is the water going now?

Hot Under the Hood

Dear Mrs. Under the hood.

I'd like to congratulate you on your knowledge of the cooling system.  Unfortunately, I can only offer some guesses as to what happened to the coolant initially.  One possibility is that the thermostat stuck closed, causing the first overheat, and at that time it boiled the coolant out of the overflow--and possibly out of the recovery tank.  Another possibility is that there is a small pinhole leak in one of the hoses that only leaks when the engine is running and pressure builds up.  The vibrations caused by the running engine may be needed to cause the leak that is so small that the coolant evaporates before you can spot it.  Over time a little coolant loss can add up and when it gets low enough, the truck overheats.  You have been following the maintenance schedule haven’t you?  I believe it calls for coolant system service at five years or 75,000 miles.  Another possibility for the first overheat is an earlier overheating caused a crack in the head or block.  I have seen a crack in the head that would open up and leak only when the engine was warmed up and running.  As a matter of fact, I suspect that is what is causing your current loss of coolant.  It could be leaking into the engine crankcase and then evaporating when the oil gets up to temperature.  Usually that type of leak will cause some contamination of the oil and show up on the dipstick.  It could also be a crack in the combustion chamber and when combustion occurs, the coolant is boiled into a vapor and goes out the tailpipe.  One note of caution here; when the truck is first started, the hot exhaust gasses go into a cold exhaust system.  Some condensation coming out of the tail pipe just after start up and before the exhaust gets hot is normal.  White smoke or steam coming out of the exhaust pipe after a good ten-mile warm up is not normal and can be an indication of coolant getting into the combustion chamber.

I suggest a visit to your favorite mechanic.  He should have tools that can detect coolant leaks, even if it is just putting the cooling system under pressure and watching for a pressure loss, both with the engine off and with it running.  The newer cars and trucks use a thin-wall casting process to help reduce weight for better fuel mileage.  Unfortunately, the thinner castings can be more prone to cracking in an overheated condition than were the older, heavier castings.  When that happens, replacing the cracked part is about the only way to fix it, and about the only way to find the crack is an engine teardown.

Herr Professor Nuzanbolts