A few days ago I fired the car up and the coolant light came on so I put about a gallon of 50/50 coolant in the car and ran it on a trip about 50 miles one way. I was driving back and made it about 25 miles and suddenly the low coolant light came on and the car started overheating. I filled it up again and drove the rest of the way home with no problem. Then I let it idle in the driveway for about 25 minutes and it did not overheat or have any leaks. I drove it around for some short trips (less than 15 miles each way) and didn't have any problems. I then drove it on a 50 mile trip and checked the fluid level once I got there and it was full. I then started on my way back and about 15 miles into the trip the low coolant light came on and the car overheated. I filled it up with about a gallon of antifreeze and made it about 5 more miles before the coolant light came on again and it overheated and it looked like all the coolant was forced out through the overflow tank. Any thoughts on what could be happening here?

The car is a 1995 LT1 with 160k total miles and about 10k miles on the latest waterpump.

 

If coolant is blowing out of the overflow, chances are it is being forced out by the cylinder pressure of a blown head gasket. It doesn't overheat until the coolant gets low, right? I would do a compression test and if inconclusive, I would do a test for CO in the coolant.

RACE ON!!!

 

CFI is right as usual but first when was the last time you replaced the cap on the surge tank? It should be done every so often they wear out. It will allow the front tank to fill too easily if it is weak.The tanks also have a tendancy to leak around the collar.

 

i had a similar problem once.... the impeller in the wp is pressed on iv had them come loose and even come off check the water temps in the rad and the hoses to make sure you have good flow ( hand held pyrometer ) or just pull the front off the pump and check it .. 10 min job...

 

I am in no way an expert on this but I’ve had this happen to me before!
If you had just added antifreeze you may have an air bubble stuck and you will need to bleed the system!
Good luck
Mike

 

Bleeding trapped air is absoluetly essential with a 95 LT1. If you don't bleed it properly it will keep overheating no matter how many times you top it off.

 

I think part of the diagnosis is in whether the over heating causes the engine to barf coolant or if it over heats due to being low on fluid. I read, "A few days ago I fired the car up and the coolant light came on so I put about a gallon of 50/50 coolant in the car and ran it on a trip about 50 miles one way. I was driving back and made it about 25 miles and suddenly the low coolant light came on and the car started overheating." to indicate that the low fluid level was the cause of the overheating. Maybe I read it wrong, or he didn't state it exactly as he should have, but I took it as the low coolant level being the cause of the high temps and not vice versa. The excess pressure of combustion was the first thing I thought of, that would expel coolant before it over heated. As also suggested, however, a weak cap could allow normal operating pressures to force coolant out. Therefore, if I've read correctly, the first test should be the cap, then follow my first post. If I've misunderstood... Never mind.

RACE ON!!!

 

If you pick up a copy of May 2008 Chevy Hi-Performance it has an article about doing a leak down test. If the cap is NOT the culprit, maybe a leak down test could pinpoint your problem? Read through the article and see for yourself. Bubbles in the coolant, air leaking into adjacent cylinders ETC....

 

I would first check the thermostat. Just replace it with a new one and go from there.

 

I agree with CFI. You must find out where the water is going. Head gasket is number one suspect.

Get a shop to test for CO in the coolant.

 

We know where the water is going. It's blowing out of the overflow. The question is, where is the pressure that is pushing it out, coming from? If it has a bad pressure cap, normal cooling system pressure could be forcing the water past the cap. If the cap is good, then it is likely that pressure from the cylinder(s) is escaping past the head gasket seal and over powering the radiator cap relief valve. I am curious to find out if over heating is causing the low coolant level or if the low coolant level is the cause of the over heating. In addition, the OP can check his oil for the "milk shake" syndrome and watch the exhaust for "white smoke".

RACE ON!!!

PS. Meanwhile, a day and 11 posts later, there is no sign Sawbone has seen any of this brilliant discussion.

 

I say the thermostat or air in the system is #1 suspect. Check these first.

 

Sorry for the delayed response time

Let me see if I can shine more light on this subject....

When I first got the low coolant light I added a little over a gallon of coolant. I always open the small bleed valve on the water pump and also open the tube on the top of the radiator to bleed air out of the system. Once I get a solid fluid stream coming from both places I assume enough air is out of the system. Prior to overheating, I've always seen the low coolant light first, then an instant temp increase until overheating (~240 - 245 degrees). As I said before, I ran the car several times on short trips and in the driveway with no signs of leakage or overheating. It seems as though everything is fine, then I get sudden loss of coolant that leads to overheating. I had no real sign of where it was leaking from until I noticed that after filling the radiator several times and losing coolant, that the overflow tank completely filled up and blew fluid out of it to the point that the reservoir area was full of fluid, the front right tire was covered in coolant, etc. I am concluding that the fluid is being pushed out of the overflow. I'm not sure if this is a possible symptom of just a thermostat or if it's definitely a head gasket issue.

 

A thermostat will do exactly what you are describing.

 

If was a head gasket you would see white smoke coming from the exhaust or have coolant in your oil

 

An old surge cap will cause what you are seeing. I just replaced mine for the same reason. Take the cap off and look at the rubber if it is indented and nasty looking replace it. You need to do this regularly as I already said it is part of regular maintenance. Follow what CFI said look for discoloration of fluid and if it looks ok continue on........CHANGE THE CAP FIRST. Also start the car and watch for leakage from around the brass part on the surge tank you can blow fluid out with a small leak and suck air back in causing the level to drop. It is easier to suck air than fluid back into the tank with a neck leak.

 

Just for clarification, the coolant is overflowing out of the tank down by the passenger side headlight (overflow), not the fill tank up closer to the heater core. Both of these tanks have caps, so I'm assuming you are referring to the pressure cap on the tank up by the heater core, but I don't see how this cap would cause coolant to overflow out of the tank down by the headlight.

 

C'mon get a grip the hose from the surge tank goes to the fill tank. The surge tank IS NOT the fill tank. It only is filled there when you get a low coolant light period.Do you have a FSM? When the fluid heats up and expands under pressure it flows to the fill tank. If the cap is bad instead of it pulling the fluid up hill it will just not do it.It keeps pumping fluid out but not pulling it back. If there is a pinhole leak at the neck of the surge tank it will still flow out but suck air back in instead of fluid. Geeze a cap is a whole lot cheaper than any other reason it needs to be changed anyway and if it fixes your problem great if not it probably needed changed anyway. Idle the car and watch the neck as it warms up to see if ANY fluid bubble out ANYWHERE around it. I got my new surge tank and am changing it out tomorrow for THE EXACT SAME REASON you are seeing. I'm done.......

 

If the pressure in the cooling system exceeds the relief pressure of the cap, approx 15# - 16# water will be released into the overflow reservoir. In normal usage water is forced from the expansion tank by the the pressure developed by the heat of normal operating temps. As things cool down, a vacuum develops in the system, and draws coolant back in. The reservoir is more than large enough to handle the fluctuation of the coolant level between hot and cold operation.

If the cap is defective, it can allow coolant to be expelled prematurely by low pressures. Not only do you loose the coolant, lower operating pressures allow the coolant to boil at a lower temp. Of course your water temp will be limited by the new, lower boiling point. Then if forced from the reservoir, there isn't sufficient coolant to refill the radiator when things cool off. A vacuum leak can prevent the coolant from recovering when cooling off, but it usually takes multiple cycles for the coolant to get low enough to light the light and cause overheating.

A defective thermostat CAN cause similar symptoms, but if it fails to open, the engine heats up FIRST and then the excess pressure of the abnormally high temp forces the coolant out.

It still sounds to me that the coolant is being forced out before the engine heats up, and that the over heating is due to the low coolant level, not the other way around.

Check the cap first. If it looks bad, replace it. If it looks OK, it can be tested. I may be wrong, but unless the symptoms change, I'm sticking to my original, long distance, diagnosis. Sorry.

RACE ON!!!

 

My 1991 was dumping fluid out the surge tank past the rad cap. The garage replaced the head gaskets but it was still dumping fluid out the surge tank. As it turned out the surge tank was cracked on the inside and although the fluid was coming out at great force it was due to the crack in the tank. May be worth replacing the surge tank or look very carefully to see if there is a crack on the inside of the tank.