1992 Cadillac Deville Overheating

When there's no external leaks, the most common cause of coolant loss is a leaking cylinder head gasket. You can prove that by adding a small bottle of dark purple dye to the coolant, then searching later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. In this case you would find it inside the tail pipe.

If you don't see coolant leaking outside the engine, it has to be going through the cylinder head gasket, but the question is to where? Sometimes it goes into an oil drain back passage and into the oil, but more commonly it goes into the cylinder where it is burned, then it goes out the tail pipe. That's where the dye is of great value. The only proper fix is to replace the head gaskets.

I flush cooling systems by removing one of the heater hoses and the radiator cap, then running water from a garden hose both ways through the hose, the port it came from, and the radiator. You have to be aware though that when you put new antifreeze in, there's going to be a lot of straight water in the block that doesn't drain out. We can discuss that if necessary.

Are you asking how to drain the leftover water out of the block after flushing the cooling system? There's drain plugs for that but we never remove them. They're usually in very inaccessible places and we don't have the time for that when we have to charge by the hour.

Use your garden hose to spray inside the reservoir and blast as much scum off as possible. If you disconnect the rubber hose from the radiator and let it hang down, the water will siphon itself out. Leave it empty for now but reconnect the hose.

Don't fall for the "pre-mixed" coolant scam because you're buying only a half gallon of antifreeze and a half gallon of water. Most of the cars I flushed held about three gallons of coolant. I assumed there was about a gallon of water still in the block, so I started by adding one gallon of straight antifreeze. Next I'd add a half gallon of water, then a half gallon of antifreeze. Keep alternating like that until the radiator is full. You'll need to burp the air bubble under the thermostat. Some engines have bleeder screws on the thermostat housing. Some have a plug or sensor you can remove. Some kind of take care of that themselves. If you do unscrew a coolant temperature sensor, if it's a two-wire sensor, that's for the Engine Computer. Don't turn on the ignition switch while that sensor is unplugged. Doing so will set a misleading diagnostic fault code and turn on the Check Engine light.

Once the radiator is full and burped, run the engine through two or three warm-up / cool-down cycles to mix the coolant. Check the freeze point to determine if more antifreeze or more water is needed. You'll rarely need to add more than a half gallon of one or the other and the reservoir will hold that much. When I did this for customers I originally asked them to come back in a couple of days so I could check the freeze point and add what was needed, but after a while I got pretty good at getting the right concentration because I kept seeing the same engines over and over. For people from out-of-town, I went a little heavy on the antifreeze, then instructed them to have it checked later and add water as necessary.

Your goal is to get a freeze point of around minus 35 to minus 40 degrees. Lower is not better for a couple of reasons. First of all, you need the additives in antifreeze like water pump lubricant and corrosion inhibitors, so you need enough antifreeze, but antifreeze does a poor job of moving heat from the engine to the radiator. Water holds more BTUs of heat so it will move the heat better. Too high a concentration of antifreeze can lead to overheating because the heat isn't being taken away from the engine fast enough.

The second reason is harder to understand. When the chemical mixture is of the right proportions, roughly half and half, the freeze point will be near minus 35 degrees. Obviously if you have too high a percentage of water, the freeze point will go up closer to 32 degrees, but what isn't obvious is once you get to minus 50 degrees and begin approaching straight antifreeze, the freeze point again starts to go up. As I recall, straight antifreeze will freeze in the container at around 0 degrees or minus 10 degrees. Mixing the two chemicals is what creates the lower freeze point. Where the misunderstanding comes in is the simple freeze and boil point testers like those with floating balls or a floating pointer or stick measure the weight of the mixture compared to the weight of straight water. Antifreeze is heavier than water so the more antifreeze you have in the mixture, the more indicator balls will float or the higher the pointer or stick will float. Since it's just measuring weight, it isn't accurate once you get past around minus 50 degrees.

To measure the freeze point accurately, you need a "refractometer". You place a drop of coolant on the slide, flip a cover glass over it, then sight through the viewing window. They take some getting used to, and they're pretty expensive. Good ones go for well over a hundred bucks. As long as you don't get carried away with the concentration of antifreeze to water, you'll be fine with a standard tester. You can also ask to have the freeze point tested by your mechanic. At the dealership I used to work for, I had the owner's blessings to do that for free on my lunch hour or when it didn't interfere with a job that a customer was paying for by the hour. The test only takes a few seconds, but be sure you've gone through a couple of warm-up cycles so the coolant is mixed.

As for where the coolant is going, the least common place is into the oil. You won't always smell it coming from the tail pipe. If it isn't leaking externally or from a head gasket, you have something else leaking that you're going to have to figure out on your own. The only other suspect would be the heat exchanger inside the radiator for the automatic transmission. Usually it's more common to find transmission fluid in the coolant reservoir.