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Diagnosing car engine overheating and antifreeze leaks

Copyright 2011 by Morris Rosenthal All Rights Reserved

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The Omni Project

Cars are dangerous and can kill people who work on them - proceed at your own risk. Diamond symbols linked to decision text. I've also posted a series of flowcharts for troubleshooting laptops and PC troubleshooting.

Flowchart to diagnose radiator leaks and overheating problems water pump leaking out bleed hole or gasket surface heater core leak under dashboard engine leak at head or wet bolt hose leak or clamp loose radiator leaking with drain closed overflow dripping through top vent steam coming from pressure cap on radiator checked ignition timing and valve timing checked or replaced thermostat has the engine and radiator been flushed ensure coolant is flowing through radiator check cooling fan comes on at idle checked antifreeze level in radiator and overflow temperature goes back down while driving real temperature gauge or idiot light sweet smell of antifreeze in car or near hood steam from under hood or drip under car

Troubleshooting car radiator coolant loss and high temperature leaks

If you only see steam rising over the hood when you've been driving in the rain or through puddles, it's probably just water that splashed on the engine or radiator steaming off. The steaming we're interested in here is steam that you can see coming from the cooling system: the radiator, the overflow reservoir, any of the hoses, the engine, even inside the passenger cabin in the case of heater core problems. You'll probably be able to tell from the sweet smell that it's antifreeze, unless you are running the non-ethylene glycol stuff. Antifreeze leaks are very easy to spot when they are fresh on the ground, in the immortal words of Scotty -"It's green". When I've got a drip under the auto I'm not sure about, I taste it, and antifreeze is unmistakably sweet though somewhat poisonous.

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Can you smell antifreeze without seeing steam or finding a leak? Don't worry, it is a leak, it's just too small to find. If you smell the leak only when you get out of the car, head over to the other side of the chart and start with "cap steaming". If you smell the antifreeze inside the auto, with the windows up, that's usually a problem with the heater core or the hoses to the heater core, so you can skip down to "heater core leak" at the center of the flowchart.

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Do you have a real temperature gauge, whether a needle gauge, a digital readout, or a digital representation of a gauge? If you have a real gauge, it's much easier to troubleshoot what's going on. If all you have is an idiot light that says "Overheating" or "Temp", all you know is that the engine has gone beyond a preset temperature, or the sensor running the idiot light is failing. Check the owners manual to see if the idiot light behavior can give you any hints as to what's going on, beyond suggesting that the engine is too hot, and then continue on to check the coolant level.

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Does the temperature return to normal? If you pull over and idle, does the temperature keep going up or does it drop. Do you notice the engine is getting a little hot in a traffic jam and then it cools right off when you get up to highway speed while you're still trying to decide whether or not to panic? Any time you see the temperature gauge above its normal range, it's a good reason to shut down and pop the hood to check for leaks as soon as you can safely get off the road. But there are all sorts of reasons a car can start to overheat and then cool down, such as a temperature sensor taking a little too long to turn on the electric fan (I haven't seen a fan belt since the 1970's), a thermostat that's a little slow to open, or a thermostat in a cooling system that has air in it, so the thermostat slug isn't submerged.

You can tell if the thermostat isn't opening by the temperature of the large radiator hose coming off the gooseneck at the top of the engine. If it's cold and the engine is hot, the thermostat isn't opening. Keep in mind that the thermostat is placed in the path of the coolant exiting the engine. That's how it "knows" to open when the engine warms up, the spring and slug are submerged in the hot coolant pushing to leave the engine, but are restricted by the thermostat until it opens. Note that it's normal for the radiator to have a hotter and cooler end as the antifreeze cools as it flows through. If the temperature looks like it's running away for a few seconds and then it drops right down to normal, it's sounds like a thermostat issue, and it will probably be fine for the rest of the trip. If you're stranded by an overheating auto, you can always wait for it to cool down, remove the gooseneck and pull the thermostat and then put the gooseneck back on with the old gasket. Most cars will run cold in this case even with minor leaks in the system, meaning you can get where you're going as you keep moving so that air cools the radiator. Your gas mileage will drop a little and the heater or defrost won't work for beans, but I've driven several cars that way in a pinch.

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Is the antifreeze level correct? New cars put the antifreeze level marks on the overflow tank next to the radiator. They usually have a high and low or hot and cold mark, but in all circumstances, the level should be somewhere between the two if the cooling system is operating properly. Unfortunately, the canisters age and get so dirty that it can be tough to see the coolant level without the aid of a bright flashlight or sticking something down in the overflow reservoir for visual contrast. Modern cooling systems are sealed, meaning you are supposed to make-up anti-freeze losses through the overflow reservoir, and the radiator will siphon the coolant in when it gets the air out and cools. But I never trusted the things, so I usually take the radiator cap off when the engine is cool and see whether or not there's antifreeze right up the the overflow take-off on the neck.

If you have to make up antifreeze on a regular basis, something is wrong. You may never find an external leak or see steam, but check that your motor oil level doesn't start rising and showing signs of antifreeze pollution (brown oil, white or green streaks). Likewise, some head gasket leaks may lead to the engine burning antifreeze, which might result in degrading or prematurely rusting exhaust and pollution control components that aren't expecting ethylene glycol combustion byproducts. Don't accept regular additions of antifreeze as a peculiarity of your auto. Read through the rest of the flowchart for troubleshooting ideas and try an online discussion group for the particular make and model to see if it's some characteristic failure that everybody knows about.

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Does the fan come on when the engine warms up? All the cars I've ever seen will get hot enough to require the fan to come on just sitting at an idle for five or ten minutes in normal weather. If the fan doesn't come on, make sure that the fuse isn't blown and that the connector to the fan electric motor is clean and firmly seated, whether it's on the motor itself or whether the wires just come out of the motor housing and into a connector a foot away. I can't think of any reason the fan motor would run at a different voltage than the battery, so when I want to test a fan motor, I just unplug the connector and put 12V on it through jumper cables with little conductor extensions, like nails. I try to get the polarity right (color coding usually make it obvious on the connector) and the real danger is shorting the battery with the kludged conductor extension or getting hit by the fan when it kicks on.

If the fan motor is good yet the fan never comes on, it means that the fan power circuit has a problem, which is most likely the temperature sensor. You can ring out the circuit with a continuity sensor before buying and replacing the sensor, which is often located right on the thermostat gooseneck. Corrosion in connectors or fusible links in addition to the fuse are also potential sources of open circuits.

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Is the coolant flowing through the radiator? Some people may be able to feel antifreeze flow by touching hoses, I'm not one of them. I need to look in the radiator and see the coolant streaming along. But you can't just remove the radiator cap when the engine is hot, even if you use a big towel and manage it without burning yourself, you'll get antifreeze all over the place. Wait for the engine to cool, remove the cap with slowly, so if there's pent up pressure it should vent before you turn the cap all the way to the release position, and then siphon off enough antifreeze to enable you to see the top of the radiator core, where all the tube openings come through. Then you can start the car up, and when the thermostat opens, you should see a very healthy flow, and the level will rise as the antifreeze heats and expands. It might take a couple minutes for the thermostat to open up if it's completely cooled down, you can tell it's not open when the top radiator hose coming from the gooseneck on the engine block is cool to the touch.

If there's no apparent coolant flow once the engine warms up and the thermostat opens, either the water pump is bad (check the belt tension again) or the engine or radiator is so gunked up that the water can't flow through. The radiator hoses shouldn't have blockage issues, they are too big for that, so a really gunked up radiator is is the best bet if the water pump is working. The engine passages might be gunked up if the antifreeze was badly contaminated, or if you just replaced the head and got the gasket backwards:-) I just added a page for replacing the water pump on a Chrysler 2.2, be sure to get an exact replacement.

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Have you tried super flushing the engine? Seems to me that Prestone used to advertise their superflush kits on TV during football games when I was a kid. Now nobody does their own work anymore, but you can still find superflush kits in an auto parts store. The antifreeze they put in sealed systems these days is so long lived that some people never change it, or even think about it unless they have a problem. But if you do have a problem and you're going to change your antifreeze, it probably doesn't hurt to flush out the system first. All the kit basically provides is a little plastic plumbing "T" with some clamps and a screw-on cap that you splice into your heater hose (I forgot which one:-) and attach a garden hose. Then you flush the system out. I don't even remember if they still provide a special cleaning solution to run in the engine before flushing, might be that environmental laws have led to discouraging the sale the extra chemicals.

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You can test the thermostat quite easily by removing it, throwing it in a pot of hot water and seeing if it opens. Checking the exact temperature it opens at requires you to have a thermometer you can measure the temperature of the water with as it heats up on the stove. Given the fact that a new thermostat only costs a couple bucks, it's usually not worth the bother. Plus, the advantage of a spare thermostat is if you have an unidentified cooling system problem preventing the thermostat from opening, you can drill a little hole in the spare thermostat and put it in. This may void the factory warranty, but I limped by for years with a leaking head gasket that way, and the small hole still allowed the car to get to operating temperature and run the heat and defrost. Lower temperature thermostats, like 180 degrees and lower, will open in a cup of hot water. Never put the metal thermostat in the microwave, heat up the water, remove the mug, and lower the thermostat into it.

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Have you checked ignition timing, OBD codes? Really bad timing may lead a auto to overheat in otherwise normal driving conditions, as can computer problems. Check the timing, and check the OBD (On Board Diagnostics) to see if there are any error codes saved reporting problems other than the high temperature. If you don't have an OBD meter, whether the original or the now standard OBD II, you can usually find a local auto parts store that will loan you one or bring one out to check for OBD codes if you drive to their parking lot. Also check the engine oil. If the oil level is low, the engine may be overheating as oil is another heat transfer fluid, and if the oil is dirty or degraded, the engine may be overheating from increased friction. And if the oil is brown or streaky , you may be leaking antifreeze into the oil.

But if you don't appear to have any engine operating problems, outside of the overheating or coolant loss, there's always the possibility you're driving it too hard, towing too much, or sitting in way too much traffic in the summer with the AC on. The engine is only designed for a standard range of operating conditions, so check the owners manual. There's also a good possibility that the coolant replacement procedure involved a trick you weren't aware of, like removing the temperature sensor from the top of the gooseneck and filling the cavity behind the thermostat with antifreeze, something like that. The sealed systems just don't work properly if they get air in the wrong place, though the only way for air to get into the system is a leak.

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Is the radiator cap steaming and hissing? Well, it might be a good sign, in the sense you'd rather have the pressure release on the radiator cap work than blow a gasket or a hole in the radiator. But it's a bad sign that you're overheating, and unless the car just climbed Mt. Washington or was grossly overloaded for a trip, you need to troubleshoot why it's getting so hot. It's also possible that the pressure relief has failed and is venting below the rated pressure for the cooling system, in which case replacing the cap and properly filling the system will solve the problem. Check the level of antifreeze in the overflow jug, and make sure the hose from the radiator neck to the jug isn't plugged up. If it repeats, continue with diagnostics for leaks.

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Do you see drips on the ground from overflow reservoir? Sometimes the overflow bottle will simply crack, either due to an external force or aged and heated plastic. If it cracks above the cool level for the fluid, it will only leak when the engine heats up and reaches the crack, leading you to think the auto is overheating. The overflow will release antifreeze through it's exhaust hose if it is overfilled. It's easy to do since the top line on the reservoir that many people take for the ideal fill line is the "engine hot" level. If you fill to that level with the engine cold, you'll almost certainly end up blowing antifreeze out the overflow hose. The overflow container itself has an overflow near the neck, a small tube that points down to open air. If you have excessive antifreeze in your overflow and you want to move it back into the radiator when the engine is cold, I just put a finger over the overflow vent, remove the radiator cap, and blow directly into the overflow. The air pressure forces the antifreeze back into the radiator through the same small hose that let it out.

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Is the radiator leaking from finned section? What might start as a pinhole leak in the finned section of the radiator will usually develop into a full-fledged squirting stream of antifreeze when the engine heats up. If the leak is due to damage, like it got bashed with a screwdriver or something sharp that hit it on the road, it makes sense to repair it or have it repaired. I believe shops generally do brazing rather than soldering, I'm not really sure why, but pinching off a column of fins at both ends and crimping before soldering or brazing probably works best. If the radiator has simply corroded with age, getting a new core is the right repair, except buying a whole new radiator over the Internet or through a wholesaler is probably cheaper.

Minor radiator leaks, if they aren't to far gone, are easily treated with stop-leak solutions that you add to the antifreeze and which are available at every auto parts store. But they usually aren't permanent fixes, even though you may limp by for a couple seasons. Since the stuff only solidifies when exposed to oxygen, it shouldn't gunk up your engine, and the manufacturers often claim that it's good for the water pump. I've taken apart some cooling systems that were loaded with various radiator leak products, and didn't notice any build-ups in bad places, and the water passages in engines are pretty aggressively sized to start with. And make sure you check the radiator drain, normally at the bottom on the same side as the cap. It's usually just a plast plug turned in finger tight, and it can loosen over time or get cracked open a little by driving through heavy snow after plowing, anything like that.

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Is there a pinhole leak in hose or at clamps? A hose leak doesn't indicate anything bad in most case, it's simply a worn out hose, and replacing and properly refilling the system should clear up any overheating symptoms. If the leak is coming from the end of the hose, beyond the clamp location, try tightening the clamp before you do anything else. Keep in mind that the outlet line at the radiator is often plastic, so you can't crank on it with your half-inch ratchet. If the hose develops a pinhole leak right at the clamp, you can always try cutting off the hose right at the failed spot seeing whether or not the remaining hose will reach. If you can tell it won't reach before you start, you can even leave a quarter or half inch of hose including the leaking spot, which should be beyond the clamp location when you reclamp and doesn't really matter. If you need to replace one radiator hose, you may as well replace both of them, and if you need to replace one heater core hose, you may as well do both of them as well since they age at the same rate. Antifreeze is expensive enough that you'll feel silly to have it all leak out if you cheap out and do the upper hose, and the bottom one develops a leak the next month.

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Is antifreeze leaking from any engine surface? Checking for engine leaks isn't as easy a task as it sounds, because it's tough to carefully inspect all the edges between the head and block, even with a good flashlight and a mirror, and impossible without removing any plastic covers, such as the timing belt cover for an overhead cam. If you always find anti-freeze spots under the engine after you park and you can prove to your own satisfaction that they aren't coming from the radiator or the hoses, the engine is all that you're left with. And remember that a head gasket leak may lead to internal coolant leaking with no external manifestation, leaving you with antifreeze in your oil (very bad) or antifreeze entering the combustion cylinders and getting burned (not great, results in white exhaust and eventually eats your catalytic and pipes).

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Is there a heater core leak? If you smell antifreeze inside the car or if it's squirting out from under the dashboard and burning passengers, you have a leak at the heater core. Check the hoses first, but thanks to their being sheltered from the elements, hose failure at the heater core is probably less common than the core itself developing a leak. Try turning off the heat using the dashboard control, and depending on how good a valve is installed in the feed to the heater core, it may help you limp home. Heater cores are generally a pain to remove, try to get a hold of the shop manual so you can find out if there are any tricks that will help you get it out without breaking the dashboard or pouring antifreeze all over the place.

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Check the belt tension on the water pump pulley (may share the timing belt on some autos) and make sure it turns the pulley when you crank the engine. If the gasket at the water pump is leaking, it's probably not a bad idea to replace the pump unless it's new. These simple impeller pumps are generally built for the life expectancy of the auto or better, mine lasted for over twenty six years and over a quarter million miles. Water pumps have a weep hole on the tower section that hold the pulley for the belt. When antifreeze leaks out the weep hole, it means the seal on the shaft is gone and the pump should be replaced as soon as possible. I recently posted and illustrated water pump replacement from my Omni.

If the leak is from the thermostat gooseneck, remove it and replace the gasket or use some silicon gasket sealer before you replace it. You don't need to run the bolts super tight, if the gooseneck is cast iron or aluminum, you can crack it by over tightening. If the leak is from the head gasket, it'may difficult to spot and to fix. I drove my 2.2 Liter with a head gasket leak for a couple years before locating it behind the timing cover. I went through a couple containers of powdered copper leak stopper, regular topping off of the radiator, and eventually, I replaced the thermostat with one that I drilled a small hole in. Before I took the last step, the symptoms were runaway overheating, usually right after hopping on the highway. The funny trick I developed was that by stopping hard, or starting and stopping hard a few times, the factory thermostat would open and everything would be fine for the trip. What was happening was the slow head gasket leak allowed air into the system that migrated to the top of the engine and got in the space where the thermostat slug is supposed to be submerged in antifreeze. Surging the car apparently sloshed enough antifreeze up there to open the thing! When I eventually went to the thermostat with the small hole, it let the air out and I never had further problems with the thermostat not opening. But I kept on slowly losing antifreeze.

When the head gasket leak got bad enough, going from a slight antifreeze smell from the steam to coolant dripping out from the timing belt cover, I tried some desperate patch kludges right on the block that didn't last. Finally, I replaced the head gasket which solved the problem for good. But I didn't follow the shop manuals that time, I just pulled the valve cover, unbolted the head (removing bolts in the proper order) picked it an inch or two off the engine with a cherry picker without removing the manifolds. Then I did my best to clean the surfaces and set the new gasket in, following the manual for the head bolt tightening sequence, but the major shortcut of not disconnecting the manifolds, removing the carb, etc, was a huge labor savings.

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