What are the signs of a blown head gasket?

Signs of a Blown Head Gasket

Note: You can only truly confirm your suspicion by actually seeing the gasket, although precursor signs are usually evident.

Input from s.com contributors:

• If you see coolant leaking from the water pump, I would pressure-test it and pinpoint the leak and fix that first; oil seepage isn't necessarily abnormal.
• Typical symptoms of a blown head gasket may include these: bubbles of air coming up into your radiator (remove cap before starting); a leaking radiator; milkshake-colored oil; overheating; rough running; coolant or oil running from head; spark plug(s) that have a green tint (if green coolant); white-colored or sweet-smelling exhaust.
• White smoke from your tail pipe, or loosing coolant through your overflow. Take the cap off and rev the engine: if you see bubbles, or if it comes out, you'll know.
• A blown head gasket will leave a dark smell in the radiator. And you will have high back pressure coming though your radiator cap.
• Take your car to a radiator shop to have a detector installed: If the blue liquid inside a "bulb" turns yellow, you have a leak.
• Beware that if you drive for too long and it overheats, a blown engine will be your outcome.
• A blown head gasket can go out in different areas causing different symptoms. Do a compression test to give you some idea. Don't confuse low compression for a bad head gasket, though. A bad valve can lower compression. And a bad ring.
• There are lots of clues you can look for. When in doubt and you have tried everything, have the head checked out by a well-established machine shop first, to see if the head was the problem. This way you're not wasting your time replacing the gasket.
• My car once had a blown head gasket. I had a great deal of coolant loss. The engine lacked power and ran poorly. It had white smoke coming out the tail pipe. And it overheated very quickly. Also, it had water in the oil.
• A quick way to check: Look at your spark plugs; if coolant squirts out, you definitely have a blown head gasket!
• Low compression does not necessarily mean a blown head gasket, but it is a good indicator if there is a sharp drop in compression on one or two cylinders, with no drop in the others. Sometimes a blown head gasket will cause a whistling or wheezing sound, but not always. It will not always cause water to enter the oil - or oil to enter the water - but they are signs to look for. Overheating will almost always occur, due to the exhaust entering the coolant. Check your overflow bottle for exhaust smells. Watch for bubbles or overflow of coolant from the radiator while running the engine. Check for muddy gray-looking oil or bubbles on the dipstick.
• Often (but not always), a blown head gasket will also cause deposit of water on a piece of cardboard held an inch from the tailpipe output while the engine is running (when this is happening, it is likely that the catalytic converter has been ruined and the muffler will corrode in short order as well). Sometimes drops of water will be seen dropping from the end of the tailpipe.
• Another clue: Turn on the heater; often when the head gasket is blown an odor of antifreeze and synthetic rubber will emanate from the heater vents.
• Many of the symptoms of a blown head gasket can be caused by some other problem in the cooling system, without the head gasket being damaged. Conversely, other problems with the cooling system can cause a blown head gasket and/or warped head. For example, a corroding radiator can send chunks of rust through the cooling system which take out the thermostat and water pump. If the thermostat is old, sticking and corroding, it can send those chunks through the system and take out the water pump or cause a blockage in the radiator, etc.
• Radiator leaks can be the primary cause, or a result, of failures in other cooling system components.
• Don't keep driving with the car overheated, especially if your engine has an aluminum head; you are likely to warp it. If it is warped beyond a certain tolerance, it cannot be planed and will have to be replaced when the head gasket is replaced.
• One of the most common tell-tale signs is a milky-gray ring around your oil cap. When coolant enters the engine oil through a crack in the head or through a blown gasket, it evaporates and leaves a milky ring around the oil cap. Another easy way to tell is to check your oil dipstick. Change your oil and pull out the dipstick. Make sure that you take note of how far up the dipstick the oil is. Top off your cooling system and fill your cooling reservoir to the top. Screw radiator cap back on and start engine. Run engine for about 20-30 minutes or until it reaches normal operating temperature. Allow engine to cool (engine must cool completely to get accurate oil reading). Check oil dipstick again. If the oil has a watery appearance and has risen noticeably up the dipstick, then you probably have a blown head gasket or a warped head. Also, look for a sweet-smelling liquid coming out of your tailpipe. Any of the above symptoms could be the result of a blown head gasket.
• The easiest way to tell is with a compression meter. This replaces the spark plug and lets you know what compression each cylinder is running at. If your compression is abnormally low, then you have a blown head gasket or a warped head. (Note: check the repair manual for appropriate compression of each cylinder.)
• This can be detected in a variety of ways: One way is to note whether that part of the engine block is leaking fluid. This is difficult to determine since there are many other parts of the engine nearby that can also leak fluids, especially when a vehicle is parked in one place for more than a few hours. One of the best indications of a blown, or nearly blown, head gasket in most automobiles is when the cooling system appears to be malfunctioning. The cooling system's efficiency and performance can be directly affected by the quality of the head gasket.
• If your radiator is getting low on water often, this is a sign. The water could be discharged through the tailpipe on your automobile. Another sign is if your car motor has a miss in the engine. The water could be going in on top of the cylinders. This will foul the plugs and cause it to miss.
• There are a few simple indicators you can check for with the engine cold and not running: 1) contaminated oil - it will have a milky appearance from the water mixing in the oil 2) oil on the top of the coolant inside the radiator (if your vehicle has a remote header tank you may not get this); 3) Have someone crank (remove the coil lead or disable the electronic ignition) the engine on the starter with the radiator cap or coolant jacket bleed hose/bolt removed. If the coolant pulses up and down or blows bubbles, you could be in trouble. If you find any of these symptoms move on to removing the spark plugs (label the plugs and the leads as you remove them, so you can put them back in the same place) and again crank the engine on the starter. Depending on how badly your head or gasket is gone, you may get coolant or oil coming out of the plug holes. Inspection of the plugs will also reveal problems during combustion: if you have rusty flaky deposits on the plugs, you may be burning off water; and if you have a heavy carbon, you are burning oil. If you have any of the first 3 items listed (water in oil, oil in water, or pulsing coolant - but don't get any result from checking the plugs) change the oil and water as appropriate, then warm up the engine without the radiator cap on (or the bleeder hose/bolt) and watch for bubbles as the engine warms up. Put the cap back on the cooling system and take the vehicle for a short drive, or run the engine till the entire system is up to temperature and then check the oil for contamination. Having these symptoms is not always indicative of a blown head gasket; usually if the gasket is gone, there is going to be some warping of the head and or block of the engine.
• Loss of engine coolant with no external leaks, a continuous stream of bubbles can be seen with the radiator cap off, black gummy and sometimes crusty stuff around the radiator

Several common signs of a blown head gasket:

• Blue/white smoke coming out the tail pipe which indicates oil is burning
• Dripping oil from the gasket itself
• Carbon Monoxide or hydrocarbons in the cooling reservoir
• Excessive coolant loss with no obvious source of leakage
• Loss of power or a rough engine due to compression loss
• Water mixing with oil
• Oil mixing with water
• Low compression in 2 or more adjacent cylinders
• Remove dipstick and let a drop of fluid fall on hot part of engine - oil will smoke water will "sizzle"