Your head gasket is blown. Now you must figure out why.

Even the novice driver with no mechanical ability knows when there's trouble, and smoke is never a good sign. White smoke from the exhaust can spell big problems with your engine, and the smoke's white color tells us that it's water vapor (blue = burning oil, gray or black = fuel). Specifically, coolant is escaping from the cooling system into the combustion chamber. At first, you're shocked that something as benign as steam could mean having a blown head gasket—a malady that is infamous for big damage and having a high cost of repair. A head gasket repair can cost a thousand dollars or more to fix, but if you're not careful, you'll be paying for the same operation over and over, and that's when your shock will turn into anger. White smoke from your exhaust is only part of the story—a single clue that can help point to the true cause of the failure. Don't despair! We're going to look at those clues and guide you through to the other side! Scroll through to see what's up.

White smoke from the exhaust means there is an opening in the cooling system that exposes coolant to the high pressure and temperature of the combustion process. There are only two ways that can happen: a crack in the engine (block or cylinder head) or a breach in the head gasket seal. Cracked engine blocks are exceedingly rare because the head gasket seal is designed to fail before the block fails, making head gasket failure the predominant mode for white smoke from the exhaust (cracked cylinder heads are slightly more common, and we'll discuss those unique cases later). Replacing the damaged head gasket, while expensive, is only half the battle. In this photo, the passenger-side head gasket on this 5.7-liter Hemi failed because during a previous repair the dealership used a high-speed abrasive disc to clean an aluminum cylinder head sealing surface designed for an MLS gasket—a big no-no. The subsequent lack of supporting material (a mere .004 inch) between the aluminum cylinder head, the MLS gasket, and the iron deck surface caused a breach that eventually allowed coolant to enter the cylinder. It will be up to you and/or your mechanic to not just perform the head gasket repair, but to do the necessary forensic diligence to fix it for good!

Got white smoke pouring from your exhaust, but you're still in denial about having a blown head gasket before checking the balance in your savings account? Do this simple move: Check your dipstick to see if coolant has mixed with the engine oil. In the majority of cases where a head gasket has failed, you will see that instead of the normal clear, dark brown oil clinging to the dipstick there will be froth-like bubbles in the oil, and it will have begun to take on an appearance mimicking a milkshake. Moreover, the longer you wait to repair the head gasket with the oil being contaminated the more likely you will add bearing damage and ring wear to your list of problems. Coolant is not a very good lubricant, and your oil's protection will rapidly decline. In this photo you can see how different engine oil looks after it has been whipped by the crankshaft with a heaping helping of antifreeze.

White smoke from your exhaust almost always indicates a blown head gasket, and just like there is coolant mixed with your engine oil, there will also be engine oil mixed with your coolant. This is because the high pressure of combustion introduces exhaust gas and nearby boundary-layer lubrication into the cooling circuit. As you accumulate more miles on your engine with a blown head gasket, the coolant escapes through the exhaust, and cooling system volume is replaced with a miasma of oil foam and exhaust byproducts. This can easily be seen by removing the radiator cap. You'll be able to spot the oily foam around the cap's seal and the radiator cap's neck, as seen in the photo above (this is not a St. Patrick's Day milkshake!). It will also take on the smell of combustion, which is not normal for healthy coolant. Disclaimer: Do not remove your radiator cap while the engine is hot. We told you so! As a bonus, you'll also be blessed with a profusion of exhaust bubbles in your coolant overflow reservoir.

If you find white smoke pouring from your exhaust, you'll probably want to know why, so that your head gasket problem won't recur. In most normal cases, a blown head gasket is caused by low-octane fuel in conjunction with some aggravating factor. When we say most cases, we want to be clear that we're talking about normal driving with an unmodified car or truck, not a souped-up hot rod. In a few cases, white smoke in the exhaust can indicate a cracked cylinder head, as is well-documented in 3-liter Ford Vulcan V-6s and late-model Chrysler Magnum V-8s; their thin, flexible cylinder heads can easily be pushed beyond the limit by low fuel octane, but this can happen to any engine. Blown gaskets and cracks can result when preignition occurs in the combustion chamber. These spikes in pressure result in damage when a set of factors converge. These can include towing in steep terrain and/or in hot weather, using low-quality fuel, having the ignition timing too far advanced (manually or via programming), and having an older engine with ring wear that allows oil blow-by to contaminate the air/fuel charge. Additional factors that may encourage preignition and detonation include a clogged fuel injector, a failing fuel pump, clogged fuel filter, and a low coolant level.

When white smoke comes from your exhaust, it's usually in conjunction with overheating. With a blown head gasket, you are losing coolant continuously even if this occurs very slowly. Over time, the cumulative loss of coolant will manifest itself at first as an occasional excursion of the temp gauge needle into high territory. As the loss of coolant becomes greater, the remaining coolant must do the job of the entire system. Moreover, lower levels of coolant make the cooling system itself less effective, and as this spirals out of control you'll see more frequent and more severe swings of the temp gauge. By this point, air pockets will have formed in the upper portions of the cooling system, including the water pump, which will be suffering from cavitation—the primary cause of the temperature swings. You can also use test tools to perform a cooling system pressure test, a vacuum gauge to check for swings in engine vacuum, a compression tester or even a leak-down tester to check sealing integrity. All these tools are good for validating and pinpointing the damage, but you'll already kind of know by using sleuthing tips from this story.

White smoke coming from the exhaust is almost always a sign of a blown head gasket, but the loss of coolant by itself isn't necessarily a sign of a blown head gasket. Moreover, you can have a blown head gasket and not have white smoke coming from your exhaust if the gasket breach is slow enough or if the breach is to the outside of the block and not between cylinders. If you need to keep filling your coolant (more than just a modest yearly topping-off), a pressure test is in order. Besides flowing through your engine, coolant is routed to the heater box via a system of hoses, valves, and junctions—many of which can become corroded or, if they are plastic, may break or crack. Now wouldn't that suck if you spent the money to have a head gasket replaced only to find out your coolant leak was happening elsewhere? The loss of coolant—whether from a blown head gasket, a split heater box, cracked radiator tank, damaged HVAC diverter valve, or something else—is usually accompanied by the sweet smell of hot antifreeze, which is similar to butterscotch or graham crackers.

Air in the cooling system can indicate a blown head gasket, although lots of other things can cause a low coolant level as well. When white smoke from the exhaust isn't evident and you still suspect a blown head gasket, you can count on problems with maintaining a full coolant system. Discerning a stubborn air pocket in the cooling system from a blown head gasket can be tough without a leak-down test, but short of that you can try to purge any potential air pockets first. In the case of many modern V-8 engines, such as the Chevy LS shown above, the high point of the cooling system isn't always at the fill level—usually because styling concerns limit hood height. In these cases, engineers design purge valves to clear air pockets from the cooling system during the initial coolant fill. If you don't know the location or locations of these burp valves and refill your coolant system without relieving these air pockets, the coolant won't be able to circulate completely, causing wide swings of the temp gauge, the same as a blown head gasket.

Related: Everything You Need to Know About LS, LSX, and Vortec Engines

If white smoke pours from your exhaust, it means that vaporized antifreeze will have contaminated one or more oxygen sensors. All fuel-injected cars have these sensors, which are screwed into bungs welded onto the exhaust system. Additional oxygen sensors are also located after the catalytic converter to monitor catalyst efficiency. The oxygen sensors are your car's window into regulating the amount of fuel to inject; when the sensors become contaminated by vaporized coolant they will stop operating as designed and a fault code will be stored. For this reason, when white smoke starts coming out of your exhaust, a check engine light from a slow or non-operating O2 sensor will almost always come on shortly after that. To restore proper engine function, it's important to always replace the oxygen sensors on the affected bank of cylinders after a head gasket is replaced.

When ordinary civilians experience white smoke from the exhaust, the phenomenon can be vexing and the verdict undeserved, but when it happens to a hot rodder, it's more like "I shoulda known better!" You, my friend, have committed the shameful sin of too much boost. We have to laugh here because in researching our story, we found no photos of "milkshake oil" in the HOT ROD archives; we had to mix old oil and antifreeze in a kitchen blender in order to get a good photo (you'll need to scroll up). No self-respecting HOT ROD staffer in history has ever been able to admit having it happen to them, let alone take a picture of it, but it happens a lot behind closed doors.

Related: What Is Detonation and 8 Ways to Stop It!

This author's personal experience contains at least three instances of blowing a head gasket from too much boost, and all three happened on a 302ci Windsor small-block Ford, an engine famous for its flexi-flyer fire deck and four-bolts-per-cylinder head-bolt scheme. Fixing the head gasket sealing issue permanently on a boosted 302 Ford can be done by means of O-ringing the heads or the block, but this causes other problems. If a thicker aftermarket block isn't used above the 600hp level, O-ringing the heads or block can result in the block splitting in half as opposed to merely blowing a head gasket. (The author once oiled down the track at Bowling Green, Kentucky, causing a two-hour clean-up, but at least everyone was happy that the head gasket survived—not!). For the boosted 5-liter Windsor, blowing head gaskets is just a fact of life.

Read about KJ Jones' heartbreaking encounter with too much boost in a 5-liter Mustang here.

Sometimes you blow a head gasket and there seems to be no plausible reason in sight. That's what happened to this author's wife when her 5.7-liter Hemi Dodge Challenger began belching white smoke from the exhaust pipe (turning up the radio is not a solution). Unlike the 302 Ford Windsor, the Hemi block has plenty of beefcake and a well-engineered MLS head gasket design, so what was the issue? Much forensic effort was expended to find the cause, but the scrutiny for the failure paid off when we discovered after 169,000 miles that the dealership botched an unrelated repair that involved removing the cylinder heads. Our autopsy of the blown engine revealed that the Hemi's aluminum cylinder heads had been cleaned with a high-speed abrasive wheel, a problem we mentioned earlier. The lesson: Previous repairs that are rushed can cause more problems down the road, sometimes a lot farther down the road. If your pride and joy starts coughing white smoke after a repair that involved removal of the cylinder head(s), this is your smoking gun.