Do I Have a Head Gasket Leak?

Head gasket leakA head gasket leak can often be very difficult to diagnose.  If you have a very large head gasket leak and your car won’t even start, it can be very hard to check for a leak.  On the other hand, if your head gasket leak is very small your vehicle may run normally with the exception of occasionally overheating.  In this article, we will address both situations to help you diagnose your head gasket leak.

Head Gasket Leak Symptoms if Your Car Won’t Start:

  • The vehicle has recently overheated
  • Bubbles in the radiator overflow or expansion tank during cranking
  • Low compression
  • Wet spark plugs


The most common cause of catastrophic head gasket failure is if your vehicle overheats.  Newer vehicles have cylinder heads made from aluminum which expands rapidly when overheated.  This quick expansion can destroy the head gasket or warp the cylinder head enough to allow coolant to slip past the head gasket.  If your car has recently overheated to the point where the temperature gauge was reading at the top end of its scale, and now your vehicle won’t start, it is likely you have a head gasket leak

Bubbles in the Coolant

If you’ve got a large enough head gasket leak that your engine won’t start it could be that one or more cylinder isn’t building any compression and that is causing it not to start.  The compression these cylinders would usually build is allowed to leak out into your cooling system through the head gasket leak. You would observe this as bubbles in your coolant either in the radiator or the expansion tank.  This would also cause your coolant to smell like gas since it is the air/fuel mixture that is being pushed into your cooling system as your engine is trying to start.

Low Compression

This same problem will cause low compression in one or more cylinders which you can check with a compression gauge attached to your spark plug port.  Usually, head gasket leaks show up in 1 or 2 cylinders not all of them, so comparing the compression readings from different cylinders and different banks can tell you which cylinder has the leak.

Wet Spark Plugs

When you remove the spark plugs to do the compression test, you may find one or more to be wet with coolant.  If your engine won’t start, it won’t get hot enough to burn the coolant off of the spark plugs. If you have coolant on your spark plug then you definitely have a head gasket leak.

If you’ve got a head gasket leak so big you cannot get your engine to run, it’s time to take it to a mechanic to have it checked out and the head gaskets replaced.

Head Gasket Leak Symptoms if Your Car Runs:

  • Random overheating
  • Mysterious loss of coolant
  • Milky oil
  • Exhaust gases in your coolant

Random Overheating

If you’ve got a small head gasket leak it will cause your car to overheat but not all the time.  The overheating is often due to a loss of coolant but can also be due to the extra heat the head gasket leak is adding to your cooling system.  If your car overheats you can try checking the operation of your thermostat and radiator cooling fans along with the coolant level. If everything seems to be working properly, the overheating may be due to head gasket leak.

Mysterious Loss of Coolant

Usually, coolant leaks are relatively easy to find as the coolant will leave behind evidence of the leak.  After the coolant dries it will leave white spots around the leak point making it easy to identify. If your vehicle requires you to top off the coolant system often and you can’t seem to find any coolant leaking anywhere, it’s possible you have a slow head gasket leak and the coolant is actually being drawn into your engine and burned.  You can try pressure testing your cooling system and looking for any external leaks. If you don’t find any, try leaving the pressure on the system for over an hour, then starting your vehicle and checking for a puff of white smoke. That would indicate a small amount of coolant had leaked into the combustion chamber during the pressure test.

Milky Oil

A small head gasket leak will allow coolant to leak into the combustion chamber after your turn your engine off.  This coolant will seep past your piston ring into your engine oil. When the oil and water mix it will make a white paste that you may be able to see on your dipstick, around the bottom of your oil cap, or in your oil if you do an oil change.  

Exhaust Gases in Your Coolant

If you’ve got a small head gasket leak exhaust gases will slowly build up in your coolant during the combustion and exhaust stroke of your engine.  You may be able to smell them in your coolant, but there is also a chemical test kit available to check for a blown head gasket.  If the chemical test comes back positive, you definitely have a blown head gasket.

If you have a head gasket leak small enough that your engine still runs, try using BlueDevil Head Gasket Sealer to seal your leak and get your vehicle back to running like normal.

You can find BlueDevil Head Gasket Sealer at any of our partnering local auto parts stores like:

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head_gasket_leak.jpg – By Comosaydice – Licensed by Getty Images – Original Link