Head Gasket

Functioning, symptons and replacement of head gasket

What is a head gasket?

A head gasket has one of the trickiest but most vital jobs within a car’s engine. It is the seal between the engine block and cylinder head, exposed to both high and low pressure levels in addition to a wide range of temperatures. It seals the combustion gases within the engine and keeps out the coolant that is designed to cool the engine cylinder.
 
Symptoms of a Blown Head Gasket

First Signs
What signs a blown head gasket manifest depends a lot on the engine, where the gasket has blown through and how big the blowout is. If the gasket happened to have blown through between a pair of cylinders with no water jacket between them, and the leak was small, you might not notice much apart from stumbling and misfire. If, on the other hand, the gasket blew through from the combustion chamber to an oil passage, coolant passage or both, you'll end up with leakage from any one of these to any of the others. One classic sign is white, coolant-smelling fog coming from the tailpipe. This is actually steam, the result of coolant entering one or more of the cylinders. You may also get blue or gray smoke, indicating oil in the cylinders. Black smoke that reeks of fuel indicates that one or more of the cylinders is misfiring.

Coolant in Oil
This is a classic sign of a blown head gasket, but can be difficult to recognize if you don't know what you're looking for. Unless you allow the engine to sit for days, coolant in the oil won't simply pool on top of the oil in the oilpan. If that were the case, you'd see water on the dipstick above the oil line when you checked it. Most of the time, though, the water will emulsify into your oil like vinegar in a bottle of Italian salad dressing after you shake it. The tiny bubbles will cause the oil to get lighter in color and go very opaque -- this is the dreaded "chocolate milk" of head gasket failure. If your oil is cleaner because you just changed it, it will be the same color, but be very opaque and hazy. Water-tainted oil also tends to run down the dipstick in odd ways, parting, separating and beading off instead of coating the dipstick smoothly. You may also see and smell steam coming from inside the engine when you remove the oil fill cap.

Exhaust Gases in Coolant
The old mechanic's spot-check for combustion gases in the coolant is to remove the radiator cap -- with the engine cold -- start the engine and smell the gases coming out of the radiator. Sometimes leaks like this won't become apparent until the engine heats up and the metal has expanded, so it may have to idle up to to temperature first. Exhaust gases in the coolant are often immediately visible as fizzy bubbles rising through the coolant with the engine running, and are often recognizable by smell to a trained nose. But if your nose isn't trained, or you don't want to stick your face near churning, boiling water, you can use a "block checker"-type dye tester. These kits use a special dye that turns from blue to yellow or green in the presence of combustion gases. The kit comes with a test cylinder that you fit over the radiator cap opening; if the fluid changes color after exposure to the gases from the radiator, you've got a blown head gasket, cracked head or cracked block.

Compression Test
A compression test is often the most definitive, and can be the only way to diagnose blow-outs between cylinders. Start by removing the fuel pump relay, and running the engine till it dies -- assuming it will start. If not, unplug the fuel injectors, or disconnect the carburetor fuel line and drain the carburetor fuel bowl. Next remove the spark plugs. Check them as you do. If you see one or two that look suspiciously cleaner or newer than the rest, it's likely because they've been steam-cleaned by coolant going into that cylinder. This is a definite sign of a blown gasket. The same is true if one or two plugs are covered with wet oil, or oil-soaked carbon. Once you have the plugs out, screw the compression tester into the plug holes and check the pressure while an assistant cranks the engine over. Record the readings from all cylinders and compare them. You're looking for even psi readings from all the cylinders, plus or minus about 10 percent. If most of your cylinders are reading 180 to 200 psi, and two cylinders are reading 25 psi, you've found your blown gasket.

 
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How to install a New Head Gasket?

Tools and Materials
- A replacement head gasket that matches the old one in the vehicle
- A prybar
- The vehicle owner's manual
- A wrench set
- An RTV (room temperature vulcanant) and applicator

Access the Head Gasket
One of the most difficult parts of the task of replacing a head gasket in a vehicle is simply accessing the gasket. In order to do so, you have to carefully remove a large set of different engine components. The head gasket is located deep within the engine framework. For a specific idea of how to remove the parts necessary to access the gasket, it's helpful to use your vehicle owner's manual as a guide. Look through the manual for a breakdown of how to access the head gasket specifically in your car. When removing the components, be sure to handle each piece very carefully so as not to break it. Put them aside in the same order in which you remove them.

Remove the Old Gasket
Use a wrench to remove the bolts from the cylinder, then lift the head off of the engine. Use a prybar to remove the old gasket from the engine. Set it aside to be properly discarded later on. Gently clean off the surface of the head underneath where the old head gasket was. This will ensure that you'll have a strong seal for the new gasket.

Check the Head and Replace the Gasket
Visually examine the head of the engine for signs of damage, wear and tear or other problems. If all seems to be in order, place the new gasket securely on the head so that there is a tight seal. Be sure that there are no gaps between the head and the new gasket. Push the gasket corners together with the head firmly. If there are any areas that do not connect, use RTV to help secure the seal. Place all of the other components back into the engine securely before you close up the hood and attempt to start the engine once again.
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