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Head Gasket Failure - Technically Speaking

Tricks of the Trade - Testing for a head gasket failure

James Berry Jul 16, 2014
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Q: Hi, my name is Paul. I am new to the Corvette world but have been working on cars all my life. I enjoy the technical articles the magazine publishes. I especially enjoy the online articles. I have been searching the web for information on how to test for a blown head gasket. It seems like the information I am getting is mixed and actually evokes more questions than answers.

I own a 1975 Corvette that I purchased with the knowledge that is had been overheating and was running rough. The car has been sitting since I bought it, but I am ready to get it back on the road. One of my symptoms is that when I crank over the engine with the radiator cap off it "false boils" is that is a sign of a blown head gasket?

Why does overheating cause head gasket problems, and what other test can I perform to pinpoint if I have a blown head gasket? Thanks for any help,

Paul F.
Via the internet

A: Paul, if the engine has been overheated it is possible that you have blown a head gasket, cracked or warped a head. If a cylinder head gets too hot it can swell to the point where it crushes the head gasket. This will usually occur at the thinness point of the head gasket which is between the engine cylinders. The blown head gasket and/or cracked combined with engine combustion pressure provides a leak path for coolant and/or combustion gases.

Let's discuss the various indicators of a blown head gasket and the diagnostic techniques we can use to determine a blown head gasket and/or cracked head. You should be careful when performing these tests because when you have a blown head gasket the engine can overheat quickly and the cooling system can build excessive pressure which can cause something in the cooling system to rupture so take care while performing this test.

Indicators of a Blown Head Gasket


Coolant emanating from the exhaust will present one of the common signs of a blown head gasket. When there is a blown head gasket and/or cracked head you may notice white smoke coming from the tailpipe. Sometimes droplets of water will be coming out of from the end of the tailpipe as well and the exhaust will have a sweet smell to it. (Image A)

One simple check of a blown head gasket is oil contamination. This can be determined by inspecting the dipstick and the oil cap for a milky residue, this residue will have a creamy tan/off-white color. This milky residue is caused by coolant entering the engine oil through the leaking head gasket and/or cracked head.

Bubbles Fluid Tester 2/6



A blown head gasket and/or cracked head can cause bubbles of air to be visible in the radiator or expansion tank. To check for this, remove the radiator cap, warm up the engine and on a warm engine some head gasket problems will present with bubbles being visible in the radiator or expansion tank. It may be necessary to bring the rpms (revolutions per minute) up for the bubbles to be visible. Be careful as it is possible that the coolant could erupt out of the radiator or expansion tank much like a geyser. Take care not to get burned while performing this test. (Image B)

The bubbles seen here are visible in the combustion leak tester tool.

Universal Combustion Leak Tester 3/6


Engine Performance

The engine may possibly idle roughly or run poorly because of compression loss from a head gasket problem. Usually one or two cylinders will be affected. If you can isolate which cylinders are missing you can remove the spark plug for inspection. If your spark plug is extremely clean or has a coolant tint the problem could be severe. If you disable the ignition system so the vehicle will not start you may notice the coolant may start spraying out of the plug holes while cranking over the engine.

With all of the spark plugs removed, so you will not hydro-lock the engine, a cooling system pressure tester can be installed to see if coolant is pumping into any of the cylinders. This can be done with the coolant system pressure tester installed on the vehicle and the ignition disabled. Have a helper check for coolant to be spraying out of the plug holes while cranking over the engine.

Compression Test

What leads to most misdiagnosis of a blown head gasket is that all of the above indicators passed the test. Remember, a head gasket can get blown without causing the coolant to mix with the engine oil and without causing combustion gases from entering the cooling system. This happens when the head gasket has failed between two cylinders resulting with a gap in the head gasket that will let the compression of one cylinder to leak into a companion cylinder.

This condition can be very easily verified by doing a compression test. If two side by side cylinders have 0 psi (pounds per square inch) compression this confirms that the head gasket is more than likely burned between those two companion cylinders.

Exhaust Gas Analyzer Test

If you have an exhaust gas analyzer, head gasket diagnostic becomes much simpler. Simply use your emissions analyzer to check for combustion gasses in the cooling system. This is done on a warm running engine, by inserting the exhaust probe of the exhaust gas analyzer into the top of the radiator without allowing it to suck any coolant into the probe. If you are afraid of a coolant bubble shooting out and getting into your probe, use a no-spill antifreeze funnel or something appropriate to eliminate contamination. You may find it necessary to drain enough coolant out of the radiator so that the level is 2 to 3 inches below the neck of the radiator or reservoir so not to suck any coolant into the probe.

If you see if combustion gasses levels soar you have confirmed that the head gasket and/or cracked head is the problem, because combustion gas from the combustion chamber of the engine is now getting into the antifreeze. This, in turn, confirms that antifreeze is going into the combustion chamber and out the tail pipe.

Cost effective Block Tester (Combustion Gas Tester)

If you don't happen to have a $5,000 exhaust gas analyzer laying around in the garage you can use a Block Tester, also known as a combustion leak tester, to determine if you have exhaust gases in your cooling system. (Image C)

Exhaust Gasses 4/6


A combustion test kit can be found at your local NAPA, auto parts store. The part number is 700-1006. The price for this part is less than $50.

Vehicles with head gasket failures can overheat quickly and the cooling system can build excessive pressure which can cause something in the cooling system to rupture so take care while performing this test. Be sure to read the manufacturer's instructions before performing any test.

This is the procedure for performing a combustion leak test using a Block Tester:

1. The engine should be warm, and allowed to idle during the test. Coolant must be warm and circulating so combustion gasses can enter the cooling system while this test is in process.
2. Remove radiator cap, and check to see that radiator coolant level is low enough to prevent any coolant from getting into the test equipment during the test. It may be necessary to drain enough coolant out of the radiator so that the level is 2 to 3 inches below the neck of the radiator or reservoir to prevent contamination.
3. Check the color of test fluid before installing it into your test equipment. If the fluid is blue it is good to use. If the fluid is green or yellow, discard and obtain fresh test fluid.
4. Remove the top of the tester by twisting and pulling up. Pour test fluid from bottle into test instrument to the fill line.
5. Insert test instrument onto the neck of radiator or reservoir with a rotating motion so a seal is formed with the radiator or reservoir. This will allow radiator gases to pass through test fluid and by the squeeze bulb. (Image D)
6. Continue to force radiator gases through test fluid by squeezing bulb for about 1 minute. If fluid turns yellow a combustion leak is present.
7. If fluid remains blue, a combustion leak is not occurring while test is in process.

Test Instrument 5/6


Gasket 6/6


Paul, replacing the head gasket will cure the leak, but remember a blown head gasket is generally the result of an underlying problem generally with the cooling system that will need to be diagnosed and repaired. Otherwise, the newly installed head gasket may suffer the same fate the next time the engine overheats.

It is also a good idea to always have the heads checked by a machine shop after removing them to make sure they are not cracked or warped. This is the kind of job you only want to do once. Thanks, and good luck Paul.

Got a question for our Tech Corner expert? Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Technically Speaking, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at vette@sorc.com. Be sure to put "Technically Speaking" in the subject line.



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