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Headgasket?

5K views 20 replies 6 participants last post by  russ93spirit 
A compression test may be inconclusive for many of the 2.5 HG failures simply because the failure is often just a hairline crack in the steel fire ring. The cylinder may pass the compression test, but a small amount of combustion gas can pass through to the cooling system passageway adjacent to the cylinder. Likewise, when a warmed engine is turned off and the cooling system is still pressurized, a drop or two of coolant can pass through a split HG and get into a cylinder. A compression test to diagnose a ruptured gasket would certainly show a problem but a ruptured gasket is usually not the typical 2.5 HG failure.

The best test is to use a product that detects combustion gasses in the coolant overflow container, unless you already see streams of bubbles present in the container; then it is pretty obvious.

Sometimes the HG hairline fractures that show up in the fire ring (that surrounds the cylinder) are so small that they don't cause noticeable leakage until the engine has warmed up. When I am checking for the HG problem, I watch the overflow container right after starting the engine when it is cold and continue to watch for 15+ minutes while the engine fully warms up. Of course, you must make sure that there was no air in the system from any previous work where the system had been opened.

Keep in mind that any coolant that has passed through the exhaust may have poisoned the O2 sensor which will cause a bad running engine.
 
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