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Few things are more damaging to an engine than coolant mixing with the oil. Coolant can enter the oil by many avenues and is very hard to find, until it is too late.

The appearance of the oil is a very unreliable way of detecting glycol

Ask most enthusiasts how to identify water in their oil and the most common response is a brown-milky appearance. Enough water in motor oil may turn it brown and milky. More often this is NOT SO with coolant in today’s oil. Modern oils contain detergent and dispersants and great damage may be done, before we notice any outward appearance.

Moisture in normal operation

All engine oil is contaminated with trace amounts of water in normal operation. The heat and cool cycles allow moisture to form in the crankcase. Detergent and additives in the engine oil, pick this up and hold it in suspension, helping to prevent a sludge build up. When the oil temperature reaches 212 degrees Fahrenheit, the water boils and turns to steam. The positive crankcase ventilation or PCV system draws the moisture out and helps prevent problems. With vehicles not driven far enough to reach full temperature, this may not occur. This is why driving only short trips requires more frequent oil changes.

Problems with coolant in the oil

Engine coolant is a combination of some form of glycol and water. Glycol presents several problems when mixed with engine oil. First is, it does not boil away, like water. Instead, it forms organic acids, thickens the oil and reduces its dispersancy. Depleted oil no longer contains the soot from combustion and sludge soon develops. Acid attacks the metal components in the engine producing corrosion and debris.

Increased viscosity does not flow properly, reducing lubrication and the oil filter plugs up. Engineers design oil filters to bypass when anything restricts them. A bypassing oil filter allows debris to flow through and engine failure is not far behind.

How coolant gets into engine oil

Blown head gaskets, cracked or warped cylinder heads normally come to mind, when coolant enters the engine oil. Any of these will allow coolant to enter the oil, but other sources also exist. On engines with coolant flow through the intake manifold, leaking intake gaskets allow coolant to contaminate the engine oil.

Rough machine work causes gaskets to fail 

This is common with early model GM engines, such as the 3.1, 3.4, 4.3 and 5.7 liters. Intake gaskets leaking at the crossover port, allows coolant to flow into the lifter galley.

Leaking intake gasket allow coolant into engine oil

Poor machine work on the sealing surface of these engines results in gasket failure. Failed intake gaskets may also allow coolant to enter the combustion chamber or leak to the outside.

Stripped or improperly sealed head bolts are another source of leakage. The head bolts on many engines thread into the coolant passages. Failure to seal the threads may allow coolant into the oil. Other sources include, leaking oil coolers, water pumps and even timing cover or intake manifold bolts, on a few engines.

Recognizing the symptoms of glycol in engine oil

An early warning is the loss of engine coolant. They seal all modern cooling systems and losing coolant is NOT normal. When the coolant level drops, a problem exists and quickly finding the cause helps prevent more damage.

Glycol changes oil viscosity and a laboratory can detect its presence, with the proper test. Often the phosphorus level and certain metals present in the oil also suggest glycol contamination. Engine oil testing is not expensive and is a good way to confirm a suspected problem.

Home tests for glycol contamination

Not as accurate as a professional oil analysis, a few home-tests suggest glycol contamination. Blotter paper is available that shows the dispersancy of oil. We place a drop of motor oil on the blotter. After a while a pattern develops. Charts are available to help understand the patterns.

Fresh oil soaked into a business card

Without special blotter paper, a standard business card can suggest the same. A drop of fresh oil will soak into the card. As the oil wicks through the paper, the color diminishes and fades out evenly. This shows good dispersion characteristics remaining in the oil.

Glycol contaminated oil on a business card

Oil contaminated with glycol, is less likely to disburse into the paper. A dark sticky deposit, remaining the center, may show excessive soot resulting from the depleted additives in the oil. A pattern similar to this, combined with a loss of coolant, is a cause for alarm.

Placing a drop of engine oil on the hot exhaust manifold may also provide a clue. Oil will produce a bit of smoke as it burns away. Glycol in the oil may cause a slight sizzle. This is subtle and we need careful attention to notice. Combined with other indicators, such as the blotter test, a sizzle test reinforces suspicion.

Another home-test may also help. We should operate the engine to warm the oil and then allow it to sit without running for two hours. Remove the drain plug very slowly and catch a small sample of oil in a transparent container. Any separation of the oil or indication of water shows a problem. This test may not show a problem unless it is severe, but is useful as additional confirmation of a problem. Unfortunately, with this much contamination, damage is likely already occurring.

What to do if coolant is present in the oil

The source of the problem must be found before operating the engine. Oil flows to every part of the engine and spreads the glycol with it. A restricted oil filter and lack of lubrication cause extreme wear and engine damage is almost certain.

Learning the source of the problem is only part of the solution. An evaluation of engine condition will tell if repair is possible or if replacement is a wiser solution. Caught early, repairing the source and thoroughly cleaning the engine, give excellent results. When sludge is present or wear occurs to bearings, camshafts and timing chains, replacing the engine makes more sense. For instance, replacing a blown head gasket on an engine that will fail, is not cost effective.

Removing a valve cover or oil pan normally shows sludge buildup and wear. This is a sound precaution before investing in a major repair. Having a professional evaluate the situation may also be wise. Experience guides the professional on where to look for problems. A trusted mechanic will also advise if repair or replacement is a better choice.

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