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If your General Motors SUV or truck is losing coolant, with no visible signs of a leak, the source may shock you.  Worse, allowing the problem to go on often results in a new engine.  A cracked cylinder head is not the only potential cause, but it is a known problem on the General Motors V8 engines, in certain year models.

The symptom

Low coolant level is often the first sign of a potential problem 

Often General Motors vehicles, with the 5.3L and 4.8L engines have come to us because they continue to lose coolant.  The owner has tried in vain to find the leak and sometimes, multiple auto repair shops have been unable to find a source. It's strange because no outward signs of coolant leakage can be found.  The largest symptom noticed is the coolant reservoir keeps going low. Knowing where to look helps locate the leak.  We find many causes, but often a cracked cylinder head is the source of the problem.

The problem 

Between 2001 and 2006 GM manufactured millions of engines, many with defective cylinder heads castings. The cylinder head castings are weak in the area where the head bolts pass through. The heating and cooling cycles of the engine take a toll and the head may crack. These cracks most often appear around the center row of head bolts. This is under the engine valve cover, and we cannot see it from the outside. This makes the problem more difficult to diagnose. Coolant is leaking into the engine oil and a great deal of engine damage can quickly occur.

False beliefs that mask the problem

People often dismiss this as a possibility, because the engine oil looks normal. The old ‘water causes the oil to cloud up’ test is false. Enough water may cause oil to turn milky over time. Coolant is leaking into the oil which is a mixture of glycol and water. Engine oil temperature is far above 212 degrees Fahrenheit. Water boils, turns to steam and leaves through the PCV system. Glycol remains and mixes with the oil.

Modern oil will often not turn milky

Modern oils can hold a large amount of glycol in suspension, with few outward signs. The problems are, it changes the viscosity of the oil and inhibits the ability to properly lubricate. This mixture flows through the engine, wearing rod bearings, main bearings, camshafts, lifters, timing chains and more. A quality oil analysis is necessary to detect the presence of glycol in the engine oil.

Another misnomer is that a cracked cylinder head will cause overheating. Again this does not apply in this instance. A crack in the combustion chamber may cause overheating. This crack is in the top of the head and coolant oozes into the oil. Combustion gasses are not involved and unless the engine runs low on coolant, overheating will not result.

The cracks are small and oil in this area of the engine is not under pressure. Oil will NOT force its way into the coolant, but the pressurized coolant will flow into the oil.

Cylinder head cracking and coolant loss often begins around 130,000 miles, but could be much sooner. We have seen the problem as low as 60,000 miles and as high as 200,000. Some engine last much longer and many never have the problem.

Outward indications of a problem

Few other outward signs, other than persistent coolant losses are normally evident.  The only symptom most folks notice is a low coolant reservoir.  This can be very misleading and simply adding coolant will only allow the condition to progress.  A much worse practice is adding water to the system.  Water dilutes the corrosion protection of the coolant.  City water may also contain chlorine, fluorides and several minerals.  These can cause even more damage to the cooling system and make a serious problem far worse.  Until repaired, only Dexcool, premixed with distilled water, should be added.

GM acknowledges the following vehicle can be affected. Unfortunately, other than standard warranty coverage, they seem unwilling to do anything about it

2004-2006Buick Rainier
2001-2006Cadillac Escalade Models
2001-2006Chevrolet Avalanche, Blazer, Silverado, Suburban, Tahoe, Trailblazer Models
2001-2006GMC Envoy, Jimmy, Sierra, Yukon Models
2001-2004Oldsmobile Bravada

We have found, an easy way to identify the problem is with fluorescent dye, added to the coolant. Another method, is to remove the valve covers and check for coolant contamination. Steam in the valve cover area will make this quite apparent.  Usually there is NO clouding of the engine oil but the interior of the valve cover may appear milky.

Normal valve cover on top, coolant contaminated cover on bottom

Coolant contamination of oil in GM 5.3L valve cover

A Proper diagnosis is mandatory

Head cracking is a common issue, but does NOT occur on all engines. Many last hundreds of thousands of miles and never have the problem. Several other coolant leak areas are common on these vehicles and give similar symptoms. Many people have misdiagnosed this issue and gone to a large expense for no reason.

A very common leak point is the water pump gaskets. Leakage trickles out between the pump and the engine block and a casual inspection may not show it. The heat of the engine evaporates the liquid and it may never drip to the ground. Always carefully inspect this area, with a mirror if necessary, before suspecting the cylinder heads.

Always check for conventional leaks, before suspecting the cylinder heads 

We normally replace the gaskets and reuse the original pump, unless it is leaking. The original GM pump is much higher quality than most replacement pumps. AC Delco also offers two lines of replacement water pumps. The original equipment line is okay. Cheaper imported Professional line parts are very substandard in our experience. Many online distributors sell the PROFESSIONAL line and may represent it as OEM. Telling the difference is hard, until the cheap pump fails. Never go by brand name alone when selecting parts.

Another common leak source, that people overlook, is the radiator core. Seeing the core is difficult, with the shrouds in place. A small leak will evaporate, from the heat of the radiator and not cause a drip. GM radiators normally last eight to ten years. A dye test or careful inspection under pressure will save very expensive misdiagnosis.

The casting mark 

The affected heads may be identified by a casting mark, just above the intake port. Cylinder heads with this mark are the ones that may be prone to cracking.  The cracks usually will be found in one or more of the five head bolt/oil drain areas, under the valve cover.

Areas prone to cracking and the identifying casting mark

The crack develops due to a poor casting and allows coolant to seep into the engine oil

Coolant leaking GM cracked cylinder head, disassembled and cleaned

On this stripped-down and cleaned head we can see the crack.  With the head on the engine and assembled, the cracks may be more difficult to see.  A pressure test with the valve cover off helps to verify the problem.  With pressure on the system, we may see coolant or air, seeping from the crack.
Finding the crack is easier when we add a fluorescent dye to the coolant and use a black light to find the traces.

A proper repair involves a full inspection of the engine.  An excessive sludge buildup is often present, if the problem has existed more than a few months.  Here, engine replacement is the best policy, as internal problems are likely.

When the problem is detected early, replacement of the bad head or heads is the solution.  Doing several oil changes, in quick succession, following the repair, is a wise precaution.  This will greatly help clear the engine of coolant contamination. Changing the oil about every 1,000 miles, for three successive changes, usually works.  Any coolant or sludge left in the engine can destroy the oil’s ability to prevent wear.  Coolant in the oil will also greatly increase a sludge buildup.

Don’t take a chance, if your General Motors 4.8L or 5.3L Vortec engine is losing coolant. AGCO can quickly diagnose and repair any problem you may have. AGCO, it’s the place to go!

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