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GM V8 Losing Coolant With No Apparent Leak

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 may well mean a new engine.

On several occasions General Motors vehicles, with the 5.3L and 4.8L engines have come to use because of a coolant leak. There is no outward signs of coolant leakage, yet the coolant reservoir keeps going low. The source of the leakage is the cylinder head(s).

Between 2001 and 2006 GM manufactured millions of vehicles, several with defective cylinder heads. The cylinder head castings were not properly done and over time they crack. The cracks often appear in the area of the center row of head bolts. This is under the engine valve cover and difficult to diagnose. Since coolant is leaking into the engine oil, a great deal of damage can quickly occur.

There will be few other outward signs other than coolant loss. Since this is coolant leaking into the oil and not water, there is normally be no clouding of the engine oil. Glycol is absorbed by the oil, but destroys the lubricating qualities of the oil. The crack also does not extend into the combustion chamber. There will not be an overheating problem, unless the coolant gets low and the radiator will not be over pressurized.

GM acknowledges the following vehicle can be affected. Unfortunately, other than standard warranty coverage, they seem unwilling to do anything about it.
2004-2006
Buick Rainier

2001-2006
Cadillac Escalade Models

2001-2006
Chevrolet Avalanche, Blazer, Silverado, Suburban, Tahoe, Trailblazer Models

2001-2006
GMC Envoy, Jimmy, Sierra, Yukon Models

2001-2004
Oldsmobile Bravada


One of the easiest ways to identify the problem is with florescent dye, added to the coolant. Another is to remove the valve covers and check for coolant contamination. Steam in the valve cover area will make this quite apparent. There will usually NOT be clouding of the remainder of the engine oil. (please click images for closer view)

Normal valve cover on top, coolant contaminated cover on bottom

Coolant contamination of oil in GM 5.3L valve cover

The affected heads have a casting mark, just above the intake port. Cylinder heads with this mark are the ones that may be prone to cracking. The cracks form in the five head bolt/oil drain areas, under the valve cover.

Areas prone to cracking and the identifying casting mark

Coolant leaking GM cracked cylinder head, disassembled and cleaned
The crack develops due to a poor casting and allows coolant to seep into the engine oil
On this stripped down and cleaned head the crack can actually be seen. With the head on the vehicle and assembled, it is quite difficult to see. A pressure test with the valve cover off will verify the problem. Coolant and/or air can normally be seen seeping from the area. Florescent dye and a black light makes this much easier. (please click on image for a closer view)

Proper repair involves replacement of the bad head(s). Several oil changes, in quick succession, will be needed clear the engine of coolant contamination. About every 1,000 miles for three-changes usually works. Coolant left in the engine will destroy the oil’s ability to prevent wear. Coolant in the oil will also greatly increase sludge buildup.

If you own a General Motors 4.8L or 5.3L Vortec engine and are losing coolant, don’t take a chance. AGCO can quickly diagnose and document the problem for you and repair any problem as well. AGCO, it’s the place to go!