GM's Dex-Cool coolant has received a lot of bad press, so many consumers want to blame Dex-Cool for cooling system problems in the Oldsmobile Silhouette, but lack of maintenance really causes the problems. In general, keep the cooling system full to prevent the formation of rust and the introduction of contaminants to the system. General Motors recommends checking Dex-Cool with a refractometer to make sure you have the correct solution, since an antifreeze tester for ethylene glycol coolants may give false readings. A 50-percent solution of Dex-Cool and water should read minus 34 degrees F.
In the 1996-through-2003 Silhouette, a coolant leak -- sometimes accompanied by an oil leak -- might be present at the lower intake manifold. A redesign of the manifold gasket for the 2004 model-year corrected the problem. Technical service bulletin No. 03-06-01-010C describes the problem and solution. GM gasket kit, part No. 19169127 includes the gasket and bolts needed to make repairs.
Dex-Cool coolant is bright orange in color, but can appear green in a dark container. If you added green coolant instead of Dex-Cool, you inadvertently contaminated the coolant. Once it's contaminated, you need to do a system flush -- not simply a drain and refill -- to rid the system of the old coolant. Mixing coolants degrades the benefits of Dex-Cool, turning your long-life coolant into a two-year or 30,000-mile coolant.
After a drain or flush, bleed the cooling system of air that causes overheating after you refill it. Your Silhouette has two bleed valves: one on top of the water pump housing and one on the thermostat housing. Open both valves and add a 50-percent solution of water and Dex-Cool to the radiator until the coolant level reaches the bottom of the radiator fill neck. Wait for two minutes for the level to drop and refill as needed to bring the coolant level back up to the bottom of the filler neck. Install the radiator cap and close the bleed valves. Add the coolant solution to the reservoir bottle up to the "Full" line and start the engine. Let the engine run long enough for the engine cooling fan to cycle on and off three times, then turn off the engine and allow it to cool completely. Check the reservoir bottle and top off as needed.
The maintenance schedule for your Silhouette recommends you service the cooling system every five years or 150,000 miles, whichever comes first. As GM points out, however, that service schedule is not a warranty guarantee. It doesn't mean you can ignore the cooling system for 149,000 miles. The maintenance schedule also recommends you inspect the cooling system once a year. Don't ignore that recommendation. Look for discolored coolant, which may indicate the presence of rust or contaminants that lead to sludge build-up and inadequate freeze and overheat protection. You need to act when you see abnormalities in the condition of the coolant in your Silhouette that will lead to cooling system problems if left unaddressed.
When you do the cooling system check every year, pressure test the radiator cap and replace it as needed. GM recommends the spring-center sealed cap over the drop-center vented cap to maintain the coolant level and keep contaminants out of the system.
After a system flush, take into consideration the water left inside the engine when figuring a 50-percent solution. For example, the cooling system capacity of the 2004 Silhouette with rear HVAC is 11.9 quarts. To end up with a 50-percent solution in the system after the flush, you'd have to add almost six quarts of straight coolant and then top it off with water. If you mix the coolant with water and then add it, you'll have too much water in the system.
