Brown, Gel-Like Substance on Rad. Cap/Upper Filler Neck (Replace Cap/Clean Neck)

Some customers may comment about a brown colored gel-like substance on the radiator cap and upper filler neck.

Important: This condition is unique to the 3.1L and 3.4L engines and should not be confused with the condition described in Corporate Bulletin Number 99-06-02-012B -- Rust in Coolant System.

This condition is almost always confined to the radiator cap and upper filler neck, so it is not necessary to change the coolant. Keeping the cooling system full at all times should eliminate future recurrence. Replace the radiator cap and clean the radiator upper filler neck using the procedure and part numbers listed below.

1. Remove the radiator cap when the engine is cool.

•	Slowly rotate the cap counterclockwise to detent. Do not press down while rotating the pressure cap.

•	Wait until any residual pressure (indicated by a hissing sound) is relieved.

•	After all hissing stops, continue to rotate counterclockwise until the cap is removed.

2. Using a shop rag, clean the upper radiator filler neck of any residue.
3. Remove the coolant recovery reservoir if visually contaminated. Refer to the Coolant Recovery Reservoir Replacement procedure in the Engine Cooling sub-section of the appropriate Service Manual.
4. If necessary, clean the coolant recovery reservoir and all holes attached to it with soap and water. Rinse the reservoir and hoses with clean water and drain.

Verify that the coolant can flow freely through the hoses.

5. Install the coolant recovery reservoir. Refer to the Coolant Recovery Reservoir Replacement procedure in the Engine Cooling sub-section of the appropriate Service Manual.
6. Refill the coolant recovery reservoir with a 50/50 mixture of DEX-COOL® and clean drinkable or distilled water to the HOT mark. Fill to the HOT mark to compensate for any air that may be in the cooling system.
7. Fill the radiator to the top of the filler neck.
8. Install the new radiator cap, P/N 10409635, making certain the arrows line up with the overflow tube.
9. Instruct the customer of the importance of maintaining the proper coolant level in the radiator and coolant recovery reservoir.

Parts are currently available from GMSPO

For vehicles repaired under warranty, use: