Stop leak for heater core?

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Full Access Member
Jun 19, 2012
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Frisco, TX
4.8 5.3 5.7 6.0 GEN III Engine Cylinder Head Cup Plug Coolant Leak

The following diagnosis might be helpful if the vehicle exhibits the described symptoms in the PI.


A customer may notice a coolant leak from the cylinder head cup plugs of a GEN III Engine.


If there is evidence of a coolant leak at the cylinder head cup plugs, the area should be cleaned and inspected to determine if an actual leak is present or if it was only a coolant stain. Some engines are built with cooling system sealing tabs. If only a stain is present, it may be the result of the cup plug seeping during the first 15 minutes of operation when the engine was new. The seepage may have occurred before the factory installed coolant tabs dissolved and sealed the plug to cylinder head area. If an actual leak is present, the cooling system should be drained, refilled with a 50/50 Dex-Cool mixture and 4 small sealer tabs. Small coolant tabs are available in packages of 5 (4 oz. each) and the part number is 12378254 [in Canada, use P/N 10953473 and add 1 1/2 pellets]. After filling the cooling system, the vehicle should be operated for at least 15 minutes so the area can be inspected for leaks again. If the cup plugs continue to leak after installing the sealer tabs, new cup plugs should be installed as follows:

  1. Lower the coolant level so there is no coolant in the cylinder heads.
  2. Remove the cup plugs and clean the bores.
  3. Apply Primer N (Loctite product number 7649) to the new cup plug.
  4. Apply Loctite 620 adhesive to the cup plug bore in the cylinder head. By applying to the separate substrates, you avoid any cross contamination of adhesive and accelerator. This will also prevent any potential precuring of adhesive that is applied to an activated surface. The vehicle should be allowed to sit for a minimum of 1 hour prior to refilling and testing vehicle.

Granted, this is Gen 3, but even then they could come from the factory with the tabs.


Full Access Member
Mar 9, 2023
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What's the deal with the heater T's? one white, one black, 111mm and 113mm. ? Can I get both black?

NVM found it

white one is a 90 degrees and black one is slightly angled from 90. Ordering SKP brand from rockauto(has a heart icon next to them..?) where both are black.
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