Replaced water pump, now leaking at back of engine and overheating

  • 1999 GMC JIMMY
  • 4.3L
  • V6
  • 4WD
  • 139,992 MILES
I just changed my water pump this morning because it was leaking. After I put the new one on, I put water in to see if it was leaking. It didn't leak at the water pump anymore. Now it leaks from the back of the engine somewhere. I let it run for a little bit to see if it was overheating still. Which it did. I also noticed smoke coming from the object behind the throttle body. I could also hear a sound like it was boiling water under the throttle body, so I cut it off and the water shot out from where ever it's leaking from now. I was told it could be my heater core because my passenger side floor board stays wet and it fogs up from time to time.
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Thursday, January 30th, 2020 AT 3:40 PM

1 Reply

If the truck came with air conditioning, there will be a condensate drain tube on the passenger side of the firewall, under the hood. It will have about a four-inch-long rubber hose with a 90-degree bend in it hanging down. If water is dripping from that hose, the heater core is indeed leaking. This is common on GM vehicles, as is a leaking radiator. The biggest contributor to the corrosion is following GM's recommendation to replace the coolant every three years instead of every two years. They originally advertised it as "lifetime" coolant which led a lot of people to have leaks due to corrosion. The problem is the water pump lubricant, corrosion inhibitors, and other additives wear out in about two years, then the acids that normally build up in the coolant attack the metal parts.

The heater core is a good suspect for the wet floor and fogging windshield, but if the water is running out real fast, also consider corroded core plugs in the engine. There's usually two or three on each side of the block, and there's one or two on the back. The transmission has to be removed to replace those two.

There's some relevant articles here:

related to cooling system leaks, but at this point you already know what to look for.
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Thursday, January 30th, 2020 AT 3:53 PM

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