Coolant Leak Help | Ford Explorer Forums - Serious Explorations

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Coolant Leak Help


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May 7, 2019
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Year, Model & Trim Level
1999 Explorer LTS
1999 Ford Explorer LTS 4WD, 4.0 L V6 SOHC. Long story: engine overheated, white steam from the hood. Got it home, and saw a bunch of residue by the upper radiator hose (next to that metal bracket thing that connects both upper radiator hoses: radiator to pipe and pipe to engine). Took them off, couldn't find any cracks or leaks. Removed thermostat, and found a piece of plastic lodged in the valve. Tested it in boiling water, and it won't open or close. Check! One problem solved, so I replaced the thermostat.

Next, I reinstalled all the hoses, and flushed the radiator and reservoir a few times, so just water is in there now (for leak testing). I noticed, however, as I was topping off the radiator at the radiator cap, the water level kept going down with a spurting noise. Great, I thought, it's getting air bubbles out. Except that it hasn't slowed down taking water. No air bubbles, but water level steadily decreasing. I looked under the car, and there's water leaking (not gushing, but a steady small stream) from something under the engine. I don't know what it's called, but it's large, silver looking (aluminum?), and there are seemingly **multiple** places where leaks/drops are coming from. Some of them I can't see the source. I plan to jack it up if I need to later to get a better view.

I'm including a link to a video with the water leaking. Please tell me if you know what the part it's leaking from is. Also notice there are at least 3 or 4 places that are leaking. Is it possible that because the thermostat wasn't opening that the coolant overheated, vaporized, and blew something somewhere else in the engine/cooling system? Before all this happened, I never had a coolant leak under the car.


Lower thermostat housing is BY FAR the most common failure with the 4.0L SOHC cooling system. If that's the problem there's an OE upgrade using non threaded sensors or aftermarket single piece aluminum replacement housings. If unsure, rent a cooling system pressure test kit with a fully refundable deposit from most larger auto parts stores. Call around.

4.0 SOHC plastic thermostat housing

4.0 SOHC Metal Thermostat Housing Comparison

Go back up to the top, follow the leakage up to the source. It's most likely from around the thermostat area, but it's possible to come from the head gaskets too. Look along the edge of the heads, the seam where they set on the block.

If it still isn't easy to identify, either rent one of the radiator pressure testers that pump air in, or remove some other parts to trace the leak.

That is a substantial leak. Hopefully, it's nothing more than a cracked t-stat housing or hose. Check that small bypass hose that runs from the housing to the water pump, too.