2006 4.0 Coolant Pouring Out Coolant Reservoir - Help


New Member
Aug 15, 2018
Sacramento, CA
2006 Ford Mustang 4.0L V6. 130k Miles on the car, Engine is a Jasper Remanufactured engine with 38K Miles.

Hey Everyone. Been a long time follower of this site, but this is my first post. Really looking for some help with this issue.
Drove my car home from work on Friday and everything was operating fine until I parked my car, turned off the engine, and immediately all the coolant poured out of the coolant reservoir. Immediately took it to my mechanic and after a diagnostic they determined the thermostat Housing was the culprit and replaced it. $380 later and 2 days and I got my car back. Picked up the car, drove it to do some errands, got home and parked...no issues. Took it to work the following day (6am in the morning) and as soon as I got to work and turned off my engine, the same issue...all the coolant pours out of the resevoir The distance from my home to work is about 13 miles freeway. Here are the things I know of and have looked for:
- Thermostat housing was replaced by mechanic
- Water pump was replaced 4 months ago
- New coolant added and air was purged from the system
- I replaced the reservoir cap and its very tight
- Operating temperature never goes above the midline (I know they are inaccurate)
- Fan is operational and spinning as it should
- No leaks AT ALL from the system at idle or doing quick drives
- Coolant only boiling over from the reservoir
- The top tube going from the radiator to the block stays firm and can feel coolant making its way into the block
- Car drives perfectly fine other than this issue
- AC and Heater operate perfectly.
- No residue on the old cap or inside the reservoir or any signs of oil making its way into the coolant
- No milkiness in the engine oil, looks normal.

**Id like to make sure its clear, there is no leak, overflowing or issues UNTIL the engine is turned off and it happens within 5 seconds.

I already dropped off my car back at the mechanic since he didn't fix the issue, but before he tries and tell me some BS I figured Id ask you experienced Folks. Appreciate your time and any assistance you can offer. Have a good day!
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