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Thermostat Leak Problem


Well-Known Member
January 30, 2012
Reaction score
City, State
So. Cal
Year, Model & Trim Level
04' Sport Trac
Hey everyone my thermostat housing started leaking a few days ago and I bought a new thermostat and o ring to do the job today. I did not realize the housing was 3 parts and the leak is lower than we thought. Should I just buy the whole housing from Rockauto?

Here is a pic, where I have the screwdriver is where it is leking but it comes apart just above that-

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Thank you for the link, now I know exactly what to get.

I just replaced mine this past weekend and it's a complete PITA !!!!

I bought the complete housing, upper rad hose, throttle body gasket and bypass hose from I also replaced the heater control valve, bought that from autozone. I would suggest getting new o-ring for the red temp sensor (the new housing will come with one you can use from the block-off plug you don't needed for our engines). Trust me on this, you do not want to have to remove everything more than once. I had to three times because of leaks from the o-ring (reused old one), heater hose (where it goes onto the top of the housing ( install regular clamps, the factory ones won't hold tightly like when first installed).

I removed the T.B., alternator, alt bracket, serp belt, and upper rad hose to access the bolts but they were still a pita, take your time and don't tighten too much.

Here's the part numbers from



Radiator Upper Hose

$ 13.01



Throttle Body Gasket

$ 1.31



Thermostat Housing / Water Outlet

$ 74.79



Bypass Hose

$ 4.08

Thanks for the info. Yeah we could tell it is going to suck, we put a new thermostat in it since we already had it apart but I will be ordering the rest of the parts on Friday and replacing them next week.

Also did a oil and oil filter change with high mileage oil and a Fram tough guard filter. This weekend I plan to do a trans oil and oil filter change as well. I am trying to get everything fresh for desert season starting in October. I am also hoping to get a new water pump for preventative maintenance and a Flowmaster muffler that will exit under the cab.

I removed the T.B., alternator, alt bracket, serp belt, and upper rad hose to access the bolts but thety were still and pita, take your time and don't tighten too much.

WOW!! You took all that **** off to replace the housing? I must have been lucky when I did mine and only took off the CAI tube, and the plastic 4.0 cover, and upper radiator hose (obviously) to replace mine.

I can't remember Chad, did you have the Job 1 or Job 2 motor in your's, that might be the difference with the different intake manifolds.

Thermostat housing replacement

2004 4.0 Sport Trac 89k miles.
Just got through replacing the housing today. Here is my story'...

About three weeks ago I noticed the heat gauge going up. Coolant level was down. No visible leaking. Serviced, drove it some more. Leak started showing itself after several days, right front part of engine. Pulled the right front tire. I could see coolant (as shown below) but not the source so I shut it down and applied about 15psi to the system.
ford tstat 5.jpg

Bingo. Not easy to see here but that pressure forced the issue and I could see a small stream of coolant coming out of the back of the seam between the upper and lower housing.

ford tstat 6.jpg

When I have this sort of thing fail I generally follow two rules:
1. Use OEM parts.
2. Replace everything associated with the failed part.

I ordered all my parts from a Discount online Ford retailer.
The total for everything but new hose clamps was right at $150. As an example, the upper and lower housings were 13.51 and 36.17 respectively.

ford tstat 2.jpg

But they sent the wrong lower housing. Apparently some models take two senders (1 for the computer... 1 for the gauge???) Anyway they sent one with two holes in it.
ford tstat 3.jpg

I didn't want to wait for the 'right' lower housing so I trimmed the old sender down, mfgr'd a keeper out of some spring stock and I was back in business.
ford tstat 4.jpg

I think its pretty obvious that the housing is going to fail on any 4.0 eventually. So if you have an old one like mine with anything close to my mileage I would suggest at least getting the replacement parts and have them handy when it does fail. The image below is a good reason to replace the entire housing if mine is typical. The plastic is clearly deteriorated and breaks off easily.
ford tstat 7.jpg

I would like to add that since we purchased this vehicle new, other than normal maintenance, with this last expenditure we have now spent a grand total of $550 for repairs in 9+ years.

One last thing... they only way I know to remove those pesky OEM push clamps, especially ones as hard to get at as these, is to cut them in half with my dremmel and a cut-off wheel. Anybody out there have an easier/better way?

Inspect those new O-rings!

Thanks for the reply swshawaii but I wasn't clear... I was looking for some better shade-tree method as I am reluctant to spent bucks on tools I would seldom use. Guess I will stick with the Dremel.
Also, I was sorely disappointed to find my brand spanking new thermostat housing was still leaking. Same place! Now what?
So I removed the top and inspected everything look for some problem. Bingo. That brand new OEM O-ring had a defect! Guess I will pay more attention to new O-rings in the future.
Ford tstat 8.jpg