Ford Mustang Running Horse

Replacing Thermostat Housing on 2005-2010 V6 Mustang

Make it out of plastic, they said. Make it two pieces of plastic bonded together, they said. Bolt it to a cylinder head and run hot coolant through it, they said. It'll be fine, they said. Nope. Not fine. Stupid idea. Looking right at you, Mr. Ford Engineer Guy!

If you have a 05-10 V6 Mustang, you've either replaced a leaking thermostat housing, or you will. In my car's case, it's both. I had the water pump replaced under warranty in 2011, and it turned out the thermostat housing was leaking, and was replaced at the same time. Once again, it's leaking. The housing is two parts: The lower housing that attaches to the cylinder head, and the neck that holds the thermostat in the lower housing. Both parts are made of a fiber reinforced plastic. The problem is the lower housing being molded in two parts, then bonded together. This bonded seam is what fails, allowing the hot coolant to be pushed out. Your options were limited to the OEM Motorcraft parts (that fail), or aftermarket parts, also made of plastic, with varying degrees of quality (which will fail). Average price for the Motorcraft upper and lower come in a little over $90 at, but dealership prices can vary. Aftermarket parts run in the $50 range, but quality varies. Recently, Simmons Autosportz stepped up with a better solution: make it out of aluminum (yes, aluminum, not aluminium, because America!). This part is considerably more expensive, coming in at $175.99, plus shipping. Yes, that's a big jump in price, but it's not plastic, and the lower is cast as one piece with no seams to fail. And since this is a job that can be done with simple hand tools, the savings on labor if you do it yourself will almost cover the additional cost of the part. I also purchased a new Ford-Motorcraft thermostat (part# 2L2Z8575AA), a gallon of Motorcraft gold coolant concentrate (Part# VC7B), and a gallon of distilled water. No harm in doing some preventative maintenance while I'm at it.

First up, locate and remove the radiator drain plug. It's on the passenger side at the bottom, and can be loosened with a 3/4" or 19mm wrench. Then remove the radiator cap and let it drain into a catch pan. You're going to get around two gallons out of it.

Once it's stopped, replace the plug hand tight, and tighten slightly with the wrench. It's plastic, and has an O-ring, so no need to go all Chuck Norris tightening it. Up top, you'll need to remove the air inlet tube. Loosen the two clamps, and remove the PCV hose.

With that out of the way, you can see the offending plastic spawn of Mr. Ford Engineer Guy, but it's not coming out of that small space.

To get better access, remove the throttle body. It's held on with four bolts, and requires a 5/16" socket and extension bar. Don't forget to unplug the throttle motor and throttle position sensor.

Next, remove the small heater hose and upper radiator hose from the housing. Also unplug the coolant temp sensor at the back of the housing. Remove the passenger side PCV hose for better access to the bolts. Using a 5/16 socket, ratchet, extension bar, and swivel adapter (if you have them, swivel sockets would be even better. I don't have them. Yet.), loosen the three bolts holding the housing to the cylinder head. Use a pair of channel-lock pliers to loosen the hose clamp, and remove the housing.

And here it is. Note the seam running around the perimeter of the lower housing. This is where the two molded parts are bonded together, and this is where the failure occurs. Not cool, Mr. Ford Engineer Guy, not cool.

You will be reusing some parts. The coolant temp sensor, and the three bolts securing the water neck. If you are not going to replace the thermostat, you'll need to pull that out as well, careful not to damage it. The temp sensor is held in with the small metal clip, you will need this to retain the sensor in the new housing.

Lets take a look at the Simmons Autosportz kit. It comes with the upper and lower housing, a new Fel-Pro lower housing ring seal, new Duralast/Fel-Pro thermostat O-ring, new temp sensor O-ring, and two paper gaskets. More on the gaskets later. You can get this kit here:

Compared to the OE/Motorcraft housing:

So yes, it's a lot more expensive, but with no plastic to warp or bonded seams to fail, It sure looks like that last one I'll ever have to buy. Lets get back to it. You'll need to make sure the surface of the cylinder head and the base of the housing are clean. A bit of brake cleaner on a clean shop rag will do the job.

Back to the paper gaskets. I did use the larger paper gasket under the thermostat neck, along with the new O-ring for the thermostat. The instruction sheet calls for the smaller paper gasket to be used along with the new rubber ring seal, but the idea of stacking dissimilar gaskets just doesn't seem right. Apparently, some people had installed the housing with RTV gasket maker, and had issues with leaks, so SA added the paper gasket to address that. I skipped all that, and only used the rubber ring seal, just like Ford did.

I assembled the thermostat, upper water neck, and coolant sensor, and the rubber ring seal on the bench, and dropped it in as one unit. I tightened those bolts to 65 inch/pounds. The back bolt for the assembly won't slide past the fuel rail crossover tube, so I put it in the housing before putting it in the car. Use the pliers to open the hose clamp, slide the assembly into the hose and position it so you feel the three tabs on the bottom of the housing fit into the opening on the cylinder head. Once in place, thread the bolts in by hand, then tighten to 95 inch/pounds. Be sure to use a small amount of anti-seize compound on the threads, since they are dissimilar metals.

Go ahead and connect the coolant temp sensor, heater hose, passenger side PCV and upper radiator hose.

Reinstall the throttle body (yes, I cleaned the oil sludge out of it), air inlet tube and driver side PCV hose. Don't forget the throttle motor and throttle position sensor plugs.

Ford says the coolant is good for 100,000 miles, but after 9 years and 79,000 miles, it was definitely time to change it. This is what was left behind:

After rinsing the sludge out and reinstalling the tank, I mixed the one gallon of coolant concentrate with one gallon of distilled water, and refilled the system. It took the whole two gallons to fill it. After that, start the car, check for leaks, and let idle until the temp comes up to normal.

Turn the HVAC to full warm and to vent, with the AC off, and make sure you get hot air from the vents. Keep a check on the coolant level, and check the upper radiator hose. It will get very warm once the thermostat opens up and allows the hot coolant to flow out of the engine. the coolant level will drop a little as well. Keep checking for leaks, then go for a ride and keep a close watch on the temp gauge. Once you're back home, shut it off, look for leaks, and allow to fully cool down. Check the coolant level and add any if needed. Thant's it.

Oh yeah, about that paper gasket/RTV/rubber ring seal thing. I said that I used only the rubber ring seal, same as Ford did. I have no leaks at all. I'll certainly keep an eye on it for the next few days, but I think it'll be fine. Chalk one up for Mr. Ford Engineer Guy. I'm not a mechanic, but I'm guessing that the people who used RTV on their install, used it incorrectly, and that caused their leak issues.

Brian Fowler - Vice President
Brian Fowler
Vice President