Unusual Coolant Leak

[JBsable99]

Hello. So here's the back story--

I was sitting in my car before work last week (just catching a few more comments from the local sports radio station) when I noticed that I had steam/smoke coming from under the hood. It wasn't a whole lot but it persisted so I popped the hood and discovered that I had a coolant leak. A quick look revealed that coolant was leaking out from the gasket where the thermostat housing meets the intake manifold. After I got home from work that day I checked on how tight the bolts were and they were pretty snug. I snugged them up a bit more just to slow the leak until I could completely replace the gasket. Everything seemed to be working just fine until I was sitting at the airport waiting for a friend to arrive. While I was waiting, with my car parked and running, coolant suddenly began spewing out of the expansion tank cap!! It was not slow leak either. I lost almost a gallon of coolant in less than 1 minute. I let the car cool for a couple minutes, refilled the expansion tank, capped it, and thankfully got it back home. A quick check at home revealed that the leak at the thermostat housing had worsened after the airport incident. The next day I changed the thermostat gasket and the expansion tank cap. Everything was fine until...

...about 15 minutes ago. I've been periodically checking my work and tonight I saw that coolant has started to slowly leak from the same spot. I just changed the gasket last Saturday. I can change it again, but that does not seem to be the root cause. I know that the housing bolts are tight. Could it be that my thermostat is failing the stay open after it has opened? (maybe that could explain the fountain of coolant at the airport--a sudden pressure build up from the thermostat closing)

Anyway, does anyone have any suggestions? Has anyone experienced this before? A flush is due, perhaps I have a blockage elsewhere? Failing water pump?

Thanks!

[rickpark]

Have you changed the thermostat recently? Is it oriented correctly (not installed backward?)

[JBsable99]

Originally Posted by rickpark

Have you changed the thermostat recently? Is it oriented correctly (not installed backward?)

Yep. If it was backwards I would have some major problems keeping the engine cool.

To stir the thinking pot:
I topped off the expansion tank this morn to the high cold fill line. When I got to work I checked the level and it had actually risen over the 20 minute drive. Coolant was still dripping from the thermostat housing as a slow leak. The rising level has me wondering if the system is developing an air lock that is pushing coolant out of the system. An air lock may explain why I had coolant bubbling out of the expansion tank at the airport. Perhaps my water pump is failing and the coolant flow rate is not suitable for pushing air out of the system.

[rickpark]

Your problem with boilover is possibly due to the fact that you do not have full pressurization of the cooling system with that leak. As long as coolant is circulating through the radiator at a normal flow rate while the engine is running, it will stay below the boiling point. Once you shut off a running engine or lower the circulation flow rate, the coolant temp will rise sharply since there is no longer sufficient heat removal at the radiator and the engine block is heating the coolant within the block.

This would cause the coolant to flash to steam which then will force coolant from the block into your overflow tank. Especially if your coolant system cannot hold the pressure it was originally designed to hold. Often, the leak is at the pressure cap or from a crack in the tank.

You need to take care of that leak and a coolant change and/or flush wouldn't hurt.

[behlinla]

Originally Posted by JBsable99

To stir the thinking pot:
I topped off the expansion tank this morn to the high cold fill line. When I got to work I checked the level and it had actually risen over the 20 minute drive. Coolant was still dripping from the thermostat housing as a slow leak. The rising level has me wondering if the system is developing an air lock that is pushing coolant out of the system. An air lock may explain why I had coolant bubbling out of the expansion tank at the airport. Perhaps my water pump is failing and the coolant flow rate is not suitable for pushing air out of the system.

As the coolant warms up, it expands. That's why you have cold and hot fill marks on the reservoir. On a normal car with a completely sealed system, this expansion will pressurize the system slightly (against the air trapped in the reservoir) and will raise the boiling point of the coolant. If the engine overheats, the coolant will start boiling, and the additional pressure from the vapor expanding will eventually blow the tank cap before a hose pops off. If the system has a leak, it will not pressurize properly and you will loose the boiling point elevation afforded by the pressurization, therefore the coolant will boil sooner and the engine will be more susceptible to overheating (steam doesn't transfer as much heat as liquid). A good analogy is a nuclear reactor, because if the core loop loses pressure all the water will flash to steam and the fuel rods will overheat, creating a steam explosion and nuclear meltdown. So pressurization is very important.

What I would do is buy a new Motorcraft thermostat, bolts (before they break on you), and a Motorcraft gasket. Clean and check the mounting surfaces for flatness with a straightedge. The gasket surfaces must be almost shiny to get a good seal. When you tighten the bolts, use a torque wrench because overtightening can cause the flange to warp and leak. I think this may have happened already. Why does everyone think they have to tighten bolts when something leaks? The proper procedure is to loosen and THEN re-torque to prevent overtightening.

[sfhess]

Use a decent gasket cement (I recommend Gasgacinch) on the thermostat gasket. NOT silicone/RTV.......

[JBsable99]

Thanks for the feedback everyone. I will replace the stat and gasket with Motorcraft parts. Would any of you replace the expansion tank too? It isn't very old, maybe 1 year. I had to replace it last winter because it cracked open.

About the gasket cement, what is the purpose? To create a seal or keep the gasket stuck to one of the parts during assembly (or both)? Gaskets at O'reilly and Autozone have one side covered with adhesive.

I will hopefully be able to do it this weekend and be able to give feedback. I am still trying to wrap my head around why the leak started in the first place. I would have expected the initial leak to have developed 1 or 2 weeks after I replaced the stat back in October, not 3 months (that's not to say that it can't happen though). I keep going back to rickpark's comment on the flow rate and how that affects pressure, so I think I'll examine the water pump too. At least the temp gauge shows normal temp.

[behlinla]

Originally Posted by JBsable99

About the gasket cement, what is the purpose? To create a seal or keep the gasket stuck to one of the parts during assembly (or both)? Gaskets at O'reilly and Autozone have one side covered with adhesive.

Gasket cement is used to hold the gasket in place until you put it together. It's not a substitute for a gasket. If the thermostat comes with a sticky side, then you can just stick it on one of the parts and press/smooth firmly to remove any air bubbles.

I agree coolant leaks can be a sign of reduced flow because of a bad water pump or clogged coolant passage or hose.

[Automender12345]

Never use a gasket on my 5 Taurus thermostat housing. I used the gray RTV it is designed for these housings. Thats what Ford used on the 2001 that I bought two years ago when under used car warranty, the heater core and timing cover needed replaced.

[JBsable99]

Originally Posted by behlinla

I agree coolant leaks can be a sign of reduced flow because of a bad water pump or clogged coolant passage or hose.

Thanks for the tip on the cement. I had a hunch it was for that purpose.

Additional details: After about 30 minutes of driving I stopped the car and looked around in the engine compartment. I could hear a hiss coming from the water pump area. There is also quite a bit of residue on the engine side of where the water pump meets the engine block. The residue doesn't really have a scent that I can pick up but it looks fairly fresh. Sounds and looks like my water pump is due.