Overheating. What to check before buying parts?

I agree with first checking and then replacing if needed the engine temperature sensor. The temperature guage should read only half way. A non working fan should be checked too if it never, ever comes on. Check thermostat too.

For how little thermostat/gasket, coolant temp sender AND sensor (yes theres 2) are going to cost you I say its a good move regardless. One sensor controls the gauge and the other communicates with the computer, if the one for the computer is giving bad readings it can cause drivability and starting issues but doubfully what you're experiencing. My bet is on the water pump as you said its boiling and possibly not circulating the coolant (worn internally) so you can't rule out the radiator either, clogged internally and coolant can't pass through fast enough to cool. If clogged externally or has alot of bent fins the air cannot pass through the cores to cool the fluid. If you're able to do the work yourself you should be able to get the 2 sensors, thermostat and gasket, water pump with gasket and all new coolant for around $100, all thats left if you're still having a problem is the radiator which will probably run you a little over $100 itself.

I took off the thermostat and tested it. Works fine. Shouldn't the gauge register that it's overheating and send the message to the computer? then the computer would send the message to turn on the fans(?) It would be the same with the Temperature Sensor sending the message--which sensor would tell the computer to turn on the fans? [Or does it work that way] If the gauge isn't sending the message the system wont work right(?) I don't see a problem with the water pump, belt, or pulleys. I flushed the system about 2 yrs ago. Not having any starting issues, gas mileage getting [slightly]worse. Did a tune-up/oil change last September. Should I replace BOTH sensors?

I'd like to be able to pinpoint this down without having to do a lot of "testing". I have an ohmmeter but that stuff frustrates me and I cuss a lot. That expansion tank and cap are cheap plastic and I don't think the cap stays on tight like it should. When plastic gets hot it softens up or even melts. Why PLASTIC for something that gets hot?

Testing saves you from buying parts that are not needed. The overflow tank is high temp plastic but it should not be getting that hot to melt. I think it is bubbling but not really hot, hot boiling. That loose cap is failing to keep the system from reaching normal pressure (16 PSI) thus allow the system to boil at 210F instead of 260F with 50/50 antifreeze mixture. I think you are on the right track by suspecting the loose cap problem. Sorry about all the tech talk but I'm an engineer and that's what cars are about. Maybe you can get a friendly mechanic to offer an opinion to get you on track. I really hope you do okay with this.

Thanks. I can understand some of the tech talk. I know the system is supposed to be under pressure and that's why I was wondering about that cap. If I screw it down to a certain point it just "pops" back up, as if the threads aren't holding it.I've replaced the cap once already. I wish it had the old metal cap system with the release lever. So what about the fan? Shouldn't the temp gauge go past the half-way mark? that's only how far it goes and I know it's hot. Both that sensor and the ECT are supposed to work in conjunction with the computer, right?

[you previously stated that the gauge should only go to the halfway mark so that's confusing] I guess I should mention that it got hot and anti-freeze blew out from under the cap all over the belt and pulleys. It caused my belt to slip off! So I had to put it back on. At that time I checked the belt and pulleys and they all seemed ok. The water pump doesn't seem to be leaking--nothing coming out externally, that is .I lost almost a gallon of anti-freeze. The fan isn't coming on!

Regarding the temperature guage. Yes, it goes about midway and that is the normal temperature range in the middle. Further to the right is too hot. Now about the fan. Yes, at some point it has to come on to have air flow through the radiator. If it never comes on, it's probably burned out or a temp sensor is failing to turn it on. That's going to allow it to overheat. As a test, the fan should always come on with the A/C running. I think the problem is in the fan not coming on.

I agree with the low pressure causing an issue, replace the expansion tank, it's not very costly and replacing the cap hasn't helped. Replace the 2 coolant sensors (even though only one is for the ecu and fan) they're cheap. I've driven around for weeks without my fan working and never once had my coolant boil over, presumably because my water pump is working properly. You can't tell of the blades of the water pump are toast by looking at the outside of it, there is indeed a week hole that will leak coolant of the SEAL is worn and that's all it will tell you. Testing things is the only way aside from blindly replacing parts which i dont necessarily agree with but sometimes it's the only way, take a methodical approach to it (like starting with the expansion tank) and you're doing it the right way. If there's a issue that you know about and avoid rectifying it you'll get nowhere fast

Also please read what people that are trying to help write, I explained that one sensor controls the gauge and the other is for the way and in a response post from you you ask why the gauge didnt trigger the computer to turn the fan on....

For the ecu not way****

brianator: I got it now. The gauge is just to let the driver know whats going on. I worked for 2 days now for what most mechanics could do in an hour or two, but I wanted to make sure I did the right thing. And share my experience. The guy at the parts store sold me the wrong thermostat, but turns out my thermostat was ok anyway. I have a roll of gasket material and sealer which saved me some frustration and time. I don't like being sold the wrong part. This store has me and my car in their computer and should know better. Anyway, I went through everything, checking and looking. I was freaking out thinking my head gasket was blown or my heads cracked. Nope. The coolant is fine and the only place it came out of was from the expansion tank--because it was so hot. So I figured I would try the ECT and that was a pain to get to. I was used to working on my old DeSoto and now these cars have too much stuff on top of everything. I was standing there doing fine when my neighbor came over. He tried to "help" He ended up putting the ECT sensor in the wrong place and jamming the plastic connector on it. Of course each connector is differ so it was stuck on tight! He ended up yanking the wire completely out of the connector, so now I have to find a new one to wire back on. I tried to fix it but no go, so now my gauge doesn't work. Anyway I put the ECT in and let the car warm up. The fan came on!!! Now I need a vacation. First I have to deal with the guy at the parts store and then with my neighbor. He seems to think I owe him for helping me. He thinks I weird being a woman working on my car but I didn't see any guys offer to help. All the info you guys gave me was very helpful, I like to sort things out and the more opinions the better. I keep reading that when a car overheats, the damage is already done.

That sucks the connector is wrecked but if you have a auto recyclers near you, grab a connector off a car and solder/heatshrink it and you're good to go. I'm glad the sensor has fixed your fan issue but I still think you should replace the expansion tank also seeing as how the cap wont hold pressure, low pressure lowers the boiling point of the coolant as Hornet said in a previous post

Oh hey and btw dont worry about people, I've met plenty of women that are better mechanics than alot of fellas, something about yall being able to rationalize i reckon. Lol

To replace that little connector is going to cost me $42.00 new! So right now the sender [gauge] isn't working and I'm showing a code. Plus, my air conditioner is blowing hot. I'm not sure if its cause the computer needs to sorts things out or not, but what the heck? I was thinking of telling my neighbor he owes me $42 but not going to.(strategy) I don't like the guys at the junk yard but will go there anyway. I got into a sort of argument at the parts store. Each one of those sensors (ECT and gauge sender) look a lot alike but the connecter part for each one has a slightly different pattern. Plus they are two different colors. He tried to sell me the connector for the ECT! This was at O Reileys. Oh well, I don't need having my neighbor mad at me so I'll just let that go. Why would the air conditioner run hot? NOW what?

PS If you ask me, those things called "sensors" are nothing but fancy fuses. At about $30 on up.

My air conditioner works now and so does my temperature gauge. The cap on the expansion tank is messed up. Ford/Mercury should know better.

Nice! Glad you're getting it all sorted out =)

I have a 2003 Mercury Sable Ls Premium 3.0 V6 DOHC. I bought it in May and it had 129k and i put 5k miles on it since. I was on my way to the doc one day (20 min drive) and was in the office only 15 minutes and came out to my car not wanting to start (I always shut the heat/radio/lights and even unplug my car charger) well at least 9 ppl tried jumping me and that didn't work, so my bf got out of work, bought a new battery from Ford and put it in and it wouldn't click or crank, so we got it towed home and a friend came and looked at it and it was the solenoid. I ended up buying the whole starter and then we put it in and the anti theft came on, so we got it towed to the ford dealer bc they said to bring it with both sets of keys and they would get the anti theft off. well they called and said the starter was bad, so we took it off returned it and bought a new one that we had tested from a different autoparts, but still had the anti theft. The next morning the service guy from Ford called and said it has nothing to do with the anti theft and said i had a bad ground and wanted another diagnostic fee to bring it in and fix the ground. Well, everyone I talked to said to just bring it home if its just a bad ground. So I had it towed home and the wires were swapped from the battery to the starter, so once we switched those we thought we were good to go, so we put a slow charge for about 10 hours on the battery and put it in the next morning and the car started and shut off and my bf went to start it again and it started so he hit the gas and it shut off again, the third time he tried starting it he heard the grinding noise so he stopped trying to start it. we then took the starter off and looked at the fly wheel and that looked good, and we started the starter and the starter is good, so we put everything back together and we had no click or crank. well I was on youtube and i seen someone say something about the inertia fuel shut off switch in the trunk, and when i bought the car, the guy said I could only open the trunk by the key or the key fob because he messed with some electrical and the button on the dash or the emergency buton in the trunk won't work, and I didnt ask what he did via electrical in the trunk(i know stupid me), but the wires that come from the shut off switch I can tell were messed with due to all of the electical tape, and I unplugged it and plugged it back in and wiggled the wires a bit and tried to start the car just to see, and now I have click and it is trying soooo hard to crank over, and I heard the cranking sound 3x, then decided to quit and put a charge on the battery, but haven't put it back in yet because my dad is dead set on the motor being siezed, but if it were seized, would I hear the slightest crank noise at all? Also ANY input on what it could be is greatly appreciated because I've spent so much money(a grand) on tows and parts and only have a battery, starter, and Ford clearing my anti theft off.and about 10 hours of you tube videos and i'm no mechanic, but I'm willing to try anything because somewhere deep inside I do not think that the motor is seized.