After having overheating issues in the past I have purchased and installed a new 24 inch aluminum radiator and dual electric fans with shroud. The temperature sensor turns the fans on at 180 degrees and turns them off at 170 degrees. Now she stays nice and cool 95% of the time and the temp guage doesnt climb more than half way and she runs so much better since the radiator and electric fans.
The other 5% is when my temp gauge gets 3/4 the way to the red randomly and sometimes even a little more than 3/4 even in cool weather (all the while i can hear my fans spinning when i listen closely so i know theyre working). But right when im about to pull over and let her cool off the temp gauge finally starts dropping. After that it deosnt run as good for a while and almost dies at red lights because it got hot. It doesnt boil over either.
Before i got the shourd and fans it would boil over as the temp gauge would reach half way to the red and i couldnt fill the radiator more than just barely to the top of the fins on the bottom of the top tank. Now i have it almost filled to the cap without boiling over.
but anyway…
maybe i need a new thermostat? i replaced it when i first dropped my motor in about 3 years ago but havent really drivin it daily until now besides around the block and such maybe once every 2 weeks or so.
I think my thermostat is sticking closed sometimes.
I am going to replace it this weekend either way. but my question is this.
If my fans turn on at 180 degress and turn off at 170 degrees, at what tempurature should my thremostat open?
What tempurature thermostat do i need?
The common choices are 195F, 180F and 160F. Get one as close to the proper operating temperature as you can without going over. If you get one that is too cold, your engine will take longer to warm up or your engine will run too cool which can cause your cylinder walls (among other things) to wear faster. The hotter, the better to a point. 160F is too cold. With a 180F or 195F, you should be fine. For emissions and computer controlled fuel injection, it matters a whole lot more. Those systems require 195F. Personally, I run a 180F. When you install the new thermostat, make sure the air bleed is at the top.
BTW, great choice on upgrading your radiator and fans. Those Central Valley summers are brutal. With my 351W, 20" 3-core and a clutch fan, I always would overheat idling on those +100F days.
Total noob question… With the standard fan, what role does the thermostat play? Is there a good web resource that describes the function of the thermostat in a carbureted engine with standard or electric fans?
I will probably go for an 180 thermostat since thats the temp my fans kick on.
I dont know if I understand that question completely. But I do know that the thermostat opens and closes at certain temperatures allowing the water that has been cooled in the radiator to flow through the motor cooling it. A standard fan is spinning all the time. My fans kick on and off at different temperatures which is why I want to match it to what temperature the thermostat opens to get the best cooling. With a standard fan you shouldnt have to worry about that. Maybe with a clutch fan you will have to get the right clutch. but I dont know. I never had a clutch fan. Hope that helps.
The purpose of any fan is to pull (or push) cool air over the radiator. If water is not flowing through your radiator because the thermostat is closed, you might as well not have a fan. A fixed fan wastes power when it is not needed. This is the benefit of a clutch or thermostat controlled electric fan.
Ok so the function of the thermostat is to open or close the flow of coolant through the radiator? But with electric fans, additionally it’s used to turn the fan on and off?
The electric fans have a completely different temp censor that screws into the top of the manifold that tell them to switch on and off. I could wire them to be on constantly if need be. So ya what controls the electric fans is completely different than the thermostat. which is why i want to match them up. so when the thermostat swaps the water from the motor to the radiator the fans kick on at the same time and starts cooling. All about timing. So that the water in the radiator gets cooled before it goes back into the motor. i dont want uncooled water going back into my motor when the thermostat opens. hope that makes sense.
By the way I did feel the difference in power getting rid of the standard mechanical fan. Might have just been 2 or 3 horse power but the car did feel a little “perkier” when I first swapped them out. haha.
That makes perfect sense. Thanks for explaining guys, very helpful!
You can always check a thermostat by placing it in a pot of hot water on the stove. If you have a cooking thermometer you can watch the stat open and check it against the temp reading. Might save you a couple of dollars if it’s just air in the cooling system causing your overheating.
Steven
I’m replacing my thermostat in the vert tomorrow, I think its sticking or going south cause last time I drove it it poured a bunch of water through the radiator overflow tube. With that being said , what is “When you install the new thermostat, make sure the air bleed is at the top” ? 
On the 289/302/351W, the thermostat mounts horizontally (direction of flow). On some thermostats, there is a little hole or check ball on one side of the mounting flange. The thermostat should be rotated so this is at the top to allow air in the cooling system to escape from the block.
Ok thanks for the reply. I have a 351C and I just checked the new thermostat and there is no little hole.
Thanks for the info on the air escape hole do hicky. I didn’t get around to installing a new thermostat because im THAT broke! and to top it off my gas saver 97 saturn took a dump on me and i really dont think ill be able to ressurect her this time. So right when gas prices go up and im out of money im forced to drive my 67 cougar and I only put 91 at the pump. So even more expensive. counting every dollar until this friday. Then I will have no problems as I will be able to budget my money a little differently and be able to handle the price of gas. But on the up side I get to drive the one car I love most.
Entering my car in a local car show this weekend. going to enter “under construction” with my buddies and their 59 ranchero 53 mainline and 68 falcon. Not trying to win any awards just going to hang out drink beer and look at cars all day. My first car show that ill be entering the cougar in so im pretty excited either way.
Went to famoso march meet drag races over the weekend. Lots of fun. Ended up finding 2 origional vintage hot rod magazines one from 68 and 69. The 68 has a 7 second 68 cougar funny car on the cover and the 69 has the eliminator and talks about the rear differential mainly. There are a lot of other cougar adds in both of them too! I will have to scan them in and post them. Also found a copy of a sketch drawing with a 67. 69. and 70 cougars. will post them soon.
try using a high flow milodon thermostat (avail from summit) when open they work well—also , you should set your cooling system up with an overflow tank—the radiator s/b full to the top when cold—keeping the air out of the system really helps
doctordesoto
I’m pretty sure we’ve talked about this once before on mc.net. But with an overflow tank isn’t the radiator supposed to suck the over flowed coolant back into the radiator as it cools? If so, is it with only certain radiator caps?
Or do you just fill the radiator up to the top and after the over flow tank is full after a week or so you empty it?
Would I need the right overflow tank? or can I throw a milk jug in there or somthing? I’m sure I can find an article online but from what I’ve found in the past is all on modern vehicles and it would be nice to get a first hand opinion for the doc. lol
Thanks…
catsrock: Oh boy, you’re gonna just love this! (And I don’t know how cougarshaman or 1970fastcat didn’t get a “radar blip” when you posted that message)
The 351C uses an entirely different thermostat from almost every other Ford engine! If, when you take out your thermostat, you look down into “the hole”, you should see a flat brass ‘doughnut’ that restricts and redirects water flow until the engine reaches “operating temperature” - it keeps water circulating through the cylinder heads until the thermostat opens. The donut is called a “bypass”. When the thermostat opens, part of it pushes down to plug off the bypass. 289/302/351 Windsor thermostats don’t have provisions for closing off the bypass; you have to get the correct one for 351 Clevelands, 351M’s and 400’s.
Here’s a page that explains the 351C thermostat and the bypass restrictor plate; and even gives pictures to help you see what I’m talking about:
http://351cleveland.wetpaint.com/page/proper+351c+thermostat
There are even manufacturers’ part numbers listed for the thermostat designed for a 351C!
If you just go to http://351cleveland.wetpaint.com , you’ll find out more than you probably ever wanted to know about the 351C and its brethern, the Ford 400 and the 351M. It’s not all pretty and fun; but the Cleveland had some unique qualities that Ford, GM and Mopar didn’t have, and it made the SBC guys freak when they were staring at a 351C-equipped Ford in the “other lane” at the dragstrip.
The engine only lasted from 1970-1974; so a lot of Cougar guys don’t know about the engine, or the “cult following” it has. To quote no less a personage than this Forum’s owner/administrator (speaking in a thread on “that other Forum”), “You mean they still made Cougars after 1968?” 
Back to the “Proper 351C Thermostat” page:
If you can’t find a store that will order the correct thermostat for you; you might get one from Don and Lin (and Mitch and all the other guys) at West Coast Classic Cougars (if they’re not all gone). Look at this page: Thermostat - 192 Degree - 351C - Robert Shaw - Repro ~ 1970 - 1973 Mercury Cougar / 1970 - 1973 Ford Mustang ( 1970 Mercury Cougar, 1971 Mercury Cougar, 1972 Mercury Cougar, 1973 Mercury Cougar, 1970 Ford Mustang, 1971 Ford Mustang, 1972 Ford Mustang, 1973 Ford Mustang ) at West Coast Classic Cougar :: The Definitive 1967 - 1973 Mercury Cougar Parts Source
If even WCCC is out of them, or if the brass restrictor plate is missing; never fear! They have a ‘work-around’. Try this page: Water Restrictor Plate - Modified - 351C - New ~ 1970 - 1973 Mercury Cougar / 1970 - 1973 Ford Mustang ( 1970 Mercury Cougar, 1971 Mercury Cougar, 1972 Mercury Cougar, 1973 Mercury Cougar, 1970 Ford Mustang, 1971 Ford Mustang, 1972 Ford Mustang, 1973 Ford Mustang ) at West Coast Classic Cougar :: The Definitive 1967 - 1973 Mercury Cougar Parts Source
Armed with this info, you could go out and find the thermostat that is designed for a 289/302/351 Windsor and just use it with this new aluminum plate and keep your Cleveland cool(er).
Now you know even more that you probably wanted to know about the 351 Cleveland under your hood; and there’s even more than this. Don’t be scared… sure there’s a lot of stuff that “isn’t like your Father’s Ford engine”; but the payoffs can be pretty good as well. That engine makes your Cougar (and 71-2-3 Mustangs and Australian Fords and DeTomaso Panteras) pretty unique. It’s almost like a cult following… or a disease. And yeah Ive been infected too, ever since I bought a '73 Ford Gran Torino from my oldest brother; a geeky Registered Civil Engineer-type who bought the Torino from the company he worked for… just to have a “spare car” while he rebuilt the engines on his '68 Mustang and '67 Cougar.
Then he started researching this ‘weird 351’ (long before there were pages like I’ve listed here); and just “got into it”.
It’s madness, I tell you, madness! 
Thanks DD, and yes thats a ton of 351C info I still havent digested yet. I did just replace the thermostat ( gotten from local auto store so i’m sure from the pics its not the elusive one) but I did drill 3 1/16 holes around the inlet to let water flow without the thermostat opening at temp. I guess i’ll get back to the books and read up cause i’m still having overflow problems ( see mcnet still pushing water) and for a lucky story see “serious issue” Then again thats why I’m here so I can learn from everyone else that has a ton of experience 
Who knew replacing a thermostat was so involved