Do you have a good source for these?

Right here...I experimented with the thermostats for the 351M/400s since I had so many. I also made a thread in the 351M/400 engine forum regarding these thermostats.

https://www.ford-trucks.com/forums/8...ermostats.html

===================

The thermostat is available at rockauto.com for $5 or so.

Thanks. They are discontinued. I bought 2 from rockauto.

Where did you hear that?

I called two Ford dealers.

It hasn't had a shroud since the family has owned it. Is there anywhere I can get one for under $125?

No shroud is most likely a HUGE part of your problem, you definitely need one, check your local salvage yards for one, they'll probably let you have it for under 20 bucks, i know a lot of the aftermarket shrouds tend to be made of brittle plastic that breaks easily.

High idle temp , low driving temp is normally a sign of too little air flow. I had this problem in the past with an electric fan. I first had 2 10" fans to try to cover more of the radiator. It wasn't cooling at idle, so I got a single 16". Still wasn't cooling. Turned out my alternator was a bit weak and couldn't support the current draw the fan needed. It ran, but not as fast as it was supposed to. Upgraded the charging system and it runs around 195 at idle (180 T-stat).

The difference is, though, that it would run 170-180 driving. Yours not dropping to or below the T-stat temp while driving points to other problems. I think you do have an idle air flow issue, but I think it's only part of the problem. Could be anything from air and/or water flow restrictions to carburetor tuning.

Check your lower radiator hose (squeeze it) to see if it has a coil in it. It's always good to use a lower hose with the spring-looking coil in it. It prevents the lower hose from contracting and blocking flow from the suction of the water pump.

I'd first try addressing the idle air flow issue.

I just bought a 195F thermostat at a local CarQuest. This T-stat is made by Stant.

I brought it home and compared it to the ones Jermafenser posted about in that other link.

From what I can tell, this Stant T-stat is damn near exactly like the Motorcraft RT139. It has the flange at the bottom and what appears to be the extra copper pellet. I'm really not seeing any difference between the two.

CarQuest part #30089

the stant SUPERSTATS are perfectly good t-stats.

The two issues are unrelated. Once you get the overheating problem solved as per the previous suggestions, your motor is going to be running too cool. Your thermostat needs to be in the 190-195 range.

I guarantee you a shroud will help your over heating "situation" at idle, but you also need to put in a 195 degree thermostat, as others have said 160 degree operating temp will kill your engine over time.

1977/79 F100/350 351M/400 Fan Shroud / Thermostat // 1978/79 Bronco
 

D7TZ8146A .. Fan Shroud / Available from Ford.

Fits 351M/400: 1977/79 F100/350 & 1978/79 Bronco / Use with Standard, Xtra Cooling and A/C radiators / NOT used with Super Cooling radiator.

MSRP: $203.98 // FTEpartsguy.com price: $146.87.
---------------------------------------------------
D7TZ8146D .. Fan Shroud / Use with Super Cooling radiator ONLY / Obsolete

Fits 351M/400: 1977/79 F100/350 & 1978/79 Bronco

DON SANDERSON FORD in Glendale AZ has ONE: 623-842-8762.

VALLEY FORD-MERCURY in Oneonta AL has ONE: 800-239-3673.

GREEN SALES CO. in Cincinnati OH has ONE: 800-543-4959.

WESLEY OBSOLETE PARTS in Liberty KY has THREE: 606-787-5293.

Sanderson will be the cheapest.

No other Ford Dealer or obsolete parts vendor has any.
------------------------------------------------------
DOAZ8575A .. 351C/351M/400 Thermostat-192 Degrees (Motorcraft RT139) / Obsolete

GREEN SALES has THREE.

PARTS INTERNATIONAL in Farmers Branch TX has TWELVE: 888-727-0418.

MILLER OBSOLETE PARTS in Binghamton NY has NINE: 800-546-7278.

BOB ALLEN FORD in Overland Park KS has FIVE: 800-676-0675.

LANCASTER FORD-MERCURY in Lancaster WI has SEVEN.
-------------------------------------------------------
D7PZ8575A .. 351C/351M/400 Thermostat-180 Degrees (Motorcraft RT310) Obsolete

GREEN SALES has TWO.

DENNIS CARPENTER OBSOLETE PARTS in Concord NC has THREE: 704-786-8139.

FORD SQUARE of Mt Vernon IL has TWO: 618-242-8600.

GRIESEL MOTOR CO. in Okarche OK has THREE.

NumberD you cease to amaze me!

Again, my suggestion would be pull the electric fans, hook up your oem fan, replace the thermostat, check hoses, flush system, go from there. You can't do much until you do that. That electric fan won't work properly in the setup you have-it's starving for power.