My 78' F250 6.6l seems to have a thermostat that is stuck open. The temp never gets much into the Normal range and the heater is not very warm. There is no coolant leaking. Other than a bad thermostat, are there other things that would cause this? Does it matter which one I get-brand, 180, 190f, Etc? Thanks.

 

You need to diagnose this problem with a mechanical gauge. There is no way to reliably correlate actual temperature with which letter in "NORMAL" the needle is pointing to. For all you know your motor could be running at 220 with the heater core blocked. I'm not saying that's the case; I'm saying you don't know until you get an actual number. If you install a mechanical gauge and find that you really are running cool, then that's fine - but at least this way you'd know for sure you'd need to start tearing into the cooling system.

There are two reasons I always say what I've said when someone asks what you've asked. (1) There is no official specification on needle swing vs. actual temperature, and (2) with a 30 year old rheostat, it's anybody's guess as to what it's actually telling you. The factory gauge is not meant to tell you what your temperature is; it's meant as a "go, no-go" deal to tell you if you need to pull over. In my opinion it's about as useful as a warning light.

 

If you don't get any heat from the heater and the engine seems to be running cool it's probably your therm. I recommend the Robert/Shaw type offered by Mr Gasket and others.

A 194 degree is stock, I run a 185 in summer.

 

Makes perfect sense. Thanks for the info.

 

The 351C, 351M and 400 use a unique thermostat that was not used in any other engines.

Make sure you get the correct thermostat.

Ford offered two heat ranges: 180 & 192 degrees. The 192 degree stat was installed at the factory. The 180 was available at the parts counter.

Motorcraft part numbers: 180 = RT310 / 192 = RT139.

Autoparts stores can cross the Motorcraft part numbers over to the brands they sell.

You can test the stat to see if it's any good. Boil some water on the stove, drop that puppy in...if it's any good it will open up.
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btw: Does this truck have factory installed A/C?

If so...you have a heater water valve that is vacuum operated.

When you move the lever on the control panel to heat, the valve (is supposed) to open.

This valve is a POS and is notorious for getting stuck in the closed, open or partially open position.

If the heater doesn't work properly and you have factory A/C, the valve is most likely the problem.

D4AZ18495A .. Heater Water Valve (Motorcraft YG136)

Stll available from Ford, and why not.....it's been a best seller for over 30 years.

MSRP: $28.72 // FTEpartsguy.com price: $17.23.

 

No factory air. Thanks for the info NumberDummy.

 

If I'm not mistaken, the easy way to tell if your thermostat is stuck open is to take the rad cap off, crank it up cold, and look down the filler neck. If the therm is stuck open, you should be able to see the coolant flowing immediately.

It is also possible that you have a lot of scale in your heater core that is keeping coolant from flowing through it or is restricting the transfer of heat to the fins.

But as I said, I could be wrong.

 

X2 on the Robertshaw type thermostat. Look for a balanced type thermostat - they look like the one below. They open and close more accurately and are more reliable. And never go below a 180 deg thermostat - as an engine the runs too cool drmatically decreases engine life.



Flow Kooler 333-180 - Flow Kooler Robertshaw Series High Performance Thermostats - summitracing.com

 

I have to agree here, I always relied on, or considered factory guages as a high brow warning light.

 

Ok, I installed a new thermostat last weekend. The old one was not stuck open but had a lot of crud/deposits on it. I drove the truck a while and the temperature of the coolant at the radiator was around 180f. The gauge showed the needle just left of the N in Normal. Can someone tell me where the sending unit is on the 400 motor? I couldn't find it, thanks.

 

look around were the heater core hoses attach at the block

 

Found it. Gauge pegs hot when grounded so that works.

Any other ideas?