New radiator thicker than the original? New thermostat stuck open? Backflush the heater core.

 

New tstat was a 180. Thought maybe it was bad, so reinstalled old one which was a 195 degree, no change. Replaced radiator due to a leak that was getting worse.
New radiator has the same dimensions as old one. New radiator is aluminum with plastic ends rather than all metal.
I will try to burp it today by raising the front up and letting it run with the cap off.


Have access to an IR thermometer, so I will try that also.

 

I had a problem a couple of years ago with incorrect temperature readings on my factory gauge. I thought my engine temperature was varying wildly, but an IR thermometer aimed at the thermostat housing showed a consistent 190-195 degree reading. On a lark I replaced the (factory original) temperature sending unit, and now my gauge registers between the O and R at operating temperature.

I would follow all the good advice given up above first, but if you don't find anything actually wrong in the cooling system, at less than $10 this is a cheap thing to try.

 

Update: Got the IR heat gun and tested the thermostat housing. After running engine at high idle the T housing only showed around 135*, while the block tested around 175*. Temp gauge was up to N in the normal. Stuck a piece of cardboard in front of radiator and the temp at the T housing slowly went up to around 195*, block temp went to 213* . Gauge temp went to the M in normal. When I took the cardboard out the temps went back to the starting temps.
As far as heat in the cab, I am getting heat, but just taking longer to heat up due to lower engine temp.
Only thing I can think of is the new radiator is a lot more effective than the old.

 

The thermostat is not working. It is stuck open. Otherwise it would heat up to 195 without the cardboard. And the fact that the cardboard causes the heat to go up proves the 'stat is open - otherwise there would be no coolant flow through the radiator.

 

Parts stores thermostats are junk. Buy a Motorcraft RT1139 192*F thermostat and be done with it. Mine works FAR better than my parts store one did. Twice the price, but worth it.

Did you happen to use one of those "Fail safe" thermostats?

 

It was a thermostat from Carquest. I have tried 2 thermostats, one was new and one was the original. I will try a Ford one and see if that works.

 

I had a Motorad 180* act like a 160* thermostat. You can pick the Motorcraft RT1139 up at most parts stores, probably a 1-2 day delay or you can buy them off Ebay for slightly cheaper.

 

I hate when new parts are bad out of the box.

Imagine how a mechanic feels when he gets to do the job again... for free. (and probably loses a customer)

 

Ya, that would suck. This will be #3 for me. Picked up the Motorcraft thermostat. Will see if it works tomorrow.

 

Test it in hot water on the stove with a thermometer first.

 

Thanks, I'll do that.

 

Tested t stat, tested good. Installed new tstat and no change from before.

 

Gary is right.
The thermostat has to get to temp before it will open, and blocking the radiator to bring temps up proves it's open.

A thermostat is designed to throttle flow and keep coolant leaving the engine at a constant temperature.

If it were extremely bitter cold (like -20*) the thermostat would short cycle for quite a while until temperatures stabilized.
Reducing radiator area in a case like that leads to faster warmups and more stable temps because the delta is so large.

Start your truck cold with the radiator cap off and look inside to determine if coolant is flowing.
Drill a small hole in the thermostat flange (1/8" or less) so it does not trap any air while trying to fill the system.

Have you checked the emissivity setting on your IR thermometer?
What is it set for?

What is the D/S ratio of the unit you are using?
If it is 8:1, be sure you are within 8" and centered on the housing so it measures a 1" circle of the object being read.
Otherwise you are reading the housing, some of the alternator, some of the ground, some of the block, whatever.... and averaging those temps.

Are you pointing it at shiny metal... the black rubber hose... what?
Try applying a piece of masking tape to the metal housing and pointing it at that.
Get up close for a good reading.

Again, heat to the cab comes long before the thermostat opens.
If it is bitter cold your heater core may be shedding enough heat that the thermostat will never open.
I've seen this before in Wyoming, where I learned that -40 was the only temperature where Celsius and Fahrenheit are the same.

 

I'm starting to think there is a problem with the thermostat housing. And it would have to be the housing because it can't be the water pump, hoses, or radiator's fault as they don't decide when coolant flows to the radiator. But if there is something wrong with the t-stat housing, like severe corrosion that keeps the 'stat from sealing then coolant could flow past it at all times.

I'm not familiar with the thermostat installation on a 300 so don't know for sure, but it must go into the block. Maybe there's something there that is preventing the 'stat from sealing? It can't be in backwards or it never would open. But I can't believe there have been 3 bad 'stats.