My truck wasn't ever reaching normal operating temperature, it would barely get to the first line of the "normal" band, figured it was a stuck open t-stat, which is was. I just asked for a t-stat for a 93 351 motor at the auto store, and the guy gave me a 195 degree one. Now I should have checked to verify that's right, but I thought that seemed right for a truck motor, and I couldn't read the number on the old stat, it was all dirty and covered in crap. Installed the 195 degree one, now the temp gauge "bounces". While I was at it I installed a real temperature gauge in place of the factory one.

It rises to about 200 initially, then drops like a rock to 160. Rises back up to like 190, drops to 170. It keeps bouncing between 190 and 170, it never settles out at a constant temp. I thought there might be air bubbles in the system, ran it for a bit with the cap off, and it's been driven several times so it should bleed itself anyway.

Did I get too hot of a thermostat? It seems like the engine heats up, the valve opens and dumps all the hot water into the radiator, but then cools off to much and it shuts again.

What is the right temp for this motor? I did a search, but couldn't find anything for older 150 engines.

 

I used a 192 degree thermostat. Six months now and no problems. But i can't see how 3 degrees would make that much of a difference............ makes me wonder if i even got the right one.

 

Not sure on the 351, but the 302 stocker is a 197 degree unit IIRC. Either way a thermostat within 10 degrees or so either way shouldn't make a big difference.

I would check the rest of your cooling system really well. It's possible that you have a blockage in flow, a slow water pump, or a bad fan clutch. Any of those could cause the thermostat to work overtime compensating, which could have been the reason for the old one failing.

 

I am not sure if you can do this with your motor, but it sounds like the thermostat is installed backwards in the housing. It will cause the fluctuation in temp.

 

When you take the thermostat cover off, the longer side with the spring points in toward the motor, right? That's how the old one was set, and how I put the new one in as well.

I checked the fan clutch, that's fine. I could see a bad water pump, but that would make it tend to run hotter than normal, not cooler. And the up-down in temperature is very rythmic regardless of idling/driving or the temperature outside.

 

Autozone lists a 192 as OEM spec for the 351 in a 94.

Assuming that the gauge and sender are working properly:
Check your coolant(how old is it, is it 50/50, etc) and radiator cap(could not be holding pressure) .

 

most t-stats are marked with the direction of flow, you may want to check it, make sure you get a gasket before you tear it open again. my truck does the same thing, up/down. 192-197 is right, really want 205 as that is the optimal performance temp of the engine .