I took my thermostat out of my 305 last summer and left it out. Of course now the truck's heater wont blow hot, but I assume the engine oil is still getting hot. But is it getting hot enough? Am I damaging my motor running no thermo?

 

i would think if radiator is clean,fan clutch is working, and water pump is working, the temp would not get up to normal operating temp in winter. Let to no heat from heater.

You may have a defective gauge, install a direct connection manual gauge and see if it gives the same reading.

Truck coolant system is designed to work with 195 degree thermosat.

just my two cents!!

 

I put a thermostat back in it yesterday, but when the guage gets up to about 195-200, it suddenly withing a about 30 seconds over to 230-240. It did this before with the last thermostat, and thats why I took it out. But now I think the guage is messed up.

 

An easy way to test a thermostat is to cook it!
Get a thermometer with the apropriate range. Inspect your thermostat in room tempreature, ie. closed. The thermostat should be stamped with a nominal temperatur number. This tells when it is designed to open. This numer shuld also corrsepond with what your manual says about running temperature.
Heat up a pot of water to 5° lower than this temperature, check with your thermometer. Drop your thermostat in, it shall not open. Raise the temperature and observe on the thermometer at what temperature the thermostat open. If it opens within +/- 2° of the nominal temperature it is OK. If it opens within +/- 5° it can be accepted for trial use. If it is more off from the mark, it is junk.

If you have checked your thermostat this way, you can deduce if the gauge is OK.

Running the engine for longer periods without thermostat will probably give bad deposits in the cylinder, and as a consequence, poor performance.

 

well, guilty as charged, i have run several engines without thrmostat before. nothing BAD happened. beside obvious "my feet freeze" winter time, you are running cold engine. it'll just underperform, that's about it. your mpg will suffer. most likely, you'll clogg your emissions system due to unburnt fuel.
we used to run engines without t-stats back in russia all the time. parts market was terrible. we would just stick a cardboard in front of the radiator, to have some of it covered to raise temp up. did same here on mitsu expo lrv without any major problem. then again, we did not have any fancy emission hardware either. no cats, nothing - straight pipe and 10 roubles to bribe a cop every time you had to take emissions test.[&o]

 

Ok, lets look at this at if we have the right parts. more than likely you got a guage or a sensor problem, or even sensor wire problems. inspect all the wiring, change the sensor, if it still dont work go down and get a guage kit, it turns it into a manual guage instead of an electric gauge. there is an ohm reading for the sensor but i cant remember the exact reading it needs to be. Anyway as far as the no t-stat thing, no major damge can occur in the little time you ran it without one. but there is a reason for them. they allow the motor to warm up. they allow the heater to work, without them the motor never gets to operating temp and this will hurt things like fuel ecenomy, it can over the course of time cause engine damage,yes i have seen it, both 305 and 350 and one 396 big block. i would flush the system as well.