Erratic Temperature readings

Just installed a reman 350 5.7L in my truck, I am getting erratic temperature readings anywhere between 170- 230. Makes me nervous with a new engine. I'm guessing the temp sensor(it is the one from the old engine) Is there any other possibilities or could it be that simple just to change the sensor?
Thanks

you did not say what year this truck

more info,,, check for air pockets,and wire connections first

things to check........thermostat, air pockets(unlikley), damaged wires leading to the sensor,'s. i would start with replacing the sensor.....not knowing the year of the truck u may have 2 of them. if its old it has one......normally in the head. if its a later model it has 2 one in the head and one in the intake somewhere (this one is in the fuel injected models, it sends info to the computer to help control fuel demand)

Truck is a 1989 4WD. It does have the two sensors, one on the side of the block and the other by the gooseneck on the intake manifold. Looking at the sensor on the intake the plastic is broken where the plug goes in and the plug is not in good shape either. I'll change both sensors and the plug and see if that helps.
Thanks

Installed new sensors and thermostat(Thermostat is a 190Deg). Going by the temp gauge the thermostat doesn't open until it hits about 250deg, then drops and holds steady at 215deg. I took a thermometer and did a check in the radiator and it was reading 170deg. This tells me that the gauge in the truck is off by 45deg.
Not sure whats going on, was thinking I could hook up a mechanical gauge to the block sensor and get a accurate reading. Never heard of the gauges being off like that. (electronic type)
any ideas?

Installed new sensors and thermostat(Thermostat is a 190Deg). Going by the temp gauge the thermostat doesn't open until it hits about 250deg, then drops and holds steady at 215deg. I took a thermometer and did a check in the radiator and it was reading 170deg. This tells me that the gauge in the truck is off by 45deg.
Not sure whats going on, was thinking I could hook up a mechanical gauge to the block sensor and get a accurate reading. Never heard of the gauges being off like that. (electronic type)
any ideas?

the 2 wire intake mounted sensor is the ECT/CTS - it feeds the ECM. hook up to a scan tool and then you can see what it is thinking if you select data stream output. the 1 wire in the side of a head or block is the gauge - they tend to corrode and they jump all over on those junk chevy gauges.

First use a scan tool to determine actual temp, if you feel it is off, grade it with a temp sensor - one of the IR type

if this is ok, then go after fixing the other one.

If you are using a seal that goes around the thermosat instead of a gasket for the housing outlet, that will stop the seepage which is supposed to happen so that the thermostat recognizes newer hot coolant instead of the colder pocket of coolant. I use the seal but I drill a 1/8" hole to help it act as if the seal wasn't there and I HAVE NO PROBLEMS NOW> I may try 2 holes a I think it is still too slow.

Installed new sensors and thermostat(Thermostat is a 190Deg). Going by the temp gauge the thermostat doesn't open until it hits about 250deg, then drops and holds steady at 215deg. I took a thermometer and did a check in the radiator and it was reading 170deg. This tells me that the gauge in the truck is off by 45deg.
Not sure whats going on, was thinking I could hook up a mechanical gauge to the block sensor and get a accurate reading. Never heard of the gauges being off like that. (electronic type)
any ideas?

try changing the sending unit first. Its located on the drivers side head, don'st cost very much and is pretty easy to do. If the guage still fluctuates, then something has happened to teh guage and the only way to fix it is to send the entire cluster off. You could buy a manual temp. guage and then you would have a true reading. My truck did the same thing about 2 years ago. The temp. guage started going to about 250 or so and then back to normal and then finally just pegged out. Changed the sending unit and it started fluctating from normal to about 240, changed thermostat and water pump before we found it was the guage. Hooked a manual temp. guage up and now the truck stays right on 190 degrees.

I have found that even with manual gauges, I run the probe into the intake manifold on older trucks, just to the left side of the t-stat housing. the temp on the manual gauge, when using a 185 stat, will climb to about 230 before the stat opens and allows coolant to circulate. at first this caused some concern, but after using auto meter, stewart warner, and various other brands of gauge, just to compare, they all do the same thing.

I have found that even with manual gauges, I run the probe into the intake manifold on older trucks, just to the left side of the t-stat housing. the temp on the manual gauge, when using a 185 stat, will climb to about 230 before the stat opens and allows coolant to circulate. at first this caused some concern, but after using auto meter, stewart warner, and various other brands of gauge, just to compare, they all do the same thing.

Well, that sounds like somethign else is wrong then in your case. Mine never makes it above 190 or close to it before the stat opens. And the only time it gets above the 190 mark is when its hot out, got the a/c on and pulling a hill, adn then it just goes to 210.

Cold pocket of coolant can't reach the thermostat if seepage isn't allowed to go past the thermostat. Is your thermostat floating in the outlet housing or sealed at the thermostat flange by a rubber seal? Remove it and just rely on the housing gasket or rtv to allow the needed seepage. Let us know.

My truck heats up to 210 before the stat opens and then it runs around 180-190