Ive just put a newly rebuilt engine and new water pump as well as a new thermostat. But, still overheating?

I don't know if you put the engine in yourself but some rebuilt are not always good and if you had a shop put it in they should be able to get another one from the place they bought it if it has some kind of warranty for a short period of time. If you put it in yourself, check your paper work and the place you got it.

I am assuming you put all the fluids back in.

Have U checked your hoses, make sure they are not collapsed?

The radiator are not lasting as long, so U might check into this. Make sure there are no leaks. Your Temp. Gauge may not be working properly check this first, sometimes they can be bad even new. Check your water pump. Hose collapse slipping belt

Bad thermistate clogged catalitic converter

Cooling system leaks

Leaky head gasket

Fan not working

water pump not working

Not sure what else to say with out a look.

Just food for thought as we don't know kind of car/truck it is, or if the engine was newer or older.

GM 350's made from 68-86 had water pumps that turned clockwise, while 87-95 had water pumps that turned counter clockwise (aka reverse flow). The water pumps will interchange between these groups, but will overheat.

Maybe this could be the case. Hit us back with more details if you could. Make Model of car/truck, What make model did the new engine come from? Is the new engine hopped up with fancy speed parts? Is the radiator a single core, dual core or quad core? Electric fan or mechanical?

Does the radiator have signs of internal corrosion or deposits? Is the heater core passing coolant like it should be? How did you get the air out of the system?

yes well say good buy to the old radiator you found your problem. Radiators now are pretty cheap, the high performance ones from aluminum work great, look on the internet, you can get them wholesale, for less than 200, for a 3 core, or if you have a big block or a built 350 do the four core if you can. Do some research on the internet, there is one big company that has five shipping points in the USA one of them is in Reno Nevada and the radiator is shipped from there. I do not remember the name its been too long ago, but likely you can find out from your mechanics maybe. put a new cap on the radiator too.

Head gaskets are put in incorrect each and every of the time. even if it rather is a cracked block the place the crack only opens up whilst heat. only leave the thermostat out till you stumble on the venture. between the advantages of it rather is you will have the means to work out the water circulating. electric water pumps have been between the dumbest suggestions ever and not could have been on engines. Get 2 temperature strips made for the point and positioned them on the two radiator hoses. that provide help to comprehend if the radiator is doing it rather is activity. and don't use money as an excuse to swap that aluminum piece of crap with a good copper radiator. it rather is going to fee you lots extra money if it rather is not doing it rather is activity. positioned an exhaust gasoline analyzer on the radiator. you do not constantly see effervescent acceptable off whilst head gaskets leak.

Like Peter said, make sure all the air in your cooling system is out.You can do this by letting your car heat up and SLOWLY crack open the cap letting air out. You might wanna look up ways on your specific vehicle. If there is an air hole in your radiator or hoses, air will get in your cooling system. If this does not work make sure your radiator is not clogged and your fan is working.

bugs guts in the middle of A/C and radiator power wash in street split it open there sweep up good for fishing don't get caught.

So I have a thumbs down player -- in front of the radiator is a AC cooling unit -- and your drive at 70 miles an hour and you hit bugs for 10 years where you think they go they hit in between the AC unit and stay between it and the radiator built up is like a brick wall -- to check flashlight at night inside hood behind the radiator can you see the light. You need to look through the grill can you see the light now.

As long as you don't see white smoke its not leaking coolant into the combustion chamber, so it may be a plugged radiator or poor air circulation. Also make sure the oil looks good because a faulty head gasket can make its way to the crankcase too.

Make sure all of the air is purged out of the cooling system. Depending on the make and model sometimes this is quite difficult.

replace radiator

your radiator can be bad or even your temperature gauge