My 78 245 runs again but now it gets hot with a termostat in it even a 72C. The temp will go up to the red in about 3 min. if I take it out it runs cold and never gets hotter than about 1/8th I'm stumped i've tried 3 different ones with the same result. new water pump and flushed the system still gets hot with a thermostat whats up.
Thanks Dave.

 

You may have a partially blocked radiator. When hot, feel around with your hand to see if it's uniformly so. Outside chance of the water pump having a bad impeller. You can opt to run the car without a thermostat--the only down side is that the heater will come on slower and possibly be a little weaker but winter is over...

 

yank out the pre-heater aluminum hose from air filter box to exhaust pipe

what about your fan clutch? spins with a little or a lot of resistance? hand turn.
reservoir cap faulty?

I got a 79 242 DL and was detailing the engine and took off the radiator duct or plastic cover to clean it well and decided to leave it off and it ran a bit hotter...just in case you took yours off.

 

doesn't have a preheater hose ,the fan clutch spins with with what seems about the right resistence to me.

 

For a car to get to HOT in 3 minutes, it's got to have NO circulation in the cooling system.

 

I agree but it doesn't heat up if I take the thermostat out. and it's opening right at 160. I'm begining to think that it's either the gauge or the temp sensor but all the bad temp sensors iv'e seen when they went bad either didn't read at all or showed hot all the time. and with out the thermostat it only gets to about 1/4 at best and takes 10 min. to get there.

 

You have to make sure the gauge is not bad. If it's correct, you'll have to consider a Head Gasket problem given all the symptoms. Head Gaskets are TRICKY, they isolate water, oil, compression, and metal (iron and aluminum), and can give all kinds of crazy symptoms! Water in the oil is what everybody looks for but this is just one of the combinations you may get.

I'd still concentrate on finding a blockage somewhere. Head Gaskets problems usually take much longer than that to heat up a car to HOT!

 

It's got a new head gasket and the head was surfaced and pressure tested.
what about the voltage compensation thing on the back of the instraments I was reading that that can cause a bad gas gauge reading and it's also part of the temp gauge circuit I think.

 

The temp gauge can certainly be faulty. Yet you say that w/out the t/stat the temp doesn't go into the red zone and acts normal? You can check the actual temp with the point laser gun, Harbor Freight sells them cheap now.