Inliners International Forums General Discussion Engines cooling

I've got a 1970 Chevy 250 engine
in a 1975 chevy Truck.While idling
The temp goes from 190 to 200,
back and forth. The thermostat is
a 180 and its new,could it be
the problem or could it be something
else.

How long after you start the truck does this happen. Also, how long does it bounce between 190-200, and how long does it take it to bounce between the two numbers. Have you had any overheating problems? If you increase engine speed, does the temp drop? Is it a new thermostat? Sorry for all the questions, but I'll need some more info.

If I let my 250 idle for a while then the temp will increase to 200 or so, but it doesn't bounce back and forth. Also, she only does that if she's been idling for about 20 minutes or more. Good luck. John.

Soon as it warms up,it goes
to around 205 and then the
thermostat kicks open and drops
to 190.Then it will move up to
200 with in say 10 minutes or so.
I just bought the engine and put it
in the truck 3 weeks ago. The strange
thing is the other engine was doing
the same thing.I swaped the water pump,
thermostat,and the fan from the old engine.

Is your temp sender in the side of the head? I recently started having the same problem but it's due to installinging my temp sender on the side of the head(I hope!)

I had to swap covers to the thermostat housing and the one I'm using now doesn't have a hole punched in the side for the temp sender, so I installed it in the head. My guage now does the same thing you're describing. Some day I'll punch a hole in the housing and go back.

I think what's going on is that theres not much coolant flow in that area and so the metal heats up which is what the temp sender is sensing and viola!
Anyone else have an opinion?

you are right Brad the location is the diference--- the water temp is slightly different in differnt locations throughout the motor. The type temp sender used here will not sense metal temps-- it has to have coolant circulating around it before it will work. Volkswagen and Audi motors used a temp sensor the sensed metal tempratures. Next time you boil a pan off water take notice just before it boils it forms bubbles in the bottom of the pan-- the water closest to the burner is hotter than the water in the top of the pan. Likewise the water closest to the combustion chamber is hotter than other parts of the motor.The water in the head area is going to get hotter quicker and should drop when the thermostat opens and the coolant begins to circulate through the radiator.Just my thinking--Paul