I have an 86 TA (carburated) with aluminum headed ZZ383, dual fans (from some minivan, really strong) and aluminum radiator.

When I start driving the car the temp climbs normally and stays around 180-190. After some driving (and the temp has been steady) the needle starts to jump up and down (really fast!) probably ~20 degrees. Some times it goes all the way to highest and stays there for some time but then goes down again. Then engine seems not to be boiling over and seems to be running fine.

The wire to to temp sending unit seems to be ok and the gauge should be ok because its works when I havent driven for long. Could it be the sending unit?

 

You may want to check into the possibility of an air pocket in the block which needs to be "burped". One other suggestion would be that the lower radiator hose may be collapsing under suction. The fix for that would be to get a radiator hose spring. When you say the needle jumps really fast do you mean within a split second or like just a few seconds? If it jumps literally within a split second then you have electrical issues. If the jumps take a few seconds then it is probably heat related. Good luck.

 

Thanks for the reply, its within a split second so its probably electrical.

 

Could a lower hose from the radiator be causing this? Its showing a normal temperature untill it has been warm for some time and then its starts to act like that.

I saw the other day that (my goodyear silicone with no spring in it) hoses where a little bit suched in after I had been driving for some time (did not chech in right after I stopped).

Could it be that this causes some air pockets or that cold water does not get into the engine?