as the title reads... my engine temp needle is bouning between cold and normal operating temp... and i mean literally bouncing.. not a gradual sweep. could this be a sign of a bad themostat or an ecu problem? the motor is a built b18c5... im not too sure but it may or may not have a spoon T-stat... my brother and his shop built the motor so im not totally familiar with all the parts used for the build up... thanx for any info in advance fellas

 

It's jumping too quickly to be a TRUE temperature change, right? So it wouldn't be the thermostat.

That gauge circuit has nothing to do with the ECU...

Look at the connection at the temperature sender. It's the small 1-wire sender near the upper radiator hose in the head. I think it would go cold if it's unplugged, so a loose connection would jump between cold & true. If it's OK out there, look for a loose connection at the cluster. I don't know if there's a plug somewhere in between??

 

Also check for air in the system and the seal on the radiator cap. It has happened to me and it was air in the coolant system due to a bad seal on the cap....

 

Replace the Temp Gage mechanism in the cluster!!!