My temp gauge on my Sonoma flucuates wildly. Immediately after startup, it goes all the way to the top turning on the check gauges light. Then it will drop to normal seconds or minI know it is not hot, it sat all night in 30 degree weather. I changed the coolant temp. sending unit and it still does it. Can't think of what it is, any suggestions?

What engine?

4.3l

1991 4.3L has two temperature sensors. The sensor in the intake manifold near the thermostat is for the computer controls, (OBDI). The sensor in the cylinder head, most likely driver side between the spark plugs, is for the dash gauge. Gauge fluctuations can be caused by several things. If it happens when the engine is cold, as you indicated, it's probably not coolant temperature related but rather electrical. It could be a loose connection or a poor ground. The system is designed to indicate HOT when there's an open circuit. Check it out and post your results.

Will do, thanks for the reply

Every since your reply, never got the chance to look at the truck again, but it has stop fluctuating. I will leave well enough alone till it does it again. Thanks for your help