1994 B4000 temperature gauge fluxuation-engine has 85,000 miles
 

1994 B4000 temperature gauge fluctuation at idle and on road-has a brand new radiator and cap also new thermostat. Gauge fluctuation off idle in park, and when at driving speed. What's the cause? The clutch fan was checked and it was tight on the shaft w/no front to back 'wiggle'. At idle the gauge stays in one place. If the engine is revved the gauge climbs and when rev goes back to idle the gauge returns to its lowest reading. On the road the gauge seems to 'cycle' from its lowest reading (just below 1/2 when driven 20 miles or more), to about 3/4 'hot'. Before the stat was replaced there was no cab heat at all; since the stat was replaced cab heat is comfortable even at 35F outside air temp. The fuel gauge does not have any fluctuations at all.

If it runs hot around town and at idle it would usually be the fan clutch. If it overheats at highway speeds a clogged bypass hose. You don't say if it is fluctuating rapidly or just an inconsistent guage reading. The temp guage and fuel guage share a voltage regulator so if they both have a problem then it is an electrical issue. The voltage regulator for those guages keeps the voltage to them at a constant 12 volts so the voltage fluctuations from the charging system won't cause them to fluctuate.