Coolant Temp Gauge Problem

Without watching the live data from the ECU on a capable scan tool you really don’t have any idea what’s going on. In most of these modern engines the ECU has the coolant temperature “sensor” telling it what the coolant temp is. Then the gauge on the dash is fed information by a coolant temp sending unit. The two sensors are usually separate.

The gauge fluctuations could be caused by anything from a bad sending unit, fluctuating voltage from the ECU-B, bad cluster, or an actual temp fluctuation.

Not sure what transmission you’re truck has but mine is a manual and the idle speed is quite sensitive to coolant temp until the engine is warmed up. That is, in neutral (when cold) the engine revs higher to achieve warm up faster than the 750RPM it idles at when warm.

Does your idle fluctuate any if you’re truck is in neutral, meaning, does the idle on your truck drop to 750ish like normal when warm, then rise above that level like the ECU is commanding a higher RPM to warm the engine again (best to check this when the gauge is indicating cold)?

 

I have such a scan tool and would be willing to check the live data on your truck if you live anywhere near NC.

 

You could hook up an ohm meter to the sensor to see what it's spitting out. Should read 0.31-0.326k ohms at 176f. Also Just taking a laser temp gun to the rad hose a few times and see if you're getting any wild temp fluctuation. Would at least start to narrow it down if it's a mechanical or electrical issue.