Temp Gauge Problem 4.3 liter

[stugots]

Hi all,
My 1999 Sonoma 4.3 automatic with 203,000 miles has a temperature gauge problem. At least I think its a gauge problem.

Gauge goes to normal operating temp ( halfway)
and then after a few minutes it pegs to the right and the check gauges light comes on. Then it goes back to the halfway point and the light goes off, then it goes to three quarters, then to halfway point. All within a few minutes.

When the gauge acts erratic, i notice that only sometimes the heat blows cool, sometimes blows warm, sometimes it is stays blowing hot.

Radiator replacement 2008
Thermostat replacement 2010 (I replaced it then, due to the same problem as i have now)

full coolant level, coolant is clean, no leaks, no blowing out the tailpipe, no mixing with the oil. no bubbling from the overflow tank either.

recently replaced the smog pump and check valves, but i doubt this has anything to do with it.

Any advice is appreciated.
Thank you.

 

[dirtclod]

temp guage

Most likely it's the temp sending unit. Haynes manual shows it's location on drivers side cylinder head, on top of engine, near firewall and below brake master cylinder. Cheap fix. Outside chance of problem being a bad connection at the sending unit. So check for a bad connection first.

 

[stugots]

Hello and thanks for the reply,
Located the sending unit, noticed it has one wire with one terminal on the female end ,coming out of the sending unit, but the harness it connects to has two wires, with two holes on the male end.
Seems odd to me, but has anyone else noticed this?

 

[dirtclod]

temp gauge

Haynes shows: yellow wire to control module, and a 2nd wire (black) to ground.

 

[swapdip]

My truck's temp gauge used to do that too because the heater core was totally clogged up. Try back-blowing yours out with an air compressor and see if your temp gauge/cool heater output problem improves.