'93 GMC Sierra fluxuating temp gauge

My '93 6 cly GMC truck temp gauge started indicating very cold temp from startup to highway driving. I replaced the therm. and no change. Then I change what I thought was the sending unit, the one on top of the engine
near the thermostat. No change. Now my temp gauge is fluxuating from cold to pegged in the hot range and check engine light is on and off. The truck temp is running normal and I know this is an indication problem. I have read other posted messages and someone mentioned a sending unit on the right side of the block. Is there one for the 6 cylinder 4.3 eng? If so, let me know and I will check that next time get to the truck.
Thanks
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Greetings,
First you will have to have the error codes pulled from the ECM and determine why the SES light is coming on. As for the wide temperature fluctuations on the gauge, just exactly how do you know that your engine is running at a normal temp if you gauge is bouncing up and down? I would start thinking mechanical items if your gauge turns out to be right. Try checking the fan clutch, and bleeding the radiator of air (both possible causes of temp fluctuations). You could also have a failing water pump or serpentine belt system.
Cheers - Jonathan

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I know the truck is running at the correct temp because the everything Jonathan mentioned as in belt, water pump, thermostat and sending unit have been replaced or checked. The radiator is not clogged either. Bleed the air out of the radiator? That's a new on me. The temp. gauge needle would bounce from cold to high, not a gradual move. The check engine light only came on when the temp needle bounced into the hot range. The needle is now in the hot range with or withot the key on now.
So I still think it is the gauge or can anyone tell me if there are 2 temp sending units. I read in another persons question that his GMC engine had 2, one in the manifold by the therm. housing and one near the #2 and 4 cyl. I am not at the truck but will check it this weekend.
Thanks
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Greetings,
Air in the radiator is a very common cause of fluctuating engine temperatures. Although you say these items have been checked, even a small leak can let air in so I would purge the air from the cooling system and see if you get any improvement. I would also make it a priority to change (not just check) the radiator cap.
The only true way to verify that you motor is running in a normal range is to have a seperate, secondary temp gauge put in, or have the temp checked manually and verified against your in-dash gauge. Just assuming the engine is at the proper temp because you've had several of the items that could affect the operating temperature checked is a sure-fire way to overlook something - possibly even the real cause!
Cheers - Jonathan

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Checked the truck today and there is a second temp. sending unit on the head between the #3 and #5 spark plug. Disconnected the wire and the temp gauge went to zero. So off to the part store, bought a new one and engine temp. is back to normal. Thanks Jonathan for your advice. Fooled me on 2 temperature sending units.
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Fooled me too, but I'm glad you got it back in order.
Cheers - Jonathan

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