temp gauge is all over the place, please HELP!!!!

[Mythic]

Hi, so I have had this truck for a little over a year now and the heater/cooling system has been a constant problem. I replaced the heater core yesterday and now the temp gauge is acting really weird. It warms up fine, gets to about 200 degrees and stays there when idling. But when I start driving the temp climbs all the way up to 260, making the check gauges light come on, and then rapidly drops back down to 210. this repeats over and over and over.

Last year I replaced the radiator and the thermostat. My initial thought is that maybe the thermostat is bad, but that seems unlikely since it is not terribly old. Is there some wiring in the enclosure for the heater core I might have messed up?

The truck is a 1990 2 wheel drive, short bed standard cab. I believe it is the 5.7 engine.

ANY advice would be very greatly appreciated and thank you in advance.

 

[cjman250]

sometimes thermostats can be bad right out of the box, regardless of age. Could it be in backwards? that could cause an issue

 

[Scrufdog]

could be air in the cooling system

 

[IOWNJUNK]

My finger points to thermostat

 

[Mythic]

Thanks everybody.

How do I get air out of the system if it is in there?

Also, which way does the thermostat go?

 

[IOWNJUNK]

Pointy side up. I have never had a problem with these trucks getting air trapped in them.

 

[Scrufdog]

go ahead and buy a new thermostat and gasket. Remove the thermostat from the engine, and top off the coolant through the hole where the thermostat goes. Install new thermostat and gasket. Fill radiator and start engine with radiator cap off. Wait about a minute and fill radiator again, then reinstall radiator cap. Go for a 10 minute drive, come back and let the truck sit for 10 minutes. Carefully remove the radiator cap and fill if needed. Also make sure resevoir is full. Should be pretty good at that point. Check it again after the next time you go for a drive.

If the engine itself isnt full of coolant, then there wont be any coolant to heat up the thermostat.

 

[Mythic]

I have replaced the thermostat and gasket and properly bled the system. Still having the issue. I am going to try the temperature gauge sender now and also do the coolant temp sensor since it is so easy. I am getting pretty frustrated.

 

[Alljackedup408]

Sounds like the gauge to me as well.... if everything above was tried.

 

[Mythic]

Hi again fellas, thanks for all the help. The issue is still not resolved. New thermostat, new coolant gauge sender, new coolant temp sensor and new thermostat. All fluids are topped off and bled. Could it actually be the gauge itself? I have never had an issue with gauges on any chevy I have ever owned, but I am kind of hoping that is the case. I can't imagine it is actually overheating. The thermostat is brand new and a totally mechanical device and there is plenty of coolant, so it should be fine. At least I hope it is. But I hate seeing that needle go all the way into the red and that check gauge light come on.