I have a 98 Silverado with the 350 Vortec and the temperature gauge is acting funny. It takes less than 2 mins for the gauge to read 210 or higher, the truck isn't sending back any signals that anything is wrong with the motor etc. The gauge will also go back and forth (up to 210, back under 210, over 210). I've replaced the thermostat as well. Anyone have any ideas??

 

I might just be a failing sending unit, not quite dead yet. When was the last time you flushed the cooling system?

 

I've owned the truck for a little over a year now and have not had the coolant flushed. Where would the sending unit be so I could replace it??

 

On your truck I am not sure where the sending unit is. It should be on the block somewhere. Argon would know for sure. Old, dirty coolant will cause fluctuations in operating temps, especially in the summer. If it goes from like 160 to 210, then your thermostat is bad. If its only a few degrees like you are saying then you need to get some fresh coolant in there. Use a flush kit. It will help get the crap out of the engine and radiator. Afterwards you will see the fluctuations will be slight, if any. If the problem persists then look into replacing the sending unit.

 

Ok, so I changed the temp. sending unit. And it still gives me the same problem. When I am parked the temp will stay at 210, but when I start driving again the temp. will climb up to 230 and eventually go back down, really starting to get frustrated[:@][:@]. What else??

 

On my 1990 model Chevy 5.7 I had a similar problem. Are you loosing any coolant? I replaced the radiator cap, then the thermostat and then my friend and I were pressure testing the radiator and I had a hole in the back side of it. We looked for a puncture on the front side, but there was not one. Come to find out it had Electrolosist or what ever it is called. It is something that basically eats the aluminum radiator until it leaks.Which is why I was lossing coolant and had my temperature fluctuating. It drove me crazy!The only cure was to replace the radiator and flush it really good.
Now I do not know if your 98 model is the same set up as my 1990 with a 5.7 was, but when I used the flush kit I flushed it through the heater core instead of just the radiator. I put a radiator flush kit T in the heater core line and back flushed it through that way. I washed all kinds of SH@T! out of the system. Hope this helps.

 

If your thermostat is stuck partially open then this could be the problem too. Try flushing first. Check for leaks too like timber74wolf says.

 

I replaced my water pump and put in some new antifreeze. The temp. gauge still fluctuates somewhat and gets over 210 sometimes, which still bothers me a little, but for the most part the motor sounds as if it is running normal. Does this sound like just a bad gauge?? Any other suggestions?

 

Did you also change the upper and lower rad hoses? when old they tend to collapse, and restrict flow. heater hoses also.

 

Make sure that your fan is kicking on and off the way that it is sapposed to. If it is electric fans than you will be able to tell right away if they are working or not, if it has a belt driven fan with a fan clutch then check the clutch and make sure that it is not worn out and not engaging when needed. If you need further guidance on the fan clutch let me know. Good luck!