So I've got a 89 Chevy 4x4, 5.7 liter and I had to replace the temperature sensor that operates the gauge. After replacing it and burping the system it wouldn't go back to original operating temp., Sits at half way(210) and goes up to 3/4 line then back to half but it has always stayed below the halfway (210) mark, that was its max high temp. So I've now flushed the entire system, backflushed the heater core, I have a new radiator and water pump, changed the thermostat, replaced both sensors, and installed a new fan. Now I've filled it with new antifreeze and burped the system properly but the operating temp is still running at 210+ and when I test drove it again back to the 3/4 mark and back to 210. This is just driving me nuts, I've worked on vehicles for as long as I can remember and never had an issue like this. Anyone have some idea of what would cause this or if I've just missed something? Thanks in advance for any advice.

 

maybe it's the gauge

 

I live in NW Florida and have a 1995 K1500 whose temperature gauge MOST always read 212 or slightly higher after 30 or so minutes of driving. Lots of us run weaker thermostats too. I do! Your stat is opening and closing as antifreeze cools or warms going through new radiator. This is why gauge goes up and down. After May, it should always read high. However, any chance you used Teflon tape sealant on temperature sensor threads???? DON'T!

 

I just acquired a 1991 3500 with a 7.4L that runs just over the halfway mark at all times and it makes me quite nervous. I just submitted a new post about the issue. When I lift the hood the engine compartment is so hot I cannot touch anything in there. That does not seem normal to me. Also the oil pressure drops to around 25 or 30 when it reaches these high temps and it makes me very nervous.

 

Might be you have air in the system. I've noticed that lots of new thermostats no longer come with the 1/8th inch hole in them. It would be easy to drill one into it. Purging all the air out of some vehicles seems to be a chore, but it doesn't have to be. You could legally drop a 190-95 F thermostat in your vehicle without any issues, but perhaps your radiator needs to be flushed out before all of that.Too, check
your hoses as the interior of them can come apart causing a blockage in your cooling system, particularly in an older vehicle like yours.

 

Yeah I know the feeling. on my 97 454 I have replace every sensor, hose, T-stat, radiator, quick connector ,heater core, water pump and clutch, belts. I have also flushed the system. Bled the system and like his my gage says 210-220. I went and also added some mechanical gages and wired the water temp to the R/S block. It runs higher but doesn't move usually. My live feed on my OBD II reader shows my ECT at 199. I don't know if that's because its the real temp for my 7.4 or its the limit of my Craftsman OBD reader.? My only thought is when Im sitting in traffic it can get high but as soon as I start moving it drops. One more thing. My auxiliary fan wont engage when the temp climbs or when the A/C is turned on. I hardwired that in to test and the fan motor is fine.
Im at the point where I just ignore it.