I'll try to keep this simple. My temp gauge was reading approx 150, i know this was a false reading so I replaced the sending unit. After I replaced the unit the check gauge light was popping on and now the gauge was reading 250 and close to the 260. Well, I freaked out and replaced the thermosat, this didn't change the reading on the gauge. I can tell by touch that the engine is no where close to 250 so this makes me think it was in the sending unit. I replaced the new sending unit with another same brand and the temp now reads about 230 and the check gauge light comes on but not as often. So between the 2 new gauges there is a 20 degree tolerance, I am starting to think it might be in the connector. One thing that I have noticed it the temp will hover around 210 when in park and at idle but jumps up under normal driving. This doesn't sound like a connector problem, more a flow issue. I know the old thermosat was 180 and I installed a new 195. Any ideas?

 

Here's a thread that might help you.

https://blazerforum.com/forum/1st-generation-s-series-1983-1994-tech-40/92-blazer-s10-4-3-vortec-cpi-temp-gauge-37915/

 

I doubt 2 sending units can be faulty so I am going to say the temp is above normal. There is a pronounced difference between the 2 (new) sending units, almost 20 degrees. The temp hovers around 220 when at low speed and/or idle but reaches 240+ when at highway or normal driving speeds. This sounds like a flow issue to me. Any ideas are welcome.

 

I would put a scan tool on it and read the temp. If false gauge reading, check out the wire that plugs on the sender. Jerry

 

I did some basic stuff...

burped the rad, had the truck on a hill and tried to release any air.
replaced the rad cap (someone had a 13lbs cap on it)
pressure washed the rad and AC condensor
flushed the system with water and super flush (someone had added the tee before me)
replaced thermosat with 195 (someone had 180 thermo in it)
replaced the gauge sending unit X 2

With what I have found, the 180 thermo and the flushing tee I suspect someone before me had overheating issues. Surprising to me there is no rust or debris of any kind in the cooling system. What baffles the hell outta me is the truck ran at temp, 210, and suddenly one day the gauge started reading 150 and that's when I replaced the sending unit and all this went downhill.

I think my next step is to get a mechanical temp sending unit and gauge and see what reading I get with that.

 

You can take the radiator cap off and stick a thermometer, like this one in and see what temp it is. Just don't take the cap off when its hot.

 

The engine temperatue for the ECM is read by it's own sending unit. If you have access to a scan tool you can read the temperature that the ECM is seeing. ECM sending unit is located by the water inlet. The gauge sending unit is located on the passenger side cylinder head.

 

Well, I am way past the point of trying to figure out which sending unit is connected to the dash gauge. Right now I have several options for testing the temp of the engine/sending unit.

OBD1/2 scan tool ($199.00) I don't have access to one so I would be buying one
Infrared thermometer ($39.99) would have to buy this also
Mechanical sending unit and gauge ($12.99 - $20.00)

I think I am going to mount the mechanical gauge on the exterior cowling so I can read it during driving conditions, just for test purposes.

 

Kind of new at this stuff, but you say you replaced the sending unit, did you try replacing the temp sensor??

 

I might be wrong but think those are the same thing.

Here's the update: Installed the mechanical temp gauge and guess what the temp is holding @ 195-200 (total investment $15.00). Which makes sense since there is a 195 thermo in it now. So where does this leave me, I have a temp gauge mounted to the outside of my truck on the cowling, a dash gauge that is about 40-50 degress off and checked over cooling system. Not sure where the problem can be now, it's either the connector to the sending unit, $25.00 part or the dash gauge not reading properly. I am at the point now where I might just route the mechanical gauge thru the firewall and install on the dash - still thinking about that. The new gauge would fit on the lower right corner of the dash but would cover 1 and 2 of the gear selector...Also I would have to drill a hole in the bezel for the sending unit line.