Yeah the temp sensor next to the stat is for the injection and the one on the passenger side by the exhaust manifold is to the gauge.

I don't think my gauge is incorrect though. Being mechanical (uses a capillary tube) I would think it just wouldn't work if something was wrong with it. When it has indicated hot and I've had the opportunity to open the hood at that time I've noticed the upper hose collapse some when I throttled it but not all the time.

I'm not used to mechanical (actual accurate gauges) so I don't know what's normal as far as how the temp should fluctuate while driving. If it gets to 240 and I throttle it to about 1500 to 2000 rpm, should it drop to 200 in 5 seconds? That's how fast it will drop at any point when I put it in neutral and rev it.

Also when it does indicate hot and I can feel the upper hose I can feel it boiling, so that's the other reason I think the gauge is accurate.

I appreciate your help, never had problems at all with temp for years until I had to change the heater core. I'm not a pro mechanic but could rebuild an engine of I had to. So it's boggling mind right now.

If I can figure out a way to post a video of the temp and rpms while I'm driving I'd do that if it might help figure this out.

 

This may or may not be the thing, but when I changed my heater core, I started the truck with the radiator cap off and waited for the thermostat to open while the heater ran full blast. In my experience, one of two things will happen. 1, (after it's topped off) the coolant will hold steady and then suddenly drop, indicating the air has been pushed out, so top it off and replace cap. 2, the coolant will not drop at all , but slowly rise until it pours out of the radiator filler neck. At that point, all the air should be worked out of the system. This has worked for me for years now. something to try anyway. Sounds like air, unless you can prove a head gasket or such, you know?

 

Ive watched this post since it started and if your sure you have all the air bled out and have good cooling air flow thu the radiator i would check and see if the new water pump is rotating the right way - my toy in my avatar is counter clockwise rotation because of the serpentine belt - the multi belt pumps rotated clockwise - while building the toy I replaced the water pump and they gave me the wrong one and I had similar over heating problems - just a thot
edit
upon further research with a serpentine belt rotation can also be either way - a water pump with a smooth pulley [no groves] is reverse rotation [counter clockwise] and if water pump has a pulley that has the belt grooves its a standard rotation type [clockwise]

 

Sounds like a bad ground somewhere causing the gauge to read high when alternator is not meeting vehicle electrical demands (at idle). This theory can be checked by observing operation of the gauge with the small connector on the alternator unplugged. Did you remove any of the grounds at the back of the heads or inside the dash during heater core replacement? I bet that you did.

Need a wiring diagram and check all the grounds you may have disturbed or left disconnected. I think you will find your problem there somewhere. All grounds should check within about 0.2V of the negative post of the battery at all times when the vehicle is operating.

 

The bad ground would probably explain the stock dash gauge to read the way it was. Now that the weather hasn't got above 60 it seems it doesn't go higher than 240, but it will hit 240 usually on the highway. I'll be posting a you tube video of the temp with a view of the rpms and speed and that might give a better example of what it's doing. I'll see if I can post the you tune link tonight to show what is doing on my 8 mile drive back from work.

 

OK Here's the first clip, a bit long but I'm not exactly youtube proficient when it comes to posting so you may want to skip a bit. You'll see it get up to temp at about 220-230. Traffic was crap so I couldn't really get up to speed on the freeway for very long but the temp went up to about 250 and dropped back down pretty fast. Excuse the sun glare and rough vid, ****ty se portland streets... After this I stopped at a store real quick and drove for another 5 minutes which was a good indication of how quick the temp would fluctuate. .. https://youtu.be/ghGlaplAu7I

 

This is the second much shorter clip
https://youtu.be/5pAfWCJ65lk

Hopefully these worked. You can hear the rpms and and see them along with the temp. A couple times I down shifted to get the rpms up to show how fast the temp would drop.

 

Ah, nice to see it in video. An image is better than 1000 words.

Seen similar things on this instrument cluster. I think that a bad ground or simply voltage drops or resistance increase can mess up the indication.
The two sensors are separate and one of the two will illuminate the overtemp spy on the dash, while the other is for the temp indication. Not sure which also supplies signal to the TBI and other stuff. Would have to look it up in the manual.

My oil pressure is doing the same. Only at over 2000 rpm it stabilizes which could be a problem with my cabling as well.

I would try to locate and bypass the wire altogether to get the reading directly to the gauge and find out if the original cabling or a connector is the troublemaker.

 

Im thinking this is the issue at this point , haven't read the rest of it but im thinking its pump rotation or the pump is faulty.

 

Can you post your VIN and make sure the engine in the Blazer is the stock one?

Then - any modifications to the accessory drive? A/C removed or such?

I think - but not sure - with the long belts it is possible to route them incorrectly, or when A/C has been removed and using a shorter one it may reverse the water pump direction.

Coming back to the funny reading of the temp dropping 40 deg. in 5 seconds measured with the separate gauge. Where do you measure exactly?
Because I think that such a pronounced drop would indicate that cooler coolant is drawn from somewhere. Also have you checked, that the heating is actually working with a reasonable heat output?

Hmm - what else could there be?

Keep us posted