Engine Temp-Too Cool

[CBT83]

1984 -C10 with a 4.1(250c.i.d.) inline 6. Was doing fine, then one day it would not show warm on the gauge. Replaced thermostat, seemed to work for a little while then back to showing cool. Gauge will not move off of "C".

We redid the interior on the truck and my son broke the gauge needle. I replaced the gauge, another thermostat, and a new sending unit.
I can ground the gauge out and it will peg to hot so I think it is good.
Sending unit is making contact etc. And after 3 thermostats, it is still not working.
I am at a loss here. I can't pinpoint it as an electrical issue, I think the T stat is working as it should.
Engine has a fresh tune, all else normal.
Fully warmed up....suspected anyway...a laser thermometer shows 135degrees or so on engine head and block.

Any ideas, or input on what this could be?
I have seen plenty of engines run to hot, but none too cool. Unless the gauge has to be calibrated I am stuck scratching my head on this.

 

[CSFJ]

Iirc, there were two or three different gauges used over time in these trucks. They all need to be paired with a sender designed for the gauge. The resistances changed between the different ones, so mixing and matching will net you an inaccurate reading. I may be mis-remembering but I thought one or two of the senders were getting more difficult to find.

 

[CBT83]

That is good to know. I will double check but the gauge was purchased for this specific model as was the sender. The other sender is larger as confirmed at Napa. We had the same issue prior to the new guage….the new gauge just showed up as part of a mistake on cleaning the old one and breaking the needle....at that time I thought the new Tstat went bad...that was not it.
Plus the engine block only reading 130 deg. mimics what I am seeing....just seems to cool. Truck runs fine but I do not want to run without an accurate gauge, a kid driver, and an old truck.
I want to milk the old 250 all I can before dropping in a V8.

 

[CSFJ]

Is there any chance something caused the new t-stat to stick open? Is there any rust or scale buildup in the water jacket?

 

[75Monza]

Another way to test is buy a cheap mechanical gauge and just hook it in at sender and set gauge under hood. I always keep a mechanical around for testing various rigs with issues. Using laser thermometer is not the greatest way to get a temp sometimes, have to remember that the laser is just a pointer of where the instrument is taking its reading from...can be above, below or around dot depending on make. Second, it's only surface temp, so not very accurate of what is in the motor. Last is a thing called emissivity; basically, it's the ability of any given material to emit infrared energy, real shiny objects will reflect ambient instead of emitting its own infrared radiation.

 

[CBT83]

Monza, yeah I agree. Good idea on the mechanical gauge. I have one but will need to find it. I still do not believe it is warming up...the upper hose never gets to hot to hold. Never.
CSFJ, this is the 3rd Tstat. Last time I made certain it was all good and clear. None of the others looked hung open, nor dirty. I am wondering if somehow the tstat might be getting lifted off of the seat and water bypassing it. Everything looks good, tight etc. Guess I will pull the upper hose and crank it and see what comes out. It would tell the tale pretty quick.