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1958 1/2 ton with new 160 thermostat, new sending unit in intake manifold. Needle starts out at the bottom with the truck cold. after a couple minutes starts to climb but goes all the way to the top before the thermostat opens. Is there a way to adjust the temp gauge? There is a little metal tab on the internal workings which looks like it might be adjustable. in or out? how much? any ideas before I wing it?

Is this a stock guage set-up?
SOunds like you may have a malfunctioning guage or sender.

Did you accidentally put the thermostat in backwards?

Yes, stock set-up, I assume the brand new sending unit is OK. The wiring is new also. The temperature gauge is 44 years old but it does register cold when the engine is cold and slowly moves up as the engine warms but is near the top of the gauge before the 160 degree thermostat is opened. Therefor I couldn't tell if the truck did overheat. No sign of that problem but it would be nice to fix it if possible without buying a new gauge.

The replies are coming fast, thanks. No, when I went from a new 180 to 160 thermostat I made sure I was in the correct way. I think its the temperature gauge itself.

How do you know that the temp gauge is not working correctly ? What if it is working and the truck is overheating ?

You may have an air pocket . That keeps engine coolant from hitting the thermostat and opening it at the correct temperature..

Try removing the thermostat and drilling a 1/8 inch hole in it.

air pocket? wouldn't that go away when the thermostat opens? would it recur every time I stop the truck? I may be running at a high temp but I figured if I can feel the thermostat open at what would be expected to be 160 degrees my gauge should be lower at that point? Thanks

If you want to know what the temp really is, take the cap off with the engine idling and put a thermometer in the water. (A candy thermometer will work fine - but don't tell my wife.)

If you think you might have an air pocket, loosen the sending unit and see if water starts seeping out around the edges. If it does, you're OK.

I would suspect the gauge, although I'd want to verify the temp with a thermometer first. Until you know for sure that the problem is an incorrect reading not an overheating problem, I'd err on the side of caution.

I had a similar problem though, my gauge didn't read quite as high. I had a 350 Olds in a 63 C20 and the gauge would read about right most of the time. When I put it under load on a long hill or the weather was hot, hot the gauge would start to climb. I flushed the radiator ran coolant, ya da ya da. Finally put in a aftermarket mechanical gauge. The added gauge would read 20-30 degrees less than the stock gauge after about 160 degrees. I never did replace the sending unit, I may have replaced the temp guage trying to find the problem.

Larry

63 C20 292 daily driver
65 C10 Custom in process

Any chance it's the "NEW" wiring? Maybe too small a guage and too much resistance? Just a thought... Best o' luck!

There was a similar discussion here a year ago about inaccurate electric temperature guage readings when installing a new sender .Is the sender NOS GM, or unmarked reproduction?

Thanks for the replies, was out of town for awhile. Not absolutely sure the new wiring isn't a problem and I think the new sending unit is just an unmarked reproduction?I think I'll check the temp with a thermometer as suggested and see if it really is running hot. If its at 160 then start looking at these other issues. thanks again

It could be several things. Start by pulling the thermostat, and putting in a pan of water on the stove, along with a thermometer. Heat the water, observe. It's surprising how many thermonstats don't behave as you'd expect. ;-)

Second, I alway drill a ~1/8" hole in the thermostat flange, in such a way as to allow water (and air) to 'leak' by. This solves trapped air problem.

Do those, see if the problem goes away. Remember rule 1: trust your gauges. If the gauge says the motor is hot, maybe it's really hot..... 8-)

Cheers!
George

Electric gauge , right ? add a grounding strap from the cylinder head to the firewall , see if that helps .

I have found aftermarket sending units to have incorrect resistence compared to GM. If using an aftermarket sender you may need a resistor in line from sender to compensate. By checking the temp with a thermometer in radiator and trying different resistors you can set the correct possision on gauge.
Dave

Thought I would give a follow-up to this posting and also thank all for the advice. THE ANSWER IS- those new after market sending units do not always work. I was about to start putting resistors in line when I found the old sender and put it in. Now the gauge sits at the low mark and doesn't budge at 160degrees. I did check with a thermometer in the radiator- 160 degrees on the button. Thanks again and stay tuned for my next ?