SW Water Temp Guage inaccurate
 

As mentioned in another thread, my new engine is in and running great. I took it on it's first cruise yesterday, and the temp guage isn't accurate. (I verified temps with a laser thermometer pointed at the sending unit under the hood.)

Bad sender? Bad guage? The sending unit is brand new. Guage was on the car when I bought it?

Where to start?

By how much do you think it's off? Lots of variables, start with the laser thermometer will vary by as much as 5 degree depending on the surface finish of the target. I'd guess the gauge is probably only good to +/- 5% so potentially 10 degrees off there. And where it is in the thermostat cycle will matter as the gauge will lag behind temp changes in the coolant.

But if the laser says 180 and the gauge says 200 .. or vice versa .. yeah, probably a problem somewhere.

Electric or mechanical gauge?

Good comments/questions above. I'd also recommend verifying the accuracy of the "laser" (i.e., infrared) thermometer if you haven't done so already.

Make sure you've got the right sending unit.
New doesn't mean it's the right one or good.

Clay

By off, it’s reading 240 while the laser therm says the intake is 200, upper radiator hose 190, etc.

Probably dumb questions:

1. How can I tell if my laser thermometer is correct?

2. I don’t know if my gauge is electric or mechanical. (I thought it was electric, now I’m not so sure.). How can I tell?

An electric gauge will have a wire running to it. A mechanical gauge will have a capillary tube running to the gauge inside the car. It’s usually covered with a springy metal covering. It will be easy to tell the difference.

Murf

With the laser ... start by pointing it at an interior wall in your house to see if it agrees with your house thermostat. That would at least tell you how accurate it is in the lower temperature range. That would probably be enough ... because engine temps would still qualify as "low" temperature range for most laser thermometers.

I wouldn't use upper radiator hose, depending on the thermostat position it may not even have water flowing in it.

I'd guess you have an electric gauge ... the mechanical type are rarely off that far. But you know, never say never.

I e had this problem with my trans temp gauge for years I swapped the senders same, swapped the gauges same. Can’t figure it out. If I get under the car and laser the pan it’s fine. I even pulled the trans and went through it clean as a whistle and no signs of heat.
I can’t figure out why it’s doing it
These are auto meter gauges too that have always been good.

So I feel for you.

I do know with SUN water temp gauges it’s very hard to find the correct sender no other senders seem to work with the sun stuff.
Good luck w the hunt and post you findings

Many gauges are actually adjustable if you can get inside them. Either by bending a "U" shaped link in the case of mechanical gauges or a needle adjustment on electric gauges. Usually the better the gauge the better the chance they will will be adjustable.

Of course with electrical gauges you can often add the appropriate resistor to bring them into accuracy. If you have the time, use a potentiometer to find the appropriate value and replace the pot with the resistor when you have it dialed in.