2001 XC-70 temp gauge (error??)

[FunWithPaul]

Hi All,
First off, a big Thank You! to all that have posted here.
I have learned so much from you!

Now my issue.

The Temp Gauge goes from cold to 1/2 as you would expect, and then NEVER goes any higher.

Even when the coolant tank is venting fluid.

So I bought a new sensor, and put it in 100c water.

It reads 1/2 a needle above the 1/2 mark (nowhere near the red...)

Do you have any experience with this?
(I think there is a gauge error)

Here is the background:
It is a 2001 XC-70 that has been having coolant/cooling issues for a while.
I replaced the waterpump about a year ago (it was leaking from the freeze plug), but it is still having random issues.

Once, it was venting coolant (but the temp gauge was in the middle)
I changed the thermostat, and that seemed to fix things.

Then I got a "LOW COOLANT" warning, pulled over and yes, the coolant was low.
The car got towed home.

I drained the coolant, flushed the system (and rinsed twice), and refilled with Zerex G48.

I have been monitoring the cold coolant level VERY CAREFULLY!

It is still (slowly) losing coolant.

I put some UV dye in the system , and found a pinhole leak at the upper right of the radiator
(it never made it to the ground, but sometimes I got that "sweet" coolant smell)

I ordered a new radiator, and pulled the old one out today.

But what is wrong with the temp gauge, and how do I fix it?

Thanks,
Paul

[vtl]

Temp gauge is a comfy approximation for type of car owners Volvo desired to have. I never saw it moving past 12 o'clock on 4 Volvos I've had, even after climbing a mountain on a very hot day. Fairy tales say some owners got it moving, but something remarkably catastrophic happened to their cars shortly after, like a blown head gasket, or ruptured radiator hose, or stream of steamy coolant from heating core to legs.

If you want to see a real temperature, you need to read with from the CAN network.

[Vova585]

Agree with VTL on gauge.
If you are loosing the coolant slowly...I woukd check heater core area(either core itself can be leaking or o-rings in supply and return lines to it), followed by exhaust gas in coolant check.

[scot850]

If it is the heater core you will likely get a greasy film along the bottom of the inner side of the windshield unless it is leaking externally like an o-ring. Then you have to lift the carpet to find that out. You may also get a strong sweet small in the cabin.

Neil.

[jonesg]

when the engine is cold open the coolant bottle, if its under pressure thats a sign of blown head gasket.
verify with block tester fluid kit.
https://www.amazon.com/RELD-Gasket-Test ... 0aWM&psc=1

[FunWithPaul]

Thank you guys!
Those are all great suggestions.

I got the new one in, and am filling (warming up) and draining as I type (to get the yellow fluorescent dye out, that I used to find the leak).

I have run it "as hot as I can get it" (up a long freeway hill, with full A/C going, and the needle gets about 1/2 needle past 12:00!
But the engine is real hot, and the electric fan is running, so "it's the gauge"....
(as I said, I tried a new sender already, no change.)

I found a temp vs resistance chart somewhere. The resistance drops rapidly as the engine warms up, to almost zero.
At 20c (room temp) the resistance is supposed to be 2450ohms
At 80c (my thermostat) the resistance is supposed yo be 318ohms
At 100c the resistance is supposed to be 184ohms

By the time the coolant gets to 100c, that temp gauge "should be" up near the red!
(as the electric fan turns on at 194F AFAIK...) but it is till at 12:00.

[jonesg]

elec fans turn on at 220F, the gauge is buffered to not show operational differences by design.
Fan will turn off again around 185F.

If the fan is faulty/broke you will see the gauge climb in a hurry if the car is not moving.

I use a cheap elm327 for smartphone, it shows exactly what temp is reported to the ecm, then double checked with an infra red gun directly on the thermostat housing, everything matched.
I leave the elm 327 plugged in the obd port, it connects to the phone automatically. Doesn't draw the battery down that I can tell.

Torque software for the elm is free.
https://www.amazon.com/ViaBecs-Diagnost ... =8-23&th=1