I have a 56+ mile round trip commute with primarily freeway at the speed limit.

The 1992 240 GL (3.1 LH-jet, M47 II, 165,000 miles), runs up the temp gauge above the 09:00 (9AM) position to like halfway between there and the orange line about seven minutes into the drive as ambient temps drop. Turning on the heat control valve level to max heat brings heated coolant into the HVAC coolant heat exchanger. The heater control valve has been replaced by a prior owner of this vehicle.

Yet in the evening, some 9.5+ hours later, I take the 1992 Volvo 240 GL back the same route, and with heat on or off, the faker temp gauge remains steady at 9AM or slightly above.

Slapping the dash makes no difference in the temp gauge needle position no matter the drive time.

The engine seems no hotter than the other two 240s, or the four prior 240s I'd owned.

In warmer weather, the temp gauge needle is slightly above 9AM and remained.

So, while I've searched the FAQ and the bb using the google site:brickboard.com in the search query expression string, the anecdotal evidence suggests a failing thermostat.

This 1992 240 GL has a replacement blackstone radiator and the coolant hoses seem perhaps, at most, four or six years? Still supple and sturdy. The upper hose is the normal Volvo 240 warm.

There may be mineral precipitate in the lower radiator portions, yet the entire radiator gives out heat, even at the bottom.

My only worry is the added stress the coming Winter temps place on the thermostat.

Thanks,

cheers,

LOL Grumpy Cat meme.
--
LOL cats!

I am sorry but I had to listen, to what you didn't say.

A 56+ plus trip...so... For that approximate thirty mile radius are there any appreciable grades?
Like are you at a lower altitude and are climbing in one of those directions.

A climbing load requires more fire or fuel into the the engine. If the radiator is on the border line of keeping up with a normal load then the total level of heat will rise.

With a normal gauge you would see that gradual rise to what ever level that's in proportion to the amount of heat generated by hills or lack of air flow and the excess heat above the thermostat setting, is ejected.
I think that's about 20 percent of the 60 percent of the energy inefficiency. The difference is in exhaust gasses.

With a compensated board, the reading does not move in an analog manor but more digital.
In other words, it may show up in one giant step. It will not move up or down until it trips to that next step. Which might only be a little bit more heat but looks like a whole lot when it does move!

To me, Overheated is a gurgling and popping sound of very hot water deep in the engine. Now that is the orange zone which means you have just strained the cooling system, if you only caught it early!
Getting close to orange is suppose to grab your attention. If there is a lot of excess heat then the next step could be into the orange. That small difference of hesitation in some extra movement, could become too late and its whoops there goes the farm!

It is for those reasons that, most people that are reasonably handy, take out the board and put in the jumper wire. One might call it, an late warning defense system, that's been defeated into an early one! Other wise is "No better" than an idiot light.

That's another part that you did not put into your post.
The results of the heater kicked on.
But we! Know why!
That's because the board has to crash to the next step!

Having a more direct and continuous readout tells you if opening the heater valve a bunch or a little had a quicker affect or not.
Opening the heater valve adds about one third to one half of a radiator on to the cooling system depending on if you use a fan or not!

You did say, its a car that's new to you. In other words you are ironing!.... The bugs out!

If you have a normal thermostat. It is one that will get fully open up just a little higher than the normal 8:30 to 9:00 position when first started from a cold engine but drops within a few minutes a normal running spot. Tiny overshoot but repetively so!

One that is just getting lazy or going south may overshoot the normal spot by quite a bit. Something like more than a couple widths of the of needle hands to almost three quarter land but will trigger the compensated gauge all the way into that three quarter land space.

That's the quandary, is it the gauge board or thermostat?
A poster suggested changing or testing?
Its a nice subject. To test or not to test, a new replacement or the old one!

IHMO,
Boiling Water, bubbles up air pockets and does not conduct heat evenly to the thermostats wax pellet. The purpose and use of a recovery bottle/ tank and closed system!

When it comes time for me to trouble shoot a thermostat, I use vegetable oil.
Its clear enough to see through and the temperature is more stable while heating.

You can raise the temperature in small increments up or down and around the stats limits and then higher by ten or so degrees.
Stamped on the stat are degrees to where some water will star flowing but you need to see it open more. You want to make sure it does that every time.
They are designed to open a lot more for only a few more degrees. More flow is needed to obtain a point of full flow. No engine designer want to under flow coolant to his engine!

Just like an oil filter has a by pass valve. Because at higher speeds, than idle, it cannot possibly pass all the oil that the pump can provide.

I like Wahler Brand stats. As they are made of copper for the pellet housing and a "brass" sealing disc/housing which is in part copper too!
Even heating of the whole apparatus, that is under tension of a spring, just makes sense to me.

I have had/ used Caloric & Stant brands that failed in several ways. Quick (sticky hot & cold) and be lazy!
Its been my experience Wahler's go the long run as they last longer and stay more accurate of which you can see truly see without the compensation board.

In the case these possible causes, pull them out and chuck them as defective, if you have them!
They both do not know if its AM or PM or can see a compass!
Phil

Taking your thermostat out and testing it is a simple task - all you need is a new gasket and a SHORT wrench to tighten it - do not break the fitting or the fasteners. Add water, a pan, and a thermometer.

Although putting a new one in is just as easy. Many mechanics test new ones too.

Drain some antifreeze first and filter it with a coffee filter before putting it back.

Missing a day's work and having your car towed (you may have no choice on the highway) costs a lot more than even having a mechanic install a new thermostat. Probably even more than a flush and new antifreeze - Volvo dealers are running winter specials.

Then, you could blow a head gasket...

BTW using a coffee filter can be instructional - Even though I used my lawn mower all summer, it began to stall after several minutes use. I strained some fuel drained from the carb and found some clear gunk - I added a fuel filter and flushed out the needle valve - no more issues.

Have you bypassed the Temperature Compensation Gauge? You mentioned a "faker temp gauge" and "Slapping the dashboard" (that isn't affecting the gauge), so I assume you still have the TCB. Slapping the dash is NOT the definitive diagnosis of a bad TCB, as it's faults are electronic, not mechanical, flaws.

Until you determine whether you are really overheating with, e.g., a IR temperature scanner (they're cheap, $20-30, and widely available under a variety of name brands) aimed, preferably, at the cover of the thermostat housing, don't assume it's your thermostat. The TCB is notorious for varying, inscrutable symptoms such as rapidly fluctuations, rises to the red over a brief time regardless of engine load, etc.

On the other hand, thermostats usually fail in only one of two ways -- reluctant to open causing consistent overheating, or inability to close causing (in this season) cold coolant (barely moving above basepoint).

Thermostats seem to either work or not, sometimes when they don't work it is because they fall apart.

You might want to flush the suspected sludge out of the system and replace the thermostat and antifreeze.
Dan

When I replaced what was probably the original 20 year-old stat on my '90, the result was that the gauge now gets to just above its normal running point (half way up the gauge) within a mile or so, then comes back down to half and stays there. It only moves from half if I am coasting for a while (drops a little), or after a while of being in heavy traffic (rises a little). The old one took a long time to get to half way on warm up, and tended to have wider variations from normal. Temp compensation board was already disabled when I purchased the car.