Starting about 2 months ago only very occasionally, and now on just about every trip, the temp needle rises above the midpoint dot. I can drive along for half an hour with no problems, and then in the course of about two minutes the needle will rise from the midpoint dot almost to the red zone. Sometimes it will occur after driving for 5 minutes, other times not al all. If I pull over it will return to normal - sometimes in less than a minute, other times after 3 or 4 minutes. Sometimes it seems that turning the heater on full blast while continuing to drive will lower it as well, but not always. I checked the radiator hoses last time this occurred - when the temp guage read high, the top one was hot, the bottom was not. After the gauge dropped again, the top one was still as hot, the bottom one was warmer then before. When I started driving again, the gauge actually "bounced" - meaning it went from normal to hot over the course of about a minute, then back down again over the course of another minute - three times before settling in at normal again.

The thing that's bothering me here is that the behavior does not seem consistant - sometimes turning on the heater will work, other times not, etc. At first I was thinking my thermostat needed replacing, now after going through the archives I'm wondering if it is the temp gauge circuit board - which according to receipts was already replaced by the PO about 2 years ago - or something else... I did just remove and re-install the instrument cluster, though the sudden increase in problems this week didn't coincide exactly with that. I suppose I could just replace the thermostat to rule that out. Anyhow, any insights would be greatly appreicated!

My '86 does the similar thing-- occasionally reads high for no reason at all. Had thermostat replaced, no difference. coolant checked etc. Since winter's on it's way those are good things to do but likely not solve your problem.

I would just reach under my dash up over the ignition "box" and twiddle with the wires going into instrument cluster and separate the wires from each other a little and the high reading would go away . . and occasionally get weird instrument displays in the process. I suspect I have a small voltage short or leak in the wires going into cluster as I once even felt a mini shock there.

tape wires? do the compensator board as several suggested??? (I'd like Don's pics if avail...) This problem caused me a lot of anxiety and wasted shop visits with little solution. A Swedish car specialist said he could fix the electrical problem for 60 bucks.. some here says it's a little time, soldering and a few pennies. By then I would just tweek the wires a bit.... however, what I thought was a false reading was once real, I sprong a leak on my water pump. Luckily I was only driving a short distance to get oil for an oil change... and found a big puddle of coolant under car there.... if I'd been driving a long distance and ignored the high readings ... disaster might have struck my sound engine. So it's worth fixing the gauge.

The best thing done though was having a mechanic check the temperature of engine while running with a thermo. gun which literally reads and tells the heat of components of cooling system. Without that you should at least tell if your thermostat is working by seeing when it "opens" (literally). Start your car when it's cold, feel the upper hose going from engine to radiator, which will be cold. Drive for a mile and a half about and minute and a half (very approximately) with heater on. You start to feel just a little heat coming from engine, see if thermo gauge is heating higher area of gauge or stopped climbing, stop, leave engine on, get out and check engine, feel upper hose again. Should be getting hot. Definitly hot if your gauge is reading high. If not, change thermostat for sure.

I am guessing though you have a small electrical problem too common to 240 Volvos.

Fist thing I've ever done when trying to solve a heat problem is to just replace the thermostat, it's easy and cheap, and I seen sticking thermos create some weird problems on some cars.

I had similar symptoms on my '86 240 except for the hose temperature which I didn't check. The thermostat is easy to change and, since I had no idea how old it was, I replaced it. This didn't fix the problem but I was glad to have a new thermostat anyway. Next I pulled the instrument cluster and: cleaned the voltage regulator contacts, cleaned and tightened the temperature meter connections, and removed the temperature compensator board and resoldered it also cleaned the board's mounting pins. This fixed the problem.

Sound like thermostat to me. Replacing the thermostat is a cheap and easy first step and not a bad preventive maintenance measure. Avoid allowing the car to overheat as damage can result if it is severe.

Temperature compensation board sounds pretty likely. In the past, Don Foster has posted some nifty photos to locate it. Bentley has a procedure as well.

Rob