I have a 1991 volvo 240! When I start my car my temp gauge
slowly goes up. Does not stop until it get to the
top.Then it stay up there for a while. Then slowly comes
back to the middle again when I'm driving for a while
my temp gauge goes under the middle almost to the bottom again.
Could it be the thermostat?

 

Could be the thermostat. Be sure to use only a Volvo one, the parts store brands (Stant and Robertshaw for sure) are problems.

It may be the liar board (compensator board) located behind the gauge, it's design to lie to you about the engine temp and often goes bad with erratic results. It's easy to bypass.

 

Is your radiator fan working?

Regards, Andrew.

 

Step 1 is to take the temperature compensating board out of the instrument cluster and toss it in the dumpster, as Larry mentioned. Replace it with a jumper wire. I think cleanflametrap.com has a writeup. If you can't find it I'll look around.

 

I'd measure the actual temperature while this variation is going on, and this should reveal what's really going on.

 

I got an 86 240 wagon. I replaced the heads a while back but with the old heads the temp gauge would be normal...the used heads that I replaced with probably had a bad temp sensor...because when I fired it up the first time...the temp gauge needle went all the way up and back down to, this for a few days until it never did anything anymore.
The temp sensor "is" on the Cyl Head right?

 

I would ground the temp sensor wire with the key in the on position engine not running and see if the guage goes all the way up and stays there if it does then leave the sensor wire un grounded and it should go all the way back down and stay down. If that happens then replace the temp sensor it should be fairly inexpensive if that fails then look at the compensation board.

I always try to go from cheapest easiest fix first to the most expensive and complicated fixes last.

 

To Typhoon:
Yes my radiator fan is working

To 1981 240 Dl:
That was the problem the temp sensor was bad.

thank all

Now I have another question

I got a new temp sensor on 2/19/10 and have driven the car less than 50 miles.My temp sensor goes up really slow and sticks for about 5-10 min then comes down.
Is this normal for the new sensor or should i change the thermostat? If so which one should i get?