Just breaking in my rebuilt 250, and noticed that the temp guage starts at cold then slowly raises and then max to hot within 20 seconds and stays there. I immediately thought that I had an overheating problem, but the thermostat (195 deg) hadn't even opened up yet so I know it's not overheating.
I did install a new sender, could it be that they gave me an idiot light sender instead of the guage sender, could it be the guage, or is it the wiring?
Thanx in advance
Dano
(Sender is installed in the cylinder head, not in the thermostat housing.)

If the guage and wiring were not changed I'd take the sender back and get a new one, double checking that it's the right one.

Assuming the temp gauge worked before the engine was reworked and all you did was put in the new sender I'd say that's the problem area. One other thing to pay attention to is how much sealer or teflon tape you used to install it. Put enough of it on and you electrically insulate the sender from the head causing eratic readings.
A quick check for that situation is to put one probe of an ohm meter on the head and the other on the base of the sender. Resistance should read Zero or almost Zero, indicating a good electrical connection between the two. If you find that to be the problem pull it out and clean things up and use a small amount of silicone sealer on the threads to reinstall it.

Installed in the head or the thermostat housing shouldn't cause much variation on your gauge if your head locates the sender just back from #1 cylinder.

Mike

I didn't use any sealer at all, because I had read in an earlier post how this affects your temp readings. I will try the resistance test.

thanx
Dano