On my 68 400 that I just got running, the temp guage sits around the middle when the ignition is off, then spikes when the car is running. It is a new sensor. Anyone have ideas or have had a similar issue?

Did you obtain the correct sending unit (gauge type or idiot lamp type -- not interchangeable)? Did you check for shorts to ground in the wire between the gauge and the sender? Did you put any type of thread sealant on the threads of the sending unit?

I did ask for the guage sending unit. It has a blade type connector that slips over from the side. I did use teflon tape when I put the sender in. If I ground the wire while the car is running, it will spike the guage as well.

And if you disconnect and don't ground the wire, the gauge drops to the bottom of the scale with the ignition on? If so, wire is (should be) good. Gauge type sending unit is commonly referred to as "nail" type, it fits a slotted 1/4" connector.

Teflon tape may keep the sending unit from grounding properly, but in your case it appears the sending unit is permanently grounded. If you have an ohmmeter, test it by measuring terminal to thread resistance (disconnected). It should be more than zero ohms.

Maybe you have the positive and negatives on the gage swapped?

Yes I have the Nail Type connector. I will take the teflon tape off and try again. All other guages work in the cluster. I have new wiring from the firewall forward.

Instead of going though all the work of taking it out and putting it back you could ground it with something to see.

Simply disconnect the wire from the sending unit and use an ohmmeter to measure the resistance from the nail to the manifold or head. If it's zero the sending unit is bad or incorrect. If it's infinity, the Teflon tape is causing problems.

I put the Ohmmeter on the sending unit and the the ohmmeter goes up the about half. The guage sits at about 1/4 when the key if off, when I turn the key on, the temp guage goes to a half. This is with the sending unit disconnected.

Did you try grounding it?

Warm the engine up and measure the resistance of the unplugged sending unit again. It should change when hot.

If your sending unit passes this test you have a gauge problem. It does not matter what the gauge says with the ignition off; only ignition on readings are of significance.

Factory gauge or aftermarket?

It is a factory guage. I'll do the hot test this weekend. Thanks for the help. How can I fix the guage, if that is the problem.

http://www.fix-a-gauge.com/faq.php

Yes, it works.

I ordered the kit. Thanks.