sounds like a grounding problem.  you may be getting voltage feed back.  try your landing light see what affect it has on gauges.  other wise, did this begin all at once or over time, new install ? 

Time for a JPI?

On the firewall there are two cable connectors that supply

input the gauges. Recc disconnect and clean. Mooney service centers see this problem a lot.

My issue with oil temp gauge was solved after installation of a new sending unit, cable connectors were ok.

The above comment of "Time for a JPI" is advise well taken. I have both a JPI and a Shaden and both are right on accurate.

I am experiencing high oil temp issues in the M20F that I am flying. In sustained flight with cowl flaps cracked open the temp gauge is hugging the red line consistently and does not go down. Is this an engine issue or simply a sensor issue? After flight the cowling is extremely hot as if the engine has not been cooling. Any possible ideas

REGARDING HIGH OIL TEMP:

Make sure temp gauge is reading correctly. Prob. not your issue due to hot cowling.

Check baffeling. (Happens)

Make sure cowl flaps are not mis-rigged. (Happens)

Check for birds nest or other debri behind oil cooler. (Happens)

Check for hot gas ring blow by, thereby heating up the crankcase oil. (Compression check on each cylinder) (Uh-oh)

Check to make sure Vernatherm is opening correctly to allow oil to circulate through oil cooler. (Your probable cause) (Microwave, hot water and micrometer to measure movement of vernatherm.)

Make sure vernatherm has been ADJUSTED  and is seating CORRECTLY..

Easy to eyeball vernatherm seat with mirror and flashlight. (Always check seating)

Check for varnish and sludge deposits in the cooler which will cut down on heat transfer. (Seen em full of sludge before)

And always remember ancient Chinese proverb: "Oil temp. rise, oil pressure fall, Mooney land in field."  I would not push throttle forward again until issue resolved. Good luck.

Wow David you are a valuable resource!

This sounds like a problem I'm having, high oil temps, slightly low oil pressure. I've talked to my A&P about this after reading above. I don't think he's up to speed on vernatherms, said they could not be adjusted. Can you tell me how, or give me a reference for, how to test and adjust it?