Could OP follow up on this issue? My ‘54 180 with O470J is having very similar behavior - would like to know what your issue turned out to be. Thanks!

I gave up , tried everything and could not fix it, flew the engine to TBO and was perfect when removed and opened, got a new PPONK and same issues, talked to Steve and he thinks either probe location or the JPI computer and to just fly it and not worried.

What I do is use carb heat all the time and lean it, that helps.

Chances are you have an induction leak which is not showing up on the soapy water test simply because its on the back side of an induction tube or hidden between the case and an induction tube. Especially if full carb heat and subsequent leaning rectifies it. These leaks shows up best when the air is cold and dense as you mentioned. The only way to know for sure is to do an inflight induction leak test. You can do it yourself easily. Look up the procedure or PM me if you have any more questions.

Ron

Easy way to test the probes:

1. Does the temperature match OAT before the engine is started, or close to?

2. Remove the probe and submerge it in an ice bath (70% ice/30% water). You should see 32-33°F on the instrument, +/- a degree or two.

3. Remove the probe and submerge it in boiling water. You should see 212°F or so.

This will give you three separate data points. If all three are within the expected temps, your instrument is very likely reporting accurate temps.