I've been getting this nagging slow leak from the ECT sensor at the coolant manifold.

First, I replaced the sensor... still leaked. Then I replaced the o-ring with a wider one... still leaked. I even tried to put plumbers tape around it and stick it in... still leaked.

Any suggestions? Is there a safe sealant I can use?

 

I'd go back to the o ring, it should be very tight on the shaft - is it? It won't leak of its the right fit, it cant. Most people get the internal diameter too big. Amazon sell selection boxes of o rings -

419pcs Metric O-Ring Kit Rubber Washer Seals Assortment Set 32 Sizes - High Engineering Grade Nitrile (NBR/Buna) Rubber Ideal for Maintenance & Plumbing - Shipped Direct by Amazon
If you start using sealant you're just throwing plasters on a problem that could be fixed in my opinion.
I'd also look on SNG Barratts website for the exact o ring, if cheap try it. JLR have some odd sized items - 9mm allen on brake pins, very odd size aircon green o ring that i cant find in any selection pack- it might be the same with this.

 

Thanks. Now that I think about it, the larger O-ring wouldn't fit all the way in. It really is a weird size.

I'm wondering how a new sensor with new O-ring would still cause the issue. Either way, I'll keep playing around with the O-rings.

The sensor itself has always been kinda loose. It doesn't seem to want to stay tight. Maybe why its a totally different design in the 5.0 models.

 

That's not good. The o ring has to be a beautiful snug fit...and the not tightening bit sounds a concern. Could you use threadlock to keep it in place PLUS a decent o ring? That would be my approach