Oil leak from top of engine, in the rear. When I first saw the puddle under the rear of the engine my heart sank. Thought it was the rear seal, or the trans input shaft seal. Thank goodness it's neither.

I taped a piece of a rag to the bottom of the bell housing, where the inspection plate is, and drove around a while. It came out dry, but the outside of the tape had a drip on it, coming from higher up apparently. I got the idea to slip my phone camera down into the tight space behind the distributor and took a few shots. It showed me the views below, first the distributor base:


And here, the oil sending unit:



I decided to pull the sending unit because I think I see a drop forming on the threads, on the left-most part of the threads in the picture. If the leak was coming from the distributor shaft O-ring I would expect to see more oil puddled in the distributor hole. Could the intake manifold be leaking? Maybe, but I put teflon tape on the sensor threads and tightened it back in. It had some kind of semi-hard sealant on the threads which I cleaned off first. We'll see how it does. Thoughts?

 

More pics. These were taken after the truck sat for a few days, but you can still see where the oil tracked on the block. Looking down from the top of the block in the rear oil had collected in the dips/depressions of the block where it meets the bell housing:

 

Still leaking from oil sending unit threads, even though I put teflon tape on and tightened it up. Can't be simple can it.

 

You can add a bit of plumbers pipe dope on top of the tape. They sell it in small tubes these days if you aren't or don't know a plumber.

 

Try the dope....Teflon tape isn't made to seal. It's made to reduce friction on the threads so that you can screw it in tighter and get the threads to make a better seal. It's a bit of a misnomer that people believe Teflon seals. Dope serves the same purpose, but sometimes allows the threads themselves to seal better because the tape is too thick, and the dope will push out of the way. Here's a good link describing all the various types of threads and when to use tape vs dope vs nothing: http://diy.stackexchange.com/questio...d-tape-be-used

 

I hadn't heard of using pipe dope before, can it stand up to engine heat?

 

Again, the dope doesn't seal....it allows lubrication to help you screw it in tighter to where the threads seal. The reason it's preferred over tape, is that the thickness of tape can actually deform the threads and allow leaks. But neither is doing the sealing....the threads are. So, if the dope "goes away" due to engine heat, doesn't matter...it's already done its job. If you want to put some sealant in there, use high temp gasket maker. But I might try changing out the sending unit if the threads look suspect.

 

I think I'm going to use Permatex aviation sealant on the threads. It's intended for use on engines and is not affected by fuel, oil, or engine heat. I used it to seal the fittings I added to my fuel tank and it worked well there.

Good info in the link on when to use pipe dope, thanks.

 

Pipe dope works in high heat, hence using it in steam systems. It will harden but still be flexible when you go to remove it. Technically the threads do the sealing but my experience is that tape actually does help a joint seal my recent use of a gauge to test my fuel pressure. The joint at the base of the gauge where I screwed the hose in failed and leaked gas with pipe dope. I used tape and it stayed sealed. I have had other experience over the last 35 years using it on the job and while it is not meant to be a sealant it is is actually intended as a lubricant, it does help a joint seal. The addition of dope I suggested is based on my experience of what it performs like in high heat situations. I have taken many threaded connections apart some more than 50 years old and the dope had often hardened around the top of the joint where your slow leak seems to be.
It occurs to me based on how clean your motor looks to ask two questions 1. Are the threads on the sending unit and the female side good? 2. If the threads are good why is pressure so high the oil is seeping out there? I suspect the threads on one side or the other are the issue.