You guys are misreading a touch. The rear main is a leak due to pressure at the rear main. It's gonna happen anytime the crank spins although it can be worse at higher RPMs due to higher oil pressure. The oil pan gasket isn't under oil pressure. It leaks due to slosh and crankcase pressure from a plugged crankcase vent. There's less slosh and blowby/crankcase pressure at idle. So, while there is no 100% one or the other rule, it's a safe guess on which is more likely. Some leak checking is in order. Otherwise, every one of us is guessing based on opinions. As a shop owner, I won't touch a repair/estimate until I can check it out first hand. I don't mind tossing out some possibilities here, though, as long as someone doesn't go tossing parts based on opinions. I'll wash my hands of parts tossers in a heartbeat.

 

KMatch, GroHo and FAke I think you guys are understanding what i am saying. The reason the oil pan will only leak while the car is in motion is the simple fact that the oil in the pan maintains a level lower than the gasket area.
The reason if it was the rear main it should leak all the time. Of coarse it will be greater at higher rpms as the case pressure goes up but it should always leak some as the crank is in motion and there is oil in the area and the case has some pressure on it.

Are there other areas in the rear of the engine that could be leaking? I know for sure it is not the valvecovers or any of the sending units or the filter. I bought a cheap chrome pan from checker auto when i had the engine done. The dude told me that the chrome pans were the worst and were known to leak.
If i would have known that i would have baught a stock replacement.

Any way I cannot work on it right now because it is back in the body shop. The guy that painted it knew that i was not happy with the result so he had me bring it back and he is going to redo the car. What are the chances that someone is going to stand behind there work these days

thankfully it happens to be on my car. woohoo.

 

IDK, the friends diagnosis makes sense to me.

A good way to check it would be to snug up the pan bolts, if this slows the leak then he is probably right... and changing the pan gasket will AT LEAST help.

But then again, if a person is going to change the pan gasket on a 66, it is almost to the point of being a "may as well" on the rear seal.... It won't take a lot more time, effort, or money to do the seal so long as you are doing the pan anyway..

(If the 66 still has a rope seal this might complicate things a little, you might need a "sneaky pete" to get the old seal out.

Both pan gasket and seal are pretty eay on a 66 Stang when compared to this procedure on MOST other vehicles.

 

cool about the paint shop,

a place that leaks often mistaken for a rear seal leak is the back of the intake manifold. Leaks down the back of the block, into the bellhousing, and... well you get it... Probably not your problem though because of the no leak at idle.

Since your engine is rebuilt you wont have a rope seal, and since the mechanic knew the perils of the chinese chrome pans, i bet he was extra catious and you have loose pan bolts.

I betcha snugging up the bolts will slow your leak.

The cheapo pans can usually be made to not leak if extra care is used when sealing them up.

"And Duh,,, (edit) if the engine was recently rebuilt, there is a CHANCE that the seal is bad,,,, but I freaking doubt it...

Assuming you got something better than a sh!tty rebuild, the pan is loose and/or it has a bad gasket. I am going to step out on a limb on this one and give it a firm 99% on this."

 

Where is the oil sender on the 302? I haven't pulled my donor engine, so don't know off the top of my head. The Cleveland in my 72 had a leaking oil sender at the back of the engine mimicking a rear intake leak. Of course, an oil sender will leak any time it's running, but it's an easy check if it happens to be in the rear. If the engine is dry above the trans bell housing, the choices are: oil pan, rear seal, any oil galley plugs, rear cam bearing cap. The last 2 require pulling the trans. Snug up the oil pan bolts and wash things off. You'll either see a difference or not - then you'll have a better idea of what the next step is. Caution: Brake clean and other chemicals have a habit of swelling seals and sealers which can mess with you - thinking a leak is gone or lessened when it was only tampered with due to a chemical which is temporary.

Avoid ANYTHING in a can for oil leak repairs or seal repairs of all kinds. The only time something is a can is of any value for repairs is an enticement that comes in a 6 pack which is used to lure a neighbor into helping with a proper repair.

 

Oil sender is on the front lower drivers side corner of the block on a 289/302/351W.

Even so, a leak here can easily be confused for a pan leak and/or even a rear seal.

Leaking oil here will typically run/blow down the lip of the oil pan to the rear of the engine before dropping off.

It is not usually easy to find the origin of a fluid leak.

Clean/ check /wipe/ check/ clean/ check/ wipe/ check/....

 

This.