Leak from rear of 258 (not RMS or valve cover)

[DaveMueller]

I just replaced my RMS along with the one piece oil pan gasket. I'm getting a large puddle dripping from the skid plate on the oil pan. It's running down the back of the pan in front of the inspection plate. I just finished rebuilding the T-150 , the bottom of the bell housing is clean. The top of the block is clean so it's not the valve cover (new also).

PCV is new and plumbed properly.

So what else could be leaking from the back? Any other causes for the rear main to leak other than a bad seal?

 

[hole]

Base of distributor maybe? Mine seeps a little oil, but not enough to worry about fixing it yet.

 

[Posi]

I know you said you replaced the RMS but it sounds like the RMS is leaking.
If you can feel a fingernail catch on the crank surface, it's too much and typically won't seal.


Cam plug maybe?

 

[TnCJ]

I know you said you replaced the RMS but it sounds like the RMS is leaking.

Agreed!

 

[DaveMueller]

I bought 2 seals in case I damaged the first, so I can try that.
Since I'm going to have to take it all apart again, how can I check the crank to know for sure if it's the problem?

I have an older engine with the 12 counterweight crank that wasn't leaking from the RMS. What would be involved in swapping it in? If I'm going to go that route, should I have it machined and get new bearings?

The distributor has a new gasket, and I sealed the dipstick with RTV.

 

[Lumpy Grits]

When was the last time you did a leak-down and compression check?
LG

 

[JeeperChris]

A little past the dip stick is where my AMC 258 i6 / 4.2l has it's oil pressure gauge tapped into the block right above the starter motor. You might check that. Mine leaked after I installed an oil pressure switch there so I pulled it out and re-sealed it and it took care of the problem. Liquid Plumbers tape and about 24 hours to cure fixed it right up.

 

[DaveMueller]

When was the last time you did a leak-down and compression check?
LG

Some of the valves are sticking open because of the fouling due to leaking valve stem seals, so I know I'm loosing compression. I had the head off to fix all of that, but couldn't get the valve retainers off so I got nowhere.

How does that cause an oil leak from the rear?

 

[JeeperChris]

Low compression could mean pressure blowing past the piston rings into the crank case causing oil to be pushed out the RMS.

 

[DaveMueller]

Low compression could mean pressure blowing past the piston rings into the crank case causing oil to be pushed out the RMS.

So how can I tell if it's a bad ring or partially open valve?
Rings - squirt oil in the cylinder
Valve - would show up as a jumping needle on a vacuum gauge
Right?

 

[Lumpy Grits]

Vacuum gauge needle will jump for many reasons.
Give the leak-down test a try and then you will know where to look.
LG

 

[DaveMueller]

Wow do I feel stupid. It was the drain plug for the oil pan. The air flow must have been blowing the oil up the backside of the pan. I figured it was easier to put a better washer and some teflon tape on the plug than to do compression and leak down tests, just in case it really was the plug. I'll drive it a few more times before I crawl under to check if there's oil coming from somewhere else, but at least there's no big puddle on the floor now.

I am going to do compression and leak down tests, lots of smoke at startup after it sits for a few days. I'm hoping its just bad valve seals. I have a 4.0 head that I'll probably put on since I'm hoping to go fuel injected in the future.

Thanks for all the help and the lessons.

Dave