: Re-manufactured HT4100 engine, oil is leaking at exhaust manifold by cylinder 6.

12-23-12, 09:27 PM
Looking for idea at why this may be. I recently had my original engine replaced by a re-manufactured engine and had a relative do the install. During the break-in period of the new engine, noticed an oil leak. Discovered leakage on left valve cover, wasn't torqued properly, same with oil pan. Tightened both those down and that seemed to fix leaks in those areas..

Now, what I originally thought was an just exhaust leak on the right side manifold turns out to also be leaking oil. I've got UV dye in the oil and I've confirmed that the valve cover above and the spark plug socket next to the leak are dry. There's a head bolt next to the leak but it's tight and oil is not coming from the bolt. I had pulled the exhaust manifold bolts and noticed one of them (double-ended stud that the AIR pipe bracket mounts to) was covered in oil. Without pulling the entire manifold off, I cleaned off the bolts, cleaned up the threads, and retorqued them and the leak persists. I can see the oil collecting and bubbling right where the exhaust manifold meets the head (There is a gasket in place, btw). It then leaks down and burn off of the manifold.

Any idea why oil may be leaking from here? Engine seems to run fine, otherwise. I don't have oil and coolant mixing anywhere so I believe the head gasket is ok.

I don't have the means to pull the engine myself but I can pull the manifold off and investigate further if I knew what I was looking for.

12-24-12, 08:54 AM
Perhaps the valve cover gasket is not seated correctly or damaged.

12-24-12, 05:07 PM
Thanks for the suggestion, drewsdeville. I thought that, too, and that was one of the first things I checked. The valve cover area is dry as a bone and I checked all the screws for proper torque. The oil is definitely seeping up from the top side of the exhaust manifold.

Also, since my original post, I checked all the cylinders for compression and they are all good so I'm not assuming head cracks or bad pistons/rings causing blowback at this point and there's no fluids mixing so I'm not assuming a bad head gasket just yet (plus the whole manifold is above where the head gasket is anyway). Also, there's no oil fouling the plugs, they look normal.

Since the manifold bolt I pulled out was oily I'm wondering if maybe that bolt hole is threaded too long and runs into an oil galley.

I had also thought about the possibility of bad valve seals. I suppose if the exhaust valve seal was bad, then it could leak oil into the manifold without fouling the combustion chamber. But I'm an amateur in these matters, so hopefully someone more knowledgeable might be able to tell me if that theory holds merit.

There's also this some information in my shop manual about "casting porosity" where oil can just leak through porous surfaces of the cast heads or block. But I've never heard of such a thing before and not sure how common an issue it is, just trying to rule out the simpler things first.

I added picture to illustrate roughly where I'm talking about. The image is the other side of the engine from where my leak is. So basically, on the opposite side of the engine and now the I'm looking, I got my cylinders reversed so it's cylinder 5, not 6 as I stated before. Anyway, other side of the engine, the red dot represents the bolt hole that I pulled the oily bolt from, the red line indicates the area where I see oil leaking between the manifold and head.

12-25-12, 11:54 AM
The heads are cast iron so porosity is not the issue. I would remove the stud and clean out the hole with Brakleen, then run the engine for a bit. Then I would insert a rod of some sort into the hole and remove it to inspect for oil on the rod. I think it's the valve cover myself. Does this engine have sheet metal valve covers or the 1985 cast aluminum ones?

12-26-12, 06:59 AM
It sounds like the OP has done some good diagnostic work in tracking down this leak. He must have been wiping down the engine to pin point the leak and he says it is bone dry just below the valve covers. That exhaust stud must thread into the oil gallery somehow whether it is supposed to or not. All I know is to try to seal the threads on that stud like on some head bolts.

Just thinking out loud: I have taken out severely oily crusted exhaust bolts out of small block Chevys and never gave it a whole lot of thought as to where that oil and sludge came from. Those exhaust bolts on a sbc are not threaded to an oil gallery so that oil found it's way there some other way; most likely valve cover leaks

All I can think of is, if the engine has been wiped down a few times and the vc area stays clean then you may be right about the oil coming from that stud hole.

Actually the more I think about it, there can't be any oil lines or passages in any cylinder head so that can't even be possible. I think some more wiping and investigating may be needed.

12-27-12, 01:08 AM
only way I can see that bolt collecting oil is if it goes all the way into the valve train, and you would be able to visually verify that with the valve cover off

12-27-12, 05:24 PM
there's no way the exhaust manifold stud passes through the head and into the valvetrain.