Installed this Mondello intake manifold ten years ago on my Olds 455 in my '55 Chevy 4x4 half ton stepside:

The motor started using a lot of oil. Before I caught onto what was happening, I did a long trip with a fairly heavy load on the truck, and pulling a grade on a narrow road in southern Oregon in heavy traffic where there was nowhere to safely pull off the road, the oil pressure went to zero! By the time I got to an area where I could pull off the road safely, and hitch a ride into town to get more oil, the motor had run a mile or so under load with no oil pressure!

I had to add at least 3 qts to a 6 qt pan to bring it back up to full, and the motor continued to run another three years without ever using any oil, after I changed the intake manifold. These Olds motors are tough!

I pulled the intake and the inside of this manifold was soaked with oil. When I put gas in the plenum it poured out through the bottom of the exhaust crossover. You can't see a casting flaw in the bottom of the exhaust crossover, but you can see it on the inside the plenum. This sort of flaw is called a "cold joint", it comes when the melted aluminum is too cold or the mold is too cold when the aluminum is poured into the mold, and by the time the molten aluminum from two directions meets in the middle, it is too cold to make a proper joint, leaving an unsealed flaw like this that will pull oil up into the intake system via the vacuum in the intake system.

And as the bottom of the exhaust crossover leaks gas when I pour it in the plenum, and it is plugged at the Edelbrock heads, there is a vacuum in the exhaust crossover caused by the intake plenum leak, which sucks oil up into the exhaust crossover, then up into the intake plenum. It was a mess when I pulled the intake off the motor.

This view down inside the plenum from the top shows that casting flaw, or "cold joint".

When I went to the Joe Mondello group about this problem, they shined me on, and would not take any responsibility for this problem, and would not discuss replacing the manifold or helping me out in any way.

If I cleaned this intake thoroughly in solvent, probably letting it soak for quite awhile, changing the solvent a couple of times, I could probably get it clean enough to coat the bottom with an epoxy of some sort and seal it off.

As it only wants to suck oil up into the plenum via engine vacuum, I think that would solve the problem, but am thoroughly bummed out by the Mondello group, and although they had plenty of business from me over the many years I have been using Olds motors, not anymore!

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