I'd check a couple of things - check for a plugged up exhaust system, which
will occur if your catalytic converter/s self-destructs (they do!). Any
competent (independently owned) muffler/exhaust system shop should be able
to help you out - they may charge you a little for the labor, but it would
probably be money well spent. You could also simply drop the exhaust system
down a little at the exhaust manifold and if the engine runs properly (
besides being very loud), you'll know that you have a plugged up exhaust
system. That's what I think this problem is.
Second, run a compression check on the engine - to ensure you don't have a
cracked cylinder or head. Normally, cracks in the head or cylinder will let
water into the oil or oil into the water, but you never know. The only place
fuel can get into the oil is in the combustion chamber, and the only way
that can happen is if you have broken rings, which you may have. I worked on
a van, years ago, that had been run in Mexico. It had a catalytic converter,
but they ran leaded gas in it, and plugged it up solid. They kept running
it, and ended up breaking every pistion ring in the engine, simply due to
the back pressure. I hope yours is not this way, but it bears checking out.
If the compression is low, or uneven throughout the engine, consider going
further on this issue.
Hope this helps. Let us know.
r/
Steve