Volvo 240 DL 1990. When I driving up a fairly steep hill, which puts the engine under more than usual strain, smoke pores out the tail pipe. It's grey white and smells more like stinky exhaust, not like the plastic smell of burning oil. One other thing to note, the constant idle value seems to be stuck, so the idle is high, and fuel consumption is also high. Lastly there's a bit of an oil leak. The other day my mechanic replaced the O2 sensor. The idle seemed to dropped for a bit but now it's back up. One theory is, the engine is running to rich and when the car hits a hill gas pours into the engine which over loads the cylinders and causes excessive engine noise and smoke out the pipe. Of course at this angle oil could be pouring onto the exhaust manifold and smoking.

If anyone has an ideas would be great.

You appear to have about 3 unrelated issues and none of them likely point to the O2 sensor. First: pull the codes...it requires no tools and takes just a couple of minutes.

The idle is controlled by the computer unless someone adjusted the idle stop. Clean the throttle body follow the instructions the FAQ's for adjusting the throttle position switch and idle stop.

Check for vac leaks...ask here if you are not aware how.

Check to see if cold start injector is leaking (remove it, put it in a jar and cycle the key a few times assuming you get the prime action of a couple of seconds when you turn the key).

Finally, the "clogged cat" is a very likely scenario but what more often happens is the matrix of the cat degrades, falls appart and clogs something downstream of the cat. If the bolts will loosen...unbolt the flange at the front of the cat and pull the pipes apart. It is a dangerous to drive around this so I do not suggest it as a troubleshooting method but...it has been done before.

All of these hinge on the assumption that you have already check for coolant in your oil or oil in you coolant.

Rich mixture tends to produce black smoke.
Gray/bluish white is oil burning.
Pinging sounds like a hammer hitting metal.
A stinky smell, like rotten eggs, I believe is caused by a bad cat.

Pinging can be caused by excessive spark advance or high compression or hot spots do to deposits in the combustion chamber and on the piston top.

I'm not familiar with a late model fuel and ignition systems but the "computers" might be trying to compensate for a bad cat by leaning the mixture and changing the spark advance curve.

What octane are you using? You could try a tank of the highest you can find just to determine if it's pre-ignition.