The exhaust from the car stinks when engine is rev'd though not too bad while idling. Mechanic tuned the engine and checked it out but said emissions in normal range & no leaks in exhaust system (didn't try to explain the smell). I did a quick check starting the engine when cold and then rev'ing it: water from the tail pipe is sooty black and sprays out (don't remember seeing it like this in the past), and there is cloudy white "smoke" (temp was about -1C/30F so maybe ok); I can't see gas or liquid coming out anywhere around the engine or under the car. There is a somewhat rough idle and on rare occasion might stall just after starting, but very smooth once driving such as last week with two 300 km drives and a fair amount around town - like new mileage and power. Looking for suggestions on what the issue might be. Here has been my thinking:
* Smells could come from combustion gases, coolant, burnt oil or excess gas. Since not sweet, black or blue, I am guessing there is a rich gas/air mix resulting in excess combustion gases that smell, or some issue in the catalytic converter. Can't see a back pressure issue without poor mileage, more stalling or lack of power.
Possible culprits:
* O2 sensor but from what I read mileage should have dropped.
* Regulating valves but again looks like I should have seen a mileage drop.
* Doesn't sound like electronic ignition issue and I just replaced plugs before the 300 km drives.
* Catalytic converter is pretty old but I expected mileage problems if getting plugged. Seems as good a prospect as any.
* Muffler?? (resonator was replaced a year or two ago)
* Exhaust gas recycle issue??
* ECU issue??
* Head gasket - white smoke but exhaust should have had a sweet smell from coolant rather than plainly stinking.
Engine was pulled and new gaskets a couple of years ago. The smell issue started some time after that though not sure exactly when.

 

My opinion:

Charcoal canister (behind driver side head light or under the car behind the same headlight.it has small lines going to the intake)

Cat Converter: if you don't live in California, you are in Converter heaven as I pay a higher price for one. OBD1 in your case...if you have the oe that bolts up...take it out (lots of wd40) and clean it.
My way of cleaning:
1 bucket
4 to 5 gallons of "distilled water"
Laundry soap, 1 scoop...

pour soap, pour regular water(war or hot) let it soak for 1 hour
grab converter and pull out of water and empty out, submerge and repeat process a few times.
change regular water with "distilled water" add a bit of soap
let it sit for 30 minutes....then submerge pull out, submerge pull out...a few times..
shake it until most water leaves cat
grab a blow dryer and duck tape it to one end or just hold it at either end and run it till cat inside is dry...you will see extra water that is stuck in the honeycomb.
steal your mothers/wife/sister/girl's acetone and pour liberally...you will notice the areas that has penetrated all the way through when you hold it up to the light and see blocked areas with open areas...aim for the blocked areas....blow dry and there you go....NOW THIS IS IF YOU NOTICE THAT WHEN YOU FIRST TAKE OUT THE CAT AND HOLD IT UP TO THE LIGHT YOU CANT ANY LIGHT PIERCING THROUGH.....IF THE HONEYCOMB IS MISSING THEN YOU NEED A CAT..OR IF IT RATTLES...

 

Sulfur smell is a sign of a failing catalytic converter.

BTW, since about 1976 all states have catalytic converter requirements...

 

I mean take it out to clean it and put it back on...not to leave it out....if you don't live in California, purchasing a new cat is cheaper elsewhere. In Cal I pay around 180 for a cat...not installed...everyone else can buy an OBD1 or universal NON CARB compliant for around 60 bucks...