Exhaust odor from engine

1998 Nissan Altima 2.4, 150k.

I have noticed a strong exhaust odor coming from the engine compartment/front of my car. The odor is choking (no sweet smell, no smoke). It is especially noticeable when I start the car, but is present even when idling, though I am not able to smell it while driving. No fluid leaks. Exhaust pipe looks good.

Where should I start for a diagnosis? I have thought of injectors, possible vacuum leak, and timing, but not sure where to go.