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Check your fuel pressure regulator , i had the same problem.
if the smell is coming from the engine compartment it could be the batteries going bad.
A sweet smell indicates engine coolant. It you can't see any leaking from the hoses in the engine compartment, it's probably the heater core. With the smell coming from the vents too, it's most likely the heater core, but rule out easy fixes first.
Because You Do How The Hell Will I Know That : )
Try replacing your Oxygen sensors they are located on the catalytic converter. I had the same problem with my 94 Caprice and that turned out to be the problem.
There is a fuel leak in the engine compartment. Have a mechanic look at the engine.
A smell coming from a reverse cycle air conditioner may be a signal that the filter needs to be cleaned. A smell like engine exhaust might be present if the air conditioner motor is in need of repair.
Have a look for slow oil leaks - the oil leaks then when the engine gets very hot the oil burns.
Check for a sickly sweet smell. The heater core is probably leaking coolant into the floor. The core is not repairable but can be replaced.
build up or clucth
engine timing
You may have a bad cat (and/or engine is not properly tuned). Make sure engine is properly tuned and do not use heavy acceleration until engine is warmed up.