Your 1991 Volvo 740 station wagon's high NO emissions would normally be due
to defective EGR system components, i.e. the EGR valve, EGR solenoid, and/or
EGR exhaust passages. IF AFTER... all emissions systems are inspected and no
faults found, consideration should be given to the CAT. We can not recommend
replacing the CAT before a through diagnosis.
The EGR system is prone to collecting carbon build-up. Since the EGR valve
pulls exhaust gasses from your exhaust and introduces them back into your
engine's intake air over time it will become clogged or stuck open/closed
from the carbon deposits.
Some other faults which could cause your 1991 Volvo 740 to fail the smog
check for high NO are:
1. Bad Engine Cooling - If your Volvo 740's cooling system is not working
efficiently, chances are there is an excessive amount of NOx being created.
NOx (Nitrous Oxides) is created only when an engine's combustion chamber
temperatures reach over 2500F. A bad cooling system will create NOx. For
this reason you want to make sure your vehicle's temperature gauge is always
normal and that your cooling system is working properly. If your vehicle has
an overheating problem, be sure to perform the necessary repairs.
2. High Cylinder Compression - Carbon build-up within your Volvo's engine
combustion chambers will cause increased engine temperature and high NOx.
Carbon build-up normally develops in an engine's combustion chambers over
some period of time. High increase in carbon build-up causes increased
cylinder compression, which causes high temperatures, which result in high
NOx emissions. Keep in mind this problem is usually seen in vehicles with
over 200,000 miles. The solution to this problem is called an Engine
DeCarbonizing. It usually costs around two labor hours at a smog repair
station. It will remove a good amount of carbon of your engine's piston
heads and valves. This will increase combustion space, lower compression and
lower NOx.
3. Lean Fuel Mixtures - Lean fuel mixtures cause high NOx also. A lean fuel
mixture is when the engine receives less fuel then is necessary to obtain
efficient combustion. Lean fuel mixtures are often due to vacuum leaks.
These leaks will allow the suction of additional and un-metered air into the
combustion mixture or exhaust (depending on where the vacuum leak is
located) disturbing pre/post fuel combustion and increasing NOx emissions.
Vacuum leaks are often hard to diagnose as they may be present at locations
not easily viewed.
4. Defective CAT - This is your Volvo's final emissions component to be
suspected after all else fails by California Law. The Catalytic Converter,
commonly referred to as the CAT is a component designed to continue the
combustion process within itself and emit a more thoroughly burned and less
harmful emission containing exhaust. It is normally placed between your
engine's exhaust port and your car's muffler. It allows the engine exhaust
to pass through while it reduces all emissions. A good CAT should
dramatically reduce HC, CO and NOx. The CAT is a catalyst, it plays a very
small part on how well your engine runs and it only begins working after
slightly rich exhaust has exited the engine's combustion chambers.
