Smogtips
CHOOSE YOUR CITY OR ZIPCODE:
Within:
SELECT SMOG CHECK TYPE:  

Join the community and post your question. Use our Ask-A-Tech for free now! ASE experts standing by.

Forum > Failed Smog Test > Question

1991 Volvo 740 station wagon failed the enhanced smog test for high NO. What do I do?

My '91 volvo 740 wagon failed the enhanced smog test, slightly over the max on NO(PPM) emissions. I am from out of state trying to get registered here, so am unfamiliar with what next. I have only owned the vehicle for 3 years so don't really know its maintence history. The technician suggested an overall tune-up (spark plugs air and fuel filters..) or more expensively may be the catalytic converter. Do you know what would cause particularly the high NO admissions when everything else passed? And/or what I can do to avoid exorbitant repair fees for an old vehicle with over 230,000 miles? Thanks and hopeful, Robin
Answer:

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.


posted by SmogTips Support

 

View SmogTips.com in: Desktop / Mobile