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Smog Test Prep, Part 2

More on passing with distinction

In our first installment on getting ready for an emissions exam, we covered the many splendid facets of smog check and its application to your vehicle. We also touched on the first part of the test: the Comprehensive Visual Inspection (CVI). We are now ready to tackle the other two parts, the Functional Check (FT) and (Tailpipe) Emissions Test (ET).

Check Light

We should first point out that if you have a newer model vehicle ('95 or later), there's a little light on the dash that can tell you whether you will pass these two parts of the test or not. From that year forward, auto manufacturers were required to comply with new government rules involving on-board diagnostic capabilities of passenger car and light-truck engine management systems. This plan, known as OBD II (On-Board Diagnostics, Phase Two), required engine management systems to monitor emissions very closely.

On these vehicles, if the Check Engine MIL (Malfunction Indicator Light) is functional, and doesn't illuminate while the car is being driven, it's almost a slam-dunk that they will pass the next two phases of the test. (It still is possible that they could fail the first part of the test, though.)

You can see if you have an OBD II vehicle by looking near the top of the underhood inspection sticker. It should read "OBD II Certified," or the equivalent. If the sticker reads "OBD I Certified," or OBD is missing entirely, then you do not have a "Big Brother Equipped" vehicle. That is, it still may fail the smog test even though the check engine MIL doesn't illuminate while driving.

Individuals who own one of these earlier vehicles should continue reading for information on preparing for the next two parts of the test. Here are some areas of concern, and things to check and repair to maximize your chance of passing.

Functional Test (FT)

Check Engine MIL — Must illuminate when key is turned to "On" position; must not be illuminated while driving (testing). Only those fairly skilled DIYers should attempt to correct a problem here. Instrument cluster removal may be necessary to replace a burnt-out light. Access to OBD failure codes, either through a repair manual, or using a "Code Reader" (or both), will be needed to diagnose a "lamp illuminated" condition.

Fuel Filler Cap — Often overlooked by vehicle owners, an incorrect or malfunctioning cap will result in a test failure. Replace it if it has been in service for more than five years. (With fuel prices being what they are, consider upgrading to a locking style unit.)

Engine Idle Speed — Should be at or close to manufacturer's specifications. On later model vehicles, it may not be possible to actually adjust the idle speed, so a diagnostic/repair procedure will be required in such cases. Vacuum leaks, an improperly set throttle stop, or a malfunctioning Idle Control Unit can cause a problem here — often without a check engine lamp signal.

EGR Valve — Must be properly connected, functional, and the passages to and from the valve must be open. See our article, The ABC's of EGR, for more in-depth information on identification and testing. Service manuals are always handy, too.

Emissions Test (ET)

Engine Tune-Up — If your vehicle is about due, then by all means, do it before the test. When restricted by dirt and/or debris, the air, fuel, and PCV breather filters will have a detrimental effect on carbon monoxide and hydrocarbon emissions. Worn ignition system components can cause high hydrocarbon emissions. A worn or malfunctioning oxygen sensor will cause high carbon monoxide and hydrocarbon emissions. Follow manufacturer's recommendations regarding replacement of any tune-related component.

Oil and Filter Change — Old, contaminated engine oil often will cause higher carbon monoxide and hydrocarbon emissions. Change it, even if it's not due for another month.

Catalytic Converter — Even if your vehicle's engine and management systems are fine, an inoperative catalytic converter will result in a test failure. Take the vehicle for a moderate five-mile drive and let it idle (preferably on a level driveway). Put the transmission in "park" and set the parking brake. Take a very brief whiff of exhaust from the tailpipe (this is not recommended if you tend to have an unpleasant/allergic reaction to such things!). It should almost smell sweet, or neutral, at least. If it is somewhat acrid and irritating, then it is likely that the Catalyst is inoperative. You'll need to have a service pro check it and replace it, if your suspicions are confirmed.

Gasoline/Additives — A lot has been said about this subject as it relates to the emissions test. Our experience has shown that a quality, high-octane (super) unleaded fuel makes a difference. It promotes a clean engine and fuel system, and burns cleaner, as well. If you've been using the low-grade stuff (yes, we know that the owner's manual may say that you can use it), you should tank-up with super, and use maybe half the tank before taking the test. As for additives, we would suggest trying only those that claim to help your vehicle pass the test in cases of more extreme intake and fuel system deposits. In any event, such additives won't cure any serious mechanical condition.

Well, there you have it. Did you ever think that a simple smog check could be so involved? Really though, most of the procedures listed relate as much to good routine maintenance as they do to prepping for a smog check. They're not just pointless efforts required by the powers-that-be. You may find some satisfaction in knowing that your contribution to cleaner air also has a practical benefit for your vehicle.