By DieHardFord (Steve)

I used to run smog checks at Advanced Automotive in Merced, CA. There are MANY things that most of you are capable of doing yourselves to aid your vehicle in passing smog that aren’t always things that you would think of.

Starting with the obvious:

Check plugs, wires, cap, and rotor. Misfiring and incomplete combustion is the starting component of smog failure.
-Check your Vacuum diagram, improperly vacuum hookups will not only cause you to fail, but it will usually render the vehicle in the gross polluter category.

Set your timing with dial in advance light, the tolerance you are allowed is 3degrees either way of the spec for your vehicle.

With Carburetors, if you are in doubt at all, have a rough idle, or smell fuel, At LEAST have your carburetor rebuilt.

Check your PCV , EGR valve, and gas cap, these are components that are actually tested for function. No function equals no pass.

Check your Intake Manifold and vacuum lines for leaks. This can be done easily by spraying lines and edges of intake with Carburetor Cleaner. If you notice a change in your engine’s sound, you’ve probably found a leak. Get these fixed, as they will raise your Hydrocarbons greatly.

Use a full tank of premium gas

If you have a check engine light and it stays on while running, check the code for something emissions related, because if it is emissions related you will fail on that right there even if your emissions are clean.

Changing your oil before a smog check is always a good idea, because hydrocarbons always get into your oil and they are sucked in through the PCV Valve. Not only will having fresh oil reduce the hydrocarbons that are there, fresh oil will more readily absorb the hydrocarbons that get by rather than have them get sucked in through the PCV Valve.

A few tests you can do:

Catalytic Converter- Pull one spark plug wire with the vehicle running (Carefully) and ground it. Use a raygun thermometer to measure the temperatures across your catalytic converter. If it is good, you will see a 100 degree difference between the 2 sides after a minute or 2 of run time. (If stuff starts glowing orange, your cat is GOOD, stop testing)

PCV Valve- Pull it out of the valve cover and cover the bottom of it, you should hear your engine change tones, uncover it and it should change again

EGR Valve- disconnect the end of the vacuum line that is not connected to the EGR valve and suck on it, your engine should sputter severely. If not, your EGR passage is clogged and needs to be unclogged, usually involves removing the intake and digging carbon out of the Exhaust crossover passages, this is a P.I.T.A for 2.8’s. A good way to do it is take a piece of broken speedometer cable, and stick the frayed end in a drill and stick it down into the passages (screwdrivers can be used on what they can reach).

Gas Cap- Look at the rubber seal, if it looks cracked it will more than likely fail.

If you have access to a gas analyzer, probe your tailpipe and adjust your carburetor there. Also, stick it right at the opening for your oil filler cap if you haven’t changed your oil in a while and you’ll see exactly what I’m talking about.

Add ons that help:

MSD Ignition or other module- AWESOME! Sometimes these will pass vehicles single handedly because they pretty much eliminate misfires, plus it means performance. Perfectly Legal as well.

Catalytic Converters- Your vehicle already has one, BUT, there are “Super Cats” out there. Sometimes you can put more than one in your system, this will help also. Talk to your local exhaust shop.

Add ons that can hurt:

You have to be careful what you do to a smog controlled vehicle. Mods should be something that is easily reversible for smog time if they are not legal.

Cams- Your cam should specify if it is smog approved. This will not hurt your emissions. Most cams that specify that they are ECM Compatible (Fuel Injection) are also able to pass smog. While it is not impossible to pass smog with a radical cam (I put my friend’s ’74 F100 with a ’70 460 interceptor built well through with flying colors), I can vouch that it is much more difficult.

Engine Swaps:

Basically, If an engine of the same size and family was available, you are allowed to put any version of that motor in your vehicle legally as long as it is the same year or newer. (I.E. 4.0 from a ’92 Ranger into an ’83 ranger) as long as all of the swapped in motor’s emissions components are there and working.

If you go outside this rule, you are able to go to the smog referee and have this checked out. I don’t have the contact information off hand but you can get it at any smog shop. They will give you a sticker or a plate stating that the motor has been swapped and it’s emissions approved.

As for all of you 3.0 and 4.0 Bronco II’s, you will probably have no problems getting your vehicle referee approved because you can tell them that it is the same setup as your year ranger, which had these engines as options. 5.0’s, that’s another story. Talk to the referee before hand about it because they were original in ’96 explorers, and if you use one from that year I bet you can get by with it.

As much of a pain in the ass smog is, I hope this helps you all. If you have any questions, I am “DieHardFord” on TheRangerStation, and I intend to ask the management about letting me have a Emissions section to moderate. Experience with what causes what is the only way to figure out a problem sometimes, and anyone that’s had to smog a vehicle and failed knows EXACTLY what I’m talking about. Costly repairs and throwing parts at a vehicle trying to come up with answers. Again, hopefully this helps!


Please let us know if this needs updated.


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