New 240 owner here. I just took it in for the California smog inspection and got a fail across the board. HC was 117 and 70 at 15 mph and 25 mph, max is 116 and 91 respectively. CO at 15 and 25 was .53 and .30, max is .74 and .62 NOx at 15 and 25 was 1843 and 1075, max 791 and 730. The tech also said that the idle was around 1200 and should have been around 350. ALso, on the visual inspection he failed the Fuel evaporation controls. the car has 180k miles. and passed smog in Dec of 06. I have some experience working on my previous vehicle (92 Toyota pickup) and would like to become more mechanically proficient.

I would start with routine maintenance that may need to be done anyway, so even if I did something that did not help with emissions, I would not be wasting my time and money. Of that list, I's start with the least expensive stuff.

The bad news is that getting NOx down is usually an expensive proposition. Worst case scenario is a new catalytic converter and oxygen sensor.

You need to get the idle down to where it should be, and smooth. I'm guessing that you have vacuum leak(s). An idle target of 350 RPM sounds awfully slow for a 4 cyl engine. I have never had a tach so I don't know what it should be, but I would think at least 500 RPM. Others will know better than I.

You will have to fix the visible problem with the evaporative system (that rag you are using for a gas cap ;-) If the evap system problem is under the hood, is it a source of vacuum leak?

Pull off all the air inlet tubes and examine them for cracks. Take some carb cleaner to the throttle butterfly and the idle air control valve while you have those tubes off. Clear all of the vacuum ports you can get to while you are cleaning things.

Pull off the vacuum hose to the fuel pressure regulator and make sure there is no fuel in it. Your symptoms don't point to that, but you need to take a hard look at all your vacuum hoses anyway.

My experience with a '90 and a '91 is that vacuum leaks at the intake manifold gasket are not uncommon (those gaskets last less than a decade). The leaks are hard to diagnose but fairly easy and inexpensive to fix. You don't need to pull the manifold off, you just have to pull it clear of the studs.

Make sure that the damper in your air cleaner is feeding cool air when the car is warm. If it is not, you might consider a 'permanent' fix rather than buying a new airbox thermostat. Hot inlet air contributes a little to high NOx.

Look at your plugs. If the center electrodes are even slightly rounded at the corners, new Densos or NDs are not expensive, and can't hurt.

Pour a bottle of Techron through the fuel tank. I like to do this once a year to all my cars as standard maintenance. Don't know if it helps much, but it can't hurt.

A fresh oil change is usually good for a few ppm on HC and CO. A tank of premium fuel can knock off a few ppm of NOx.

I don't know what kind of ignition you have, but if you have any control over your ignition timing, try to retard it, or discourage it from advancing at low load. If the smog check guy checks your timing at all, he will likely check it only at idle. Reduced timing advance may give you the biggest free reduction in NOx, though you must not go too far, and you will want to put it back later or it will hurt your mileage.

Thanks a lot. That sounds like a great start as replacing the cat was the only thing I thought to do with my limited experience.

High NOx: It's common for the steel exhaust pipe between the cast iron exhaust manifold and the front of the catalytic converter to crack under the shields where you cannot see it. This lets in stray air and runs NOx way up. What you lose in repair bills on an old Volvo you more than make up for by not having car payments, so I try not to worry about what I am spending making it run right. FCPGroten.com has good cheap exhaust parts.