I just bought a 93 Ford Ranger and it failed smog.it had high NOX and HC. I changed the oil, spark plugs dis cap, wires, cleaned MAS, and it stilled failed. any Ideas?

 

Important to run it at highway speeds for about 15 miles, need to get the converter good and hot to have it working at max efficiency. Even run in drive rather than OD to work the engine and converter more, you want it hot. Do not shut it off waiting in line. You can add some gas drier (aka methanol) to the fuel to help, raise the idle speed if you can. Clean new air filter? Higher idle speed does wonders, can be hard to do on IAC controlled vehicles.

 

Well, a lean misfire comes to mind. Is your timing set correctly, high NOX often indicates too much timing and or the wrong octane fuel.

Carbon build up in the cylinders can cause high combustion temperatures which will cause high NOX. It also can prevent fuel from burning properly, which could cause high CO and HC.

 

NOx is 'burned' Nitrogen ... which isn't supposed to burn as it is an inert gas. It will hook up with the O's if it gets hot enough. EGR will cool the burn down, so it is worth checking that the EGR flows properly.
HC is unburned fuel of one sort or another. If the oil was fuel contaminated by age or leakage, it would allow vapor HC's through the PCV into the intake... extra HC's the computer didn't order and doesn't know about. It will try to cut back on fuel if it finds the exhaust gas to be too rich by measuring the O2 sensor output.
You could also have a buildup of crud on the inside of the intake manifold. The coating will hold extra HC's and feed them in just as contaminated oil will. You can use a spray cleaner to wash down the intake walls.
I do make the assumption that the air filter has been inspected and replaced as needed. The system will compensate for a dirty filter, and choke down fuel flow as needed to get full combustion. There's a test out on the web showing no problem until you get to XX% blockage, IF your system is working properly, but you do lose some power.
tom

 

1) Where did you buy it? The SELLER is responsible for a smog in CA. If you can take it back to the seller and make him fix it, you'd be money ahead imo. Unwind the deal if need be.

2) If you are stuck with it, post your numbers and limits that will help.

This thing should have OBD I. You can read the codes and maybe get a direction on fixes.

Search "how to read Ford obd1 codes" and find a bunch of results including this:

Ford OBD1 Codes and Procedure