My 1989 T/A 305 TBI will not pass emissions testing. It is failing because of NOX and THC. What is wrong and how do I get these levels to acceptable? Thanks.

 

Sounds like it needs a new cat.

How old is the one that's on it?

 

I do not know for sure but would guess that it is the original CAT.

 

A new cat may lower the readings, but an old cat probably isn't the cause.

High NOx is caused by high combustion chamber temperatures. The factory contolled this via the exhaust gas recirculation valve (EGR valve), which introduces a little exhaust gas to the intake charge to dilute the mixture under high combustion temp conditions - which has the effect of lowing combustion temp. It is pretty typical to have the EGR passages plug up over time, especially older engines with compromised valve stem seals. Removing the intake manifold and cleaning the passages usually helps. It could also be a bad EGR valve.

High hydrocarbons is caused by incomplete combustion. This is typically a cylinder misfire. If you haven't done a complete ignition tune-up yet, you'll probably find the issue in the spark plugs, plug wires, cap, or rotor. It could also be a weak coil or ignition control module, but if you aren't noticing any other performance problems, it probably isn't one of those guys.

 

OK, I am a dummy. Where is the EGR Valve?

 

On the intake manifold. Beside the throttle body unit, passenger side.

 

Does it have two large round disc like objects on either side?