Toyota Repair: 89 Corolla smog failure & surging idle, intake vacuum leaks, spark plug wires

Question
This car failed the smog test with high HC and high CO on both 15 and 25 mph tests.  HC (380 & 345) and CO (6.71 & 5.04).  After several diagnostics (Toyota & others), I’ve replaced the catalytic converter, O2 sensor, carburetor and distributor cap & spark plug wires.

The car has most recently passed for both HC and CO, but failed the 15 mph test for NO (1117).  The 25 mph NO was a pass (449).  I think this failure was because the hoses from the EGR Modulator to the carb were misrouted.  I’ve corrected that and used carburetor cleaner to check for intake vacuum leaks and found none.  I’ve backed the mixture screw out ¾ of a turn from the point where the engine stumbles and dies and that seems to smooth out the acceleration.  And I’m not seeing any trouble codes.

Currently the car is surging at idle and seems to be surging a bit a speed and it dies at stops.  I’m wondering if  the EGR valve might be the problem.  What do you think?  Do you think the misrouted hoses might have also lead to incorrect readings on the last smog test?

Thanks, Ted!

Answer
High NOx readings are almost always caused by an EGR problem which will also affect the idle, the fact that the car passed at 35mph indicates this, the EGR valve may be sticking open which will affect the nox readings at lower speeds but not at higher speeds when it is supposed to be on anyway,I would try replacing the egr valve.