falcon63

Took my 88 vert to a "test only" station, per DMV for reistration renewal. On the low speed test it read 301 HC ppm, .07 CO%, and 952 NO ppm. Max allowable readings in the same order are 121, .76, and 807. On the high speed test it read in the same order 296 HC, .25 CO, 499 NO. Max allowable readings in the same order are 96, .65, and 746. Because of the high HC readings, it is now classified as a "gross polluter." I then took the car to a Bureau of Automotive Repair recommended smog repair shop. They replaced a broken engine coolant temp sensor, found the O2 sensor voltage fixed at 50 mv all the time, the EGR valve not holding vacuum, and no vacuum to the EGR. There are no rotary specialists in my area and the local dealer's mechanics don't seem to have a clue. Can anyone help me to get a handle on which way I should go with this aside from replacing parts on the possibility that one might work?

scrip7

iTrader: (6)

When the egr valve is working properly, it lowers NOx. If the egr valve does not hold vacuum, the valve itself needs to be replaced. And in order for it to work properly, it has to have vacuum to it from the egr solenoid. The solenoid could be defective or have a poor connection either at it's electrical plug or the vacuum hoses. With low CO readings and elevated HC, I suspect you have a small vacuum leak.
(1) have your egr valve replaced and check the egr solenoid

(2) do a vacuum leak test, either the propane method or compressed air method.

(3) tune the engine (spark plugs, check plug wires, etc.)

How old are your catalytic converters?

twistedriver

Check to see if the engine is over heating or running hot, that could cause high NOX.
Inspect timing which can cause hight hc, nox.
Posting the O2 and CO2 reading will also help. The higher the Co2 the better the car is running.
If the car has an orginal cat a replacement will help.

scrip7

iTrader: (6)

I agree with twisted. Over-advanced timing gives you high HC, and an O2 reading would confirm a lean mixture. A Co2 reading would help determine the converter's health.

falcon63

Gentlemen, thank you for your advice.
The cat was replaced 2 years ago at the previous smog test and every reading was well below the requirements.
The current low speed readings are 11.15 %CO2 and 5.31 %O2. The high speed readings are 12.19 %CO2 and 3.89 %O2.
The ignition timing is A22, no overheating problems at all. I don't even have the miss at 3800 RPM. The car is averaging 22 MPG on 87 octane as I'm commuting about 100 miles roundtrip at freeway speeds
Spark plug wires were replaced about 2 months ago. The plugs have about 20,000 miles on them.

drftwerks

iTrader: (1)

adv timing is also high NOX

scrip7

iTrader: (6)

Start with the obvious egr solutions mentioned in post #2 and although the mileage is relatively low on your plugs, it's not a bad idea to do a fresh tune before doing another smog check.