Well, I took my car for a smog test on Saturday, it failed. I had excessive oxides of nitrogen. I had it diagnosed by this guy I was refered to and he said I had a bad EGR valve. So I repaced it and went back for retest today, but I ended up with the same results. I'm really stumped, just need some advice of where to start with the toubleshooting. Hydrocarbons are below average so I am pretty much ruling out the cat. Overall the car runs awesome, any help would be appreciated, thanks.

 

Generally, NOx is a sign of the engine running lean and/or hot. The fact you have low HC's would seem to confirm its on the lean side. What did the CO look like? Do you have any actual numbers.... your results vs. pass/fail limits for all three - NOx, HC and CO?

EGR is there to reduce NOx.... introducing exhaust to the combustion chamber lowers the combustion temps, reducing the formation of NOx. What led him to believe the EGR was not working? Did he scan it for codes? If so, which one(s) did he get? There are several reasons it might not be working, including cracked vacuum lines, faulty EGR vacuum solenoid, plugged EGR passages in the intake manifold, insufficient backpressure on the exhaust system, or a faulty EGR valve. Replacing the valve may not fix the problem.

 

He said the egr did not hold vacuum, I got a new one on there now with the same problem. I have a scanner and it didnt give me any codes when I read it out.
15 mph: HC measured 33 max 83 CO measured .06 max .48 NO measured 1115 max 689.

25 mph HC measured 12 max 48 CO measured .00 max .46 NO measured 958 max 706.

My vacuum lines are good, I dont know how to check the egr solenoid, and the intake manifold passages should still be clear. I resealed the intake manifold gaskets last year and have since only driven about 2000 miles. I did have my cutout open for the drive to the smog shop, I closed it like a half mile from the shop. I dont think the computer would have to relearn any fuel air curves would it? Some people say that if you throw a new cat on there it will definately pass, but I dont want to buy one if I dont have to.

 

I'd say its running hot and/or lean. The HC and CO are low enough to indicate its not rich, and they would indicate that at least the oxidizing portion of the cat is working fine. The monolithic bed includes both oxidation (CO->CO2; HC->H2O+CO2) and reduction (NOx-> N2 + O2) catalysts. Its possible the reduction catalyst is weak.

To test the EGR system.... Key off/remove the EGR vacuum line from the side of the intake manifold, and pull a vacuum on it. You should be able to pull full vacuum, but not have the EGR valve move. That would confirm the solenoid is blocking the vacuum line when it is not energised. Same test - pull a vacuum, but this time supply +12V and a ground to the EGR vacuum solenoid. It should open, and it should cause the EGR valve to open fully. That would confirm the solenoid is working with 12V applied. If there was any sort of wiring problem between the PCM and the solenoid, you would have DTC 27. If the EGR valve was not opening and supplying the required volume of exhaust gas, its possible you would have DTC 32 for inadequate circulation, but in OBD-I the MAP-based diagnostic is not very sensitive. It's possible to completely eliminate the EGR valve and not set the code, because of the weak diagnostic.

 

Thank you for your help, I'm gonna check it out later this week