LT1 High NOx level
#1
03-13-2015, 10:28 PM
LT1 High NOx level
So I bought a 97 z-28 3 weeks ago 97,000 miles. This is my second so I am pretty familiar with the LT1. I took it to get it smogged and it is failing due to an elevated NOx level. Note I said elevated and not high, and only on the 15 MPH test. The levels were 669 ppm (Max allowed 419) at 15 mph and 660 at 25 mph (max allowed 706. Everything (CO, HC, Co2) else is well with in range.
So here is what I have done. New optispark, new plugs, new O2 sensors (upstream), fresh oil and fuel system cleaner. I forgot to mention that those readings were the second attempt after the repairs. Prior to the repairs the levels were actually lower. 548 at 15 MPH and 587 at 25 MPH.
I know the higher levels are caused by higher chamber temps. I pulled the EGR and checked it. It is functioning normally. It holds a vacuum and the diaphragm is working fine. I know the chambers are clear because the engine begins to stall out if I open the EGR by hand.
One thing I am thinking is that maybe the EGR solenoid is not functioning properly. I came to this determination because the levels don't seem to change with the different MPH. They are almost identical.
At this point I am stumped. I am a mechanic by hobby, but not a professional. I have read that changing the cats alone will not reduce the NOx levels, only lowering chamber temps. I thought about running some seafoam through it to see if there are deposits on the cylinders and valves causing hot spots. Other than that, I am out of ideas. The ambient temperature of the air outside was warmer during the second test, and the engine was also warmer, which may account for the higher readings. The smog tech thought I should bring it by first thing in the morning while the air temperature is cooler and the car was cold. But I don;t know if that will be enough to lower the levels enough to pass.
HELP!!
So here is what I have done. New optispark, new plugs, new O2 sensors (upstream), fresh oil and fuel system cleaner. I forgot to mention that those readings were the second attempt after the repairs. Prior to the repairs the levels were actually lower. 548 at 15 MPH and 587 at 25 MPH.
I know the higher levels are caused by higher chamber temps. I pulled the EGR and checked it. It is functioning normally. It holds a vacuum and the diaphragm is working fine. I know the chambers are clear because the engine begins to stall out if I open the EGR by hand.
One thing I am thinking is that maybe the EGR solenoid is not functioning properly. I came to this determination because the levels don't seem to change with the different MPH. They are almost identical.
At this point I am stumped. I am a mechanic by hobby, but not a professional. I have read that changing the cats alone will not reduce the NOx levels, only lowering chamber temps. I thought about running some seafoam through it to see if there are deposits on the cylinders and valves causing hot spots. Other than that, I am out of ideas. The ambient temperature of the air outside was warmer during the second test, and the engine was also warmer, which may account for the higher readings. The smog tech thought I should bring it by first thing in the morning while the air temperature is cooler and the car was cold. But I don;t know if that will be enough to lower the levels enough to pass.
HELP!!
#3
03-14-2015, 01:11 PM
It was throwing a couple of codes, one from a vacuum leak in the evap system. That was one of the first things I fixed. I have been running premium fuel in it since I got. Its in the shop right now getting new cats put on it. Since it is so close to passing I am really hoping that will put it over. I will know by tonight.
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