I am hoping there is a smog technician that can help me. I am trying to get my 1994 Z28 smogged. It has very high mileage but runs great. The car is 100% stock. Oil & filter are new. No check engine light. Smooth idle. My smog results were:
15MPH CO2-14.9 O2-.1 HC-7 CO-.01 NO-750.
25MPH CO2-14.9 O2-.0 HC-10 CO-.01 NO-529
I have checked the EGR & replaced it due to a sticky valve. The ports are open-there is exhaust & vacuum at the ports. If I open the valve manually the car runs very bad. I checked the operation of the solonoid & the vacuum hose-all ok. I also replace the catalyst with a new California approved part. The numbers seem to say that the car is running lean. I smoke tested the intake yesterday & cleaned the air mass meter-no leaks. The O2 sensors are fairly new Delphi units. They are scanning fine. Fuel trim is about 125. Any Ideas would be greatly appreciated.

 

High NOx can also be the result of high combustion chamber temperatures. Is the engine running excessively hot?

 

Thanks for the quick reply. No the engine temp is normal. The water pump, thermostat & radiator are all just a couple of years old. The cooling fans are working normally.

 

I just tried to test the new EGR valve I applied vacuum to open it with the engine off. With the engine running I can't open it with vacuum. I ran the car with the rear wheels off the ground & the valve tries to open when i accelerate but not much. Is this normal?

 

Where are you applying the vacuum? You would have to connect directly to the EGR valve, since the solenoid only opens under certain operating conditions, and is pulse width modulated, meaning its not going to allow full vacuum when PWM is significantly less than 100%.

 

You try seafoaming the motor for excess carbon build up? Little seafoam and some deep creep, 20 bucks and very easy to do.

 

I just ran the car with a vacuum gauge T'd in between the EGR solonoid & the EGR valve. The egr comes on full at lower speeds and moderate throttle. At faster speeds & more or less throttle the egr reduces or shuts off. The vacuum varies between 6 psi & 15 PSI. From what I read is the service manual this seems normal? I think the problem may be that the car is running lean? Any ideas?

 

i have the same issue well could be, I hope you can get it resolve and post the answer. what i did was disconnect the vaccum line going to the solenoid and attached another hose directly to the egr valve and it pass smog.

 

I scanned the car last night. The fuel trim looks off to me?
Blm 016
Temp 094
IAC 021
IAC learn 021
IPW1 2.5
IPW2 2.6
Idle 065
LTFT1 116
STFT1 128
MAF 056
MAPV 1.15
MAP KPA 031
LTFT2 118
STFT2 128
ADV 19
TPS .64
It looks rich but the smog numbers are lean. Any help would be appreciated
Thanks
Walter

 

Just find someone who will pass you for an extra $150...thats what i did when i lived in that tree huggin money grubbin state.

 

I know I can do that but I want to fix whatever is wrong.

 

The fuel trims are the values that the PCM is using to bring the A/F ratio back to the target 14.7:1. It would be running rich on both banks, but the PCM is subtracting 7.8% (left bank)/9.4% (right bank) of the fuel it would normally supply to keep it from running rich. Additionally, since the engine is idling in that single frame of data, you are only looking at the LTFT's for the idle condition. There are 17 other sets of fuel trims for various engine operating conditions. You have to look at the LTFT's in all the cells, particularly the cells that correspond to the RPM/engine load conditions that are used for the dyno test.

Assuming the O2 sensors are accurate, it is running at the A/F ratio the cat is designed to operate at. That's the whole purpose of the LTFT's.

Now if you somehow had a "false rich" indication from the O2 sensors, the LTFT's would be causing it to run lean.

 

Thanks everyone! I am going to Sea-foam the combustion chambers to reduce hot spots & I have ordered replacement O2 sensors. Due to a foul up at California DMV everyone got the renewals too late. For $50.00 I got a 60 day extension. Not fair but at least the time pressure is off. I will post after I do these repairs. Thanks again,
Walter

 

The cat is new. Replace the O2 sensors LTFT is now normal around 125 failed smog again. Another new EGR valve -Nox is actually worse! O2 is a little high, still lean? Help!! Has anyone had anything weird that has cured this problem?

 

LTFT for which cell(s)?