Failed Emissions Test - High NOx

[paulo57509]

Robert, Thank you for the feedback. I've already been following your diag path but in a different order.

I've replaced the O2 sensor first (I had one on the shelf) with no change.

I hooked up a pressure gauge to confirm the fuel system is holding pressure which might indicate a leaking injector. The system holds pressure longer than 30 minutes. Pressure starts to a a few pounds after this time.

The next step is to make sure the EGR is opening and the passages are clear. My scan tool doesn't support EGR demand - thanks for confirming that I can jumper the jumper the valve directly from the battery.

[1project2many]

Rich mixture is indicated when the O2 content in the exhaust is extremely low compared to O2 in free air. NOx is created when combustion temperatures are very high. If high combustion temperatures are causing free oxygen to react with nitrogen then it will not be read by the O2 sensor. The 3.1 timing maps are generally very advanced at the load and speed where your car failed and require EGR to reduce NOx. Checking EGR is important.

[paulo57509]

I removed the EGR and jumpered the terminals between 12v and ground. All the solenoids/pintles open. I confirmed that the EGR valve passages are clear with compressed air with the pintles opened.

I left the jumpers in place and re-installed the EGR. With the engine running at idle I connected the jumpers between 12v and ground. The engine would stumble and want to stall when the EGR terminals were grounded (one by one). So the EGR passages are clear.

Signs are pointing to a bad converter.

What's bothersome is that the O2 sensor is reading a rich mixture. I'd like to correct this before installing a new converter. I need to go back to my notes and confirm fuel pressures, etc. and re-read the FSM for specs that are unclear.

Thanks for all the help that's been provided.

Just a data point; I replaced the plugs (when I replaced the O2 sensor) and they appear as they always have in the past.


Plugs - 27,393 miles; in order - 1 thru 6 (5/22/16)


Typical plug (42,369 miles) with new replacement (8/20/11)

[paulo57509]

<snip>

What's bothersome is that the O2 sensor is reading a rich mixture. I'd like to correct this before installing a new converter. I need to go back to my notes and confirm fuel pressures, etc. and re-read the FSM for specs that are unclear.

<snip>

Saturday afternoon I removed the plenum in order to get to the injectors and fuel rail.

I measured the resistance of each injector and got the following values. Temperature of the injector bodies at the time of measurement was 95.5F.

#1 - 12.4 ohms
#2 - 12.3 ohms
#3 - 12.4 ohms
#4 - 12.4 ohms
#5 - 12.3 ohms
#6 - 12.2 ohms

I don't have a spec to compare these values to.

I then ran an injector balance test, the injectors pulsed 50 times for this test. The initial fuel pressure was 43 psi. The results are below.

#1 - 37.0 psi; pressure drop = 6.0 psi
#2 - 37.5 psi; pressure drop = 5.5 psi
#3 - 37.0 psi; pressure drop = 6.0 psi
#4 - 38.0 psi; pressure drop = 5.0 psi
#5 - 37.5 psi; pressure drop = 5.5 psi
#6 - 37.5 psi; pressure drop = 5.5 psi

Pressure Drop Difference: 0.5 psi/1.0 psi (min/max)

I pulled the injectors/fuel rail from the manifold and looked into the injector holes in the manifold. I think I found the reason for the O2 sensor reading a rich mixture. The backs of the intake valve heads have a coating of those hard deposits (carbon?). My thinking is the deposits get fuel soaked and add fuel to the mixture that the O2 sensor picks up.

This link (http://www.crypton.co.za/Tto%20know/...h_mixture.html) states carbon on the intake valve can cause high NOx; I'm assuming due to hot spots?....the statement really doesn't elaborate. I'm thinking this is true if the deposits are on the face....but on the neck side of the valve head?

Anyone have any luck with removing these kinds of deposits short of taking the valves out of the head and taking a wire brush to it? Short of removing the heads, I'm thinking of trying that CRC intake cleaner for direct injection engines.

I did find the exhaust passage from the EGR to the plenum restricted but it was still opened. I had to use a burr to remove the carbon build up. Other than these couple of things, I really couldn't find anything out of the ordinary.

This evening I pulled the ECM and read the stock AXKK EPROM. I just got done burning a "test only" chip. I cut the values in the Main Spark Advance Table by 50% across the board. Perhaps this will get the NOx numbers below the threshold fail limit.

[paulo57509]

Just to close this thread; I got the car to pass.

[1project2many]

Congratulations. Was the timing change responsible, or were additional steps required?

[paulo57509]

I did multiple things at once so I really can't say what really affected the change the most. At least with the tune:

1. I went ahead and reduced the timing advance by 50%.

2. I turned the cooling fans on earlier by reducing their on and off temps by 20-degrees C.

By and large, the change in timing is what probably did the trick.