Help Passing Emissions Z71

[ROAD]

Post a pic of the test results,what did it fail for

 

[L31MaxExpress]

Once you address the tune up side, EGR and possibly cat, you can also put 1 gallon of denatured alcohol into about 4-5 gallons of gas. Drive it around, get it good and hot and do not shut it off before it gets tested. The alcohol burns cooler and cleaner and will help the numbers as well as clean your fuel system and injectors. Might also help to use a can of GM Top Engine clean, SeaFoam or the equivalent 2 stroke carbon remover, Qwiktune or the like.

I use a infared thermometer to test cat function. After driving the outlet side of the cat should be a solid 100F hotter than the inlet if it is doing its job. It is possible that someone welded a case that looks like a cat in place of the cat.

 

[L31MaxExpress]

Post a pic of the test results,what did it fail for

It is in the first post, it failed miserably on HC, CO and NOx. Close to double the allowed.

 

[L31MaxExpress]

I didn't notice a "cut wire" on the EGR, but I am thinking the O2 sensor is old, and the catalyst has a pipe for air injection but it's not connected--which may mean that the catalyst is an aftermarket one-size-fits-nothing replacement. It's certainly smaller than the original, pellet-style cat on my '88

I have also seen the values sky high when the timing is off.

The catalytic converter can produce NOx on its own if it has too much fuel getting to it and the substrate is overheating.

 

[ROAD]

if the tune up is solid, put a cat on it and do L31 MAX says and make sure the cat stays hot

 

[Schurkey]

Looks to me like the EGR Supply/Vacuum tube is cut/broken. I had to zoom into the picture to see it.

I did zoom in...and didn't see a missing hose. I saw a "90 degree elbow" that wasn't there. I somehow managed to see the rim of the EGR valve as if it were a vacuum hose. My fault.

 

[Sramos03]

As above, get the engine running properly via tune-up services including likely replacement of the O2 sensor (they get lazy with age) and potentially a catalytic converter or two, depending on how your truck was equipped originally. A scan tool will be very helpful. New-to-you vehicle? A cranking-compression test is recommended.

1. High hydrocarbons (HC) is often due to misfire. The entire cylinder-full of air/fuel mix goes out the exhaust valve when that cylinder doesn't fire, and the high HC is the fuel part of the air/fuel mixture. In the process it overheats the catalytic converter causing it to fail.

2. High Carbon Monoxide (CO) is generally due to an over-rich fuel mixture. The air/fuel mixture burns, but not completely because there's too much fuel/not enough oxygen (air). In a properly-proportioned fuel/air mixture, there's enough oxygen during the burn to reduce the CO into carbon dioxide (CO2)

When a cylinder misfires, it forces the O2 sensor to send a false "lean" signal to the computer because all the oxygen (air) in the misfiring cylinder is going out the exhaust valve along with the unburned fuel. A misfire therefore causes high HC, and indirectly, high CO.

Side-note: Since there's only one O2 sensor on the TBI trucks, either the sensor is severely screwed-up, or the misfiring cylinder is on the same bank that the sensor is monitoring. While it's possible that the misfire is so severe that the excess O2 is traveling from the other bank...it's not so likely.

3. High NOx often indicates a failed EGR system. Essentially, combustion temperature is excessive. The more-expensive catalyst systems also lower NOx emissions; but I don't know if your vehicle would be equipped with such a catalyst. I'd be looking at the EGR first, ignition timing second, and other engine-tune issues third.

Ended up getting it resolve the catalytic converter was the culpri

As above, get the engine running properly via tune-up services including likely replacement of the O2 sensor (they get lazy with age) and potentially a catalytic converter or two, depending on how your truck was equipped originally. A scan tool will be very helpful. New-to-you vehicle? A cranking-compression test is recommended.

1. High hydrocarbons (HC) is often due to misfire. The entire cylinder-full of air/fuel mix goes out the exhaust valve when that cylinder doesn't fire, and the high HC is the fuel part of the air/fuel mixture. In the process it overheats the catalytic converter causing it to fail.

2. High Carbon Monoxide (CO) is generally due to an over-rich fuel mixture. The air/fuel mixture burns, but not completely because there's too much fuel/not enough oxygen (air). In a properly-proportioned fuel/air mixture, there's enough oxygen during the burn to reduce the CO into carbon dioxide (CO2)

When a cylinder misfires, it forces the O2 sensor to send a false "lean" signal to the computer because all the oxygen (air) in the misfiring cylinder is going out the exhaust valve along with the unburned fuel. A misfire therefore causes high HC, and indirectly, high CO.

Side-note: Since there's only one O2 sensor on the TBI trucks, either the sensor is severely screwed-up, or the misfiring cylinder is on the same bank that the sensor is monitoring. While it's possible that the misfire is so severe that the excess O2 is traveling from the other bank...it's not so likely.

3. High NOx often indicates a failed EGR system. Essentially, combustion temperature is excessive. The more-expensive catalyst systems also lower NOx emissions; but I don't know if your vehicle would be equipped with such a catalyst. I'd be looking at the EGR first, ignition timing second, and other engine-tune issues third.

was finally able to get it to pass with ERG replaced, O2 sensor replaced and a new catalytic converter thankfully.