Help Passing Emissions Z71

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Sramos03

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Ive been having a hard time passing my AZ emissions. I wanted my truck to be my daily driver and since I’m a teacher with a 3 day weekend I wanted to give it a go myself. I recently purchased the truck it’s a 1994 Z71 with the 5.7. I’ve done an oil change as well as a new air filter. Any other tips on things I should change would be greatly appreciated.
 

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Erik the Awful

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Check your O2 sensor. Back-probe the connector with a multimeter and you should see it swing back and forth between rich and lean 7 times every 10 seconds.

If you haven't done a tuneup lately change your fuel filter, spark plugs, and wires. Good work getting the air filter already.

Check your distributor cap and rotor. If they're not in good shape, get quality replacements. United Motor Products makes a blue cap with brass contacts that's head and shoulders above most everything else.
 

PlayingWithTBI

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^^^This, plus -

Check for any MALF codes with a scanner or the paperclip trick. Your SES light won't always stay on but the code(s) will be stored for ~30 - 40 key cycles.

Make sure your base timing is set to zero with the EST bypass wire disconnected.

You need a 194° T-Stat.

Make sure your EGR is working properly
 

Sramos03

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Check your O2 sensor. Back-probe the connector with a multimeter and you should see it swing back and forth between rich and lean 7 times every 10 seconds.

If you haven't done a tuneup lately change your fuel filter, spark plugs, and wires. Good work getting the air filter already.

Check your distributor cap and rotor. If they're not in good shape, get quality replacements. United Motor Products makes a blue cap with brass contacts that's head and shoulders above most everything else.
Awesome thanks for the tips. What spark plugs would you recommend there’s so many different options just wanted to make sure I get something appropriate.
 

Schurkey

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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.
 

Sramos03

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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.
I was able to take some picture of the items you talked about to see if there’s anything visually wrong. For some reason the EGR has like a cut wire. This is a new truck to me just bought it last month. I had the transmission rebuilt but that’s about it. 225K on the engine I believe
 

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Schurkey

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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
 

Jeff Janssen

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